Patent Publication Number: US-11665230-B2

Title: Data center network device sensing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 16/289,572, titled ROLE-BASED COMMAND DISTRIBUTION ACROSS NETWORKS, filed 28 Feb. 2019, which is a continuation of Ser. No. 15/366,528, titled DISTRIBUTED PEER-TO-PEER DATA CENTER MANAGEMENT, filed 1 Dec. 2016, which is a continuation-in-part of U.S. patent application Ser. No. 15/065,212, titled OUT-OF-BAND DATA CENTER MANAGEMENT VIA POWER BUS, filed 9 Mar. 2016, which claims the benefit of the following U.S. Provisional Patent Applications: U.S. 62/130,018, titled RACK FOR COMPUTING EQUIPMENT, filed 9 Mar. 2015; U.S. 62/248,788, titled RACK FOR COMPUTING EQUIPMENT, filed 30 Oct. 2015; and U.S. 62/275,909, titled RACK FOR COMPUTING EQUIPMENT, filed 7 Jan. 2016. The entire content of each aforementioned patent filing is hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates generally to computing equipment and, more specifically, to management networks for data centers. 
     2. Description of the Related Art 
     Data centers are often used to house and interconnect large collections of computing devices, like servers, databases, load balancers, and high-performance computing clusters. Often, the computing devices are interconnected with two different networks: 1) an in-band network for conveying the data upon which the computing devices operate, for example, content in webpages, queries and query responses, and data for high-performance computing; and 2) an out-of-band management network for conveying commands to the individual computing devices to manage their operation, e.g., for conveying information like sensor data indicative of the operation of the computing devices or for remote serial console sessions for server management. 
     Out-of-band management serves a number of purposes, depending on the context. Often, out-of-band management networks are used to manage security risk, by limiting the attack surface of a network that could be used to control the computing devices and segregating the in-band network that often receives data from the outside world. In some cases, out-of-band management networks are operative to control the computing devices even when the computing devices are turned off, for example, by accessing memory on computing devices that is persistent (like flash memory) to perform things like extensible firmware interface (e.g., BIOS or UEFI) updates, read values from registers indicative of configuration or state, and the like. Other examples of activities include booting a device that is been turned off, remote installation of operating systems, updates, setting hardware clock speeds, updating or querying firmware versions, and the like. 
     SUMMARY 
     The following is a non-exhaustive listing of some aspects of the present techniques. These and other aspects are described in the following disclosure. 
     Some aspects include a process of managing rack-mounted computing devices in a data center with a distributed peer-to-peer management system, the process including: determining roles of data-center management computing devices in a distributed peer-to-peer data-center management system; receiving, via an out-of-band network, a data-center management command at a given data-center management computing device; distributing, based on at least some of the roles, via the out-of-band network, the data-center management command from the given data-center management computing device to a first plurality of other data-center management computing devices; and for each of the first plurality of other data-center management computing devices, distributing, based at least some of the roles, via the out-of-band network, the data-center management command to a respective second plurality of other data-center management computing devices, wherein: the given data-center management computing device, the first plurality of other data-center management computing devices, and the respective second pluralities of other data-center management computing devices are each configured to manage a respective plurality of rack-mounted computing devices; each of the pluralities of respective rack-mounted computing devices executes an in-band application that communicates on an in-band network different from the out-of-band network; and the data-center management command is a command to update or otherwise configure a microkernel, a virtual machine, a container, an operating system, or an application executing on one or more data-center management computing devices. 
     Some aspects include a tangible, non-transitory, machine-readable medium storing instructions that when executed by one or more processors effectuate the above process. 
     Some aspects include one or more processors and memory storing instructions that when executed by the processors effectuate the above process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned aspects and other aspects of the present techniques will be better understood when the present application is read in view of the following figures in which like numbers indicate similar or identical elements: 
         FIG.  1    is a physical-architecture block diagram that illustrates a data center configured for out-of-band management via power bus, in accordance with some embodiments; 
         FIG.  2    is a physical-architecture block diagram that illustrates a data center configured for out-of-band management without using a power bus for communication, in accordance with some embodiments; 
         FIG.  3    is a physical-and-logical-architecture block diagram that illustrates a data center configured for out-of-band management without using a power bus for communication and with a rack controller executed by a managed device, in accordance with some embodiments; 
         FIG.  4    is a physical-and-logical-architecture block diagram that illustrates a data center configured for out-of-band management via a power bus and with a rack controller executed by a managed device, in accordance with some embodiments; 
         FIG.  5    is a flow chart that illustrates an example of a process for out-of-band management of a data center, in accordance with some embodiments; 
         FIG.  6    is a flow chart that illustrates an example of a process to manage rack-mounted computing devices, in accordance with some embodiments; 
         FIG.  7    is a block diagram of a topology of a data center management system, in accordance with some embodiments; 
         FIG.  8    is a flow chart that illustrates an example of a process executed by the system of  FIG.  7    to manage a data center, in accordance with some embodiments; and 
         FIG.  9    is a diagram that illustrates an exemplary computing system by which the above processes and systems may be implemented, in accordance with embodiments of the present techniques. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
     To mitigate the problems described herein, the inventors had to both invent solutions and, in some cases just as importantly, recognize problems overlooked (or not yet foreseen) by others in the field of data center equipment design. Indeed, the inventors wish to emphasize the difficulty of recognizing those problems that are nascent and will become much more apparent in the future should trends in the data center industry continue as applicants expect. Further, because multiple problems are addressed, it should be understood that some embodiments are problem-specific, and not all embodiments address every problem with traditional systems described herein or provide every benefit described herein. That said, improvements that solve various permutations of these problems are described below. 
     This patent disclosure describes several groups of inventions that can be, but need not necessarily be, used together. The groups are described in a sequence that generally follows their relative proximity to the edge of an out-of-band network or networks. A first group of inventions described with reference to  FIGS.  1 - 4    and  FIG.  5    relates to a system and process by which an out-of-band network may be implemented between a rack-mounted computing device and a rack controllers or other data center management computing devices. A second group of inventions described with reference to  FIGS.  1 - 4    and  FIG.  6    relates to systems and processes for managing groups of rack-mounted computing devices with rack controllers and other data center management computing devices. A third group of inventions described with reference to  FIGS.  1 - 4    and  FIGS.  7  and  8    relates to systems and processes for managing groups of rack controllers and other data center management computing devices. Additional inventions described take the form of combinations of these groups. 
     Many extant out-of-band management products have deficiencies. Among these problems, the application program interface (API) capabilities of these products are lacking. Additionally, many existing out-of-band management products rely on a wired, Ethernet-based communications bus and some variation of a baseboard management controller (BMC) per server. As a result, many of these systems suffer from reliance on an API that is difficult to implement, sensorfication that is typically limited to chassis-only (e.g., on the chassis of the rack-mounted computing device, rather than on the rack itself or related infrastructure), and reliance on wired connections and a BMC, which may have a more limited set of exposed functionality than some embodiments. 
     Additionally, many data center management products also lack the ability to sufficiently aggregate and analyze data across a multitude of services, including application, operating system, network, building management system, hardware, power, fire, and other capabilities. The ability to look “north” and “south” of the rack is an open issue in the data center market. Typically, significant hardware investment has been necessary to achieve modest monitoring and control capabilities; the ability to provide northbound and southbound management and analytics capabilities in a variety of form factors (controller, switch module, etc.) has been hard to achieve. 
     Generally, there is a need for a turn-key, easy-to-operate set of management, sensorfication and intelligence services that are well-integrated with hardware like that described herein and used in data centers. This includes supported power, fire, network, and other ancillary equipment, in some embodiments. (This is not to suggest that some embodiments do not also suffer from different subsets of the above-described issues, as the various inventions described herein may be used to beneficial effect without implementing other inventions addressing other problems. Thus, not all embodiments mitigate all described problems, and some embodiments mitigate problems implicitly described that will be apparent to one of ordinary skill in the art.) 
     In some embodiments, a Data Center Sensorfication, Control, Analytics and Management Platform, referred to as “Vapor CORE,” mitigates some or all of the problems above by providing a set of services with south-of-rack (‘southbound’) management and monitoring capabilities, as well as north-of-rack (‘northbound’) aggregation and analytics capabilities. In some embodiments, these capabilities are exposed in a ‘southbound’ API and ‘northbound’ API, and are supported by a variety of tools and deployment options that can be used to integrate, analyze, visualize and operationalize the data and decisions generated by the APIs and components. In some embodiments, Vapor CORE&#39;s API may be exposed by rack control units or other data center management computing devices, like those described below and that execute logic to implement the APIs. 
     In some embodiments, Vapor CORE is implemented as a set of microservices (e.g., each executing a server monitoring a respective port on a respective computing device and executing code responsive to communications received via that port (either loopback or external) and sending output via that port), executing on the rack controller described below, and deployed and managed automatically by the techniques described below or those the Crate Configuration, Container and File management system described in U.S. Patent Application 62/275,909, filed 7 Jan. 2016, titled RACK FOR COMPUTING EQUIPMENT, the contents of which are incorporated by reference above. The services may be organized in two categories: a southbound API and a northbound API. 
     The southbound API of Vapor CORE, in some embodiments, provides a RESTful (representational state transfer) API built atop Nginx™ and uWSGI™, using Flask™ as the microdevelopment framework. A Python™ package, in some embodiments, is used to interpret API requests and translate them into a serial protocol payload, sent over power line communications (PLC) to devices along the serial bus. Response data, in some embodiments, is read from the serial bus, interpreted, and returned to the API consumer in JSON format. The API capabilities, and corresponding implementing code, in some embodiments, include:
         Ability to scan or enumerate the devices to gather a full picture of devices and capabilities available; the results of the scan, in some embodiments, are returned in an easy-to-interpret form that allows for easy programmatic access. These results, in some embodiments, also include locational information for sensors and devices, for easy mapping and visualization of every piece of data gathered from the system, as discussed in U.S. patent application Ser. No. 15/337,732, filed on 28 October, 2016, titled SENSING LOCATION OF RACK COMPONENTS, the contents of which are hereby incorporated by reference, and elsewhere in this document with reference to location-awareness-related sensing.   Reading of analog and digital sensors. This, in some embodiments, involves translating a stream of bytes into a meaningful and human-understandable response. Sensor support, in some embodiments, includes humidity, temperature, pressure, vibration, particulate, noise, and other similar sensors.   LED (light emitting diode) control for rack (e.g., wedge) status communication. The LED light at the top of a rack, in some embodiments, may be changed in terms of color, illumination, or blinking to visually communicate rack status. Or audible or other visual indicators may be actuated.   Fan control, including, in some embodiments, getting fan status in terms of revolutions-per-minute (RPM).   Power control and status. This involves, in some embodiments, sending a command to a device, requesting power on/off/cycle or status. In some cases, power status is returned and translated from the power supply&#39;s format to a more easily machine- or human-interpretable form. This, in some embodiments, includes power status (on/off), power “ok” (true/false), voltage and current consumption, and whether the power supply registers an undervoltage or overcurrent condition.   Device inventory, including subcomponent inventory from servers and devices in the rack. The response, in some embodiments, may also include Extensible Firmware Interface (EFI) (like Basic Input/Output System (BIOS) or Unified Extensible Firmware Interface (UEFI)) information, which may include items such as Asset Tag and other device-specific information.   Boot device selection, allowing, in some embodiments, boot device to be retrieved from device, as well as for the boot target to be specified. This, in some embodiments, may be used for automated provisioning of data center devices.   Door lock control. Door locks, in some embodiments, may be controlled on a rack&#39;s front door to allow/deny physical access to rack contents, as well as to provide an audit trail of physical access to a given rack. Or some embodiments may implement U-specific locks that gate access to individual rack-mounted computing devices.   Intelligent Platform Management Bus (IPMB) Communications Protocol. Some embodiments allow sending/receiving of IPMB packets over serial bus to carry out standard IPMI commands over IPMB via a powerline communication (PLC) bus.   OCP (Open Compute Project™) debug header POST (power-on self-test) code retrieval.   Firmware flash. Some embodiments allow device firmware to be remotely updated through the API.   Version information about the endpoint, software, and API.       

     Additionally, in some embodiments, the southbound services provided by Vapor CORE include serial console access to individual devices in the rack. This access, in some embodiments, is provided over PLC as well, and is mapped to virtual TTY devices, that may be accessed locally from the rack controller described below, or remotely via secure shell (SSH) to a Transmission Control Protocol (TCP) port on the rack controller described below. This, in some embodiments, is implemented by mapping a TTY to a device ID, and communicating with rack controller described below to marshal access to the serial console of that device. 
     The services described above, in some embodiments, are distributed via a Docker™ container, managed by Crate (described below with reference to  FIGS.  1 - 4  and  7 - 8   ). Serial console access, in some embodiments, is managed by a separate operating system module that scans the bus and creates and maps the devices based on its interpretation of the scan command. 
     In some cases, the southbound API may facilitate the process of obtaining data from sensors on rack-mounted devices, like servers. Often, sensors impose a large number of steps to convert sensor readings to things useful to machines or humans, e.g., converting a voltage from a thermocouple to a temperature. To mitigate this issue, some embodiments may identify what type of sensor is present based on a code returned that indicates the sensor type, which may be obtained based on electrical characteristics and reading registers, for instance, using techniques described below or in U.S. Patent Application 62/275,909, in a section titled “External Access to Local Motherboard Monitoring and Control.” In some embodiments, based on the code, the appropriate conversion may be selected, e.g., volts to degrees Celsius. Some embodiments may use this technique to obtain temperature, humidity, particulate count, air flow rates, etc., as opposed to a voltage or a current or other scaled value. 
     The northbound side of Vapor CORE, in some embodiments, is a separate microservice that is designed in a three-level pluggable architecture including the following:
         Data source plugins   Analytics engine   Presentation plugins       

     Data source plugins, in some embodiments, may be registered with Vapor CORE given a standard data source data transformation API and may be used to gather data from a variety of sources in the data center, including, in some embodiments, the southbound API, Crate, building management systems, fire suppression and power distribution systems, Bloom box API™, other management and control systems (e.g., Puppet™, Chef™, Ansible™, etc.), IPMI/iLO/DRAC, etc. Registration of components, in some embodiments, includes storage of addressing, credentials and rules for polling, among other things; registration data piggybacks the data store in the analytics layer, and may be carried out via API or user interface (UI) (both distributed with Vapor CORE). 
     In some embodiments, an analytics engine component serves as a primary data store for Vapor CORE and stores plugin registration, as well as schematized data gathered by plugins. Pre-built analytics routines, in some embodiments, may be included with Vapor CORE to compute metrics such as price per watt per dollar (price/W/$), cost of cloud, etc. Additional analytics routines, in some embodiments, may be developed by customers or solution vendors, and snapped into the analytics engine, bound to the data source plugins registered with Vapor CORE. 
     Presentation plugins (e.g., executing on a data center management computing device, like those described below, for instance in a dashboard application), in some embodiments, may be registered with Vapor CORE given a standard presentation plugin API and may be used to export the result of analytics routines in a variety of forms (e.g. UI, comma separated values (CSV), JavaScript™ Object Notation (JSON), extensible markup language (XML), etc.). Presentation plugins, in some embodiments, are bound to a set of analytics routines and data sources stored in the analytics engine, and transform and present the data in a variety of ways. Presentation plugins, in some embodiments, are registered in a similar manner to data source plugins, and their registration also includes configuration of the output mechanism (e.g. TCP port, file, etc.). 
     In some embodiments, Vapor CORE sits atop a variety of data sources and provides an endpoint exposing raw, aggregate and computed data points that may be consumed by a higher level tool, such as an orchestration or automation engine, dashboard, or as a data source input to another Vapor CORE instance. Crate and Vapor CORE, in some embodiments, may also communicate reciprocally to inform and perform automated management tasks related to data center equipment or Vapor components such as Vapor Edge Controllers (VECs) and Vapor software. 
     Vapor CORE, in some embodiments, may exist in a hosted environment, and be used to remotely monitor and manage a data center, as described elsewhere herein with reference to Crate. 
     In some embodiments, Vapor CORE may perform or provide critical (which is not to imply that it, or any other feature, is required in all embodiments) environment live migration, data center management or devops capabilities, workload and management capabilities and the like. 
     In some embodiments, the distributed nature of Vapor CORE and the data center racks may allow for strategies for data aggregation and analysis in a decentralized manner. This is expected to allow for the computing resources of the rack controllers to be well-utilized and facilitate operations at scale. 
     Some embodiments may be configured to obtain a device inventory and boot selection from rack-mounted devices. For instance, upon scanning (e.g., an inventory scan for a particular device), some embodiments may access a system management bus (SMBUS) on the server midplane and retrieve a list of processors and device list seen by the operating system of the server. In some cases, this data may be acquired from SMBUS without using an agent executing within the operating system or on the CPU of the server. Similarly, some embodiments may access this bus to interrogate and change boot target selection or adjust BIOS (or other EFI) settings in memory, e.g., for automated provisioning that includes switching to a different boot target on the network to roll out a new BIOS. In some embodiments, the boot target can be read and set by the southbound API, e.g., with a representational state transfer (REST)-based request. Further, some embodiments may perform agentless system monitoring of the operation of the rack-mounted device, e.g., tracking a server&#39;s CPU usage rate and memory consumption, in some cases, without using a BMC. Further, some embodiments may provide for remote console access—remote TTY—over powerline communication. In some cases, because communication occurs via a web proxy, web-based security techniques may be employed, like OAuth and Lightweight Directory Access Protocol (LDAP). 
     In example use cases of some embodiments, these techniques may be used to view diagnostic information describing a boot operation. For instance, if a machine is power cycled, some embodiments may retrieve power-on self-test (POST) codes for troubleshooting. These techniques are best understood in view of an example computing environment. 
     In some cases, the features of Vapor Crate are provided by some of the embodiments described below with reference to  FIGS.  1 - 5  and  7 - 8   , the features of Vapor CORE are provided by some embodiments described below with reference to  FIGS.  1 - 4  and  6   . 
       FIG.  1    illustrates a data center  10  configured to mitigate a variety of problems, both those explicitly discussed below, and those implicit in the description and which will be apparent to those of ordinary skill in the art. In some embodiments, the data center  10  includes a plurality of racks  12  (e.g., identical racks arranged in a pattern, like a rectangular array or hexagonal packing pattern), examples of which are described in each of the applications incorporated by reference, such as those titled RACK FOR COMPUTING EQUIPMENT. 
     These applications describe, in certain embodiments, wedge-shaped racks arranged to form chambers, and those wedges may serve as the racks herein. Or the racks may take other forms, e.g., traditional racks, e.g., those with hot aisles, arranged edge-to-edge along linear aisles, either with front-access or rear-access for maintenance. 
     The racks may house (e.g., mechanically support, cool, and provide data and power to) a plurality of rack-mounted computing devices  13 , an example of which is described below with reference to  FIG.  9   . In some embodiments, the data center  10  includes a relatively large number of racks  12 , for example, more than 10, or more than 20, and each rack may house a relatively large number of computing devices  20 , for example, more than 10, and in many cases, more than 50. In some cases, the rack-mounted computing devices  13  are arranged in discrete units of space, called “U&#39;s,” for instance in a vertical stack of U&#39;s. In some cases, the rack-mounted computing devices are mounted to rails (e.g., on a slideable shelf) and can be slid horizontally outward from the rack for service. Or in some cases, the racks have U&#39;s arrayed horizontally, and rack-mounted computing devices  13  may be slid vertically, upward, like out of a bath of cooling liquid, such as mineral oil. In some cases, a cooling fluid (e.g., liquid or gas) is conducted over the rack-mounted computing devices  13  for cooling. Three racks  12  are shown, but embodiments are consistent with a single rack or substantially more racks. Each rack  12  may include the features illustrated in the enlarged view of one of the racks  12 . 
     Data centers provide and use computational resources (e.g., applications, networks, computing devices, virtual machines, containers, and the like) in two domains: (i) to provide general purpose computing power (or special purpose computing power) to a user; and (ii) to manage the data center itself. Accordingly, it is helpful to define terminology to distinguish between these domains, as commercial implementations often use different types of resources in each domain, typically with much less expensive and much less powerful resources being used for management. These different domains are distinguished herein by more broadly leveraging the “in-band” and “out-of-band” modifiers used in industry to identify networks serving these respective domains. Thus, the rack-mounted computing devices  13  may execute “in-band” applications that provide the functionality for which a data center or rack therein is built, e.g., hosting user-facing software-as-a-service applications or virtual machines, storage, or containers provided as service to remote users or applications. This is distinct from “out-of-band” resources (applications, computing devices, and networks) used to manage the rack-mounted computing devices  13 , as part of the infrastructure of a rack or (i.e., and/or) data center. In some embodiments, the out-of-band networks have less than ½ the bandwidth of the in-band network, e.g., less than 1/10th. In some embodiments, the out-of-band computing devices (or corresponding constructs, like virtual machines) have less than ½ the available floating-point-operations-per-second than the in-band computing devices, e.g., less than 1/10th. Some embodiments may keep these out-of-band infrastructure resources and in-band applications separate, either partially (e.g., with different containers or virtual machines) or fully (e.g., with different computing devices), for security purposes. 
     To this end and others, some embodiments may include an in-band-network  15  (e.g., implemented with network switches  11 , like top-of-rack switches) and an out-of-band network  17 , each having a distinct address space (e.g., a private Internet-Protocol (IP) subnet or different ranges of public IP addresses), with the former  15  conveying data between rack-mounted computing devices  13  or the public Internet  19 , and the latter  17  conveying data within the data center  10  (and in some cases, externally) for purposes of managing the rack-mounted computing devices  13  (e.g., monitoring, provisioning, load-balancing, updating, servicing, etc.) and related infrastructure (e.g., monitoring, responding to, and adjusting cooling or power delivery). Keeping the networks  15  and  17  (and related computing devices or applications) separate is expected to reduce the likelihood of a penetration of the more externally facing in-band-network  15  resulting in an attacker gaining control of data center infrastructure. That said, embodiments are also consistent with consolidating these networks or different subsets of the out-of-band resources (e.g., computing devices or applications). 
     Many traditional out-of-band networks present a number of problems in data center designs. For instance, often switching and wiring is replicated relative to the in-band-network all the way to the edge of the out-of-band networks, often doubling the networking equipment costs and wiring complexity in a data center (which should not be read as a disclaimer, as some embodiments of some inventions described herein are consistent with this approach). 
     These problems often co-occur with other undesirable aspects of data center hardware. Additional issues include added cost for power distribution and conditioning circuitry. In many cases, power is distributed within a data center via alternating current, while individual computing devices generally operate on direct current. In many cases, the transition between alternating current and direct current is made with computing-device specific AC-to-DC power converters. This architecture has the undesirable effect of multiplying the number of power converters within a data center, placing a heat source and electromagnetic radiation source near sensitive computing equipment, occupying valuable rack space, and multiplying the number of locations were failures may occur in hardware. (These discussions of problems with traditional design should not be taken as disclaimers of subject matter, as several inventions are described, and they are independently useful and may be used in environments where some problems persist while others are addressed.) 
     To mitigate these issues, in some embodiments, an edge-portion of the out-of-band network may be replaced or supplemented with a plurality of powerline communication networks that deliver both data and direct-current power to a plurality of rack-mounted computing devices. In some cases, each rack  12  may include a (e.g., one and only one, or more than one) rack-specific powerline network, which may be a DC-powerline network  16 . Or in some cases, an individual rack  12  may include a plurality of powerline networks, or a powerline network may span multiple racks  12 . Or in some cases, a rack  12  may have separate power-delivery and sub-networks (relative to the out-of-band network  15  extending throughout a data center). 
     Thus, some embodiments may include 1) an in-band-network  15 ; 2) a data center-wide out-of-band network  17 ; and 3) a plurality of sub-out-of-band networks  16 . Each sub-out-of-band network may have their own address space distinct from each other and the data center-wide out-of-band network and in-band network  15  (e.g., each using the same addresses for different devices on different racks), and each sub-out-of-band network may provide out-of-band network access for monitoring and controlling a plurality of rack-mounted computing devices  13  (e.g., a full-rack) and sensors and other actuators associated with the plurality of rack compute units  20  (e.g., associated with the rack). 
     To these ends and others, an alternating-current-to-direct-current converter  14  may deliver direct current to each of the racks  12  via a bus  16  that also conveys data. In some embodiments, each rack  12  may include its own dedicated converter  14  that services a collection of computing devices on the rack, or in some cases several racks  12  may share a converter  14 . In some embodiments, converter  14  includes a rectifier, step down transformer, and low-pass filter operative to deliver, for example, to racks  12 , direct current power. Rack-specific converters are expected to segment the media for out-of-band data signaling, reduce the number of users of the address space on the media, and permit simplified and less expensive circuitry for the devices communicating on the DC power bus, but embodiments are also consistent with busses shared across racks. Having consolidated AC-to-DC converters for a collection of computing devices (e.g., a full rack) is expected to avoid the cost and thermal load arising from performing the conversion at each computing device with a dedicated converter, though embodiments are also consistent with this implementation. Some embodiments may have one AC-to-DC converters per plurality of rack-mounted computing devices  13 , e.g., one per rack, or one per collection of racks. In some cases, data may be conveyed via an AC powerline network. 
     In some embodiments, direct-current power is distributed throughout a rack  12  via a direct current power bus  16 . In some cases, the direct-current power bus  16  includes two distinct conductors, for example, carrying ground and a 12-volt or 48-volt potential with two and only two conductors along a terminal portion of a path to a device receiving power and data at an edge of the sub-out-of-band network formed by bus  16 . In some embodiments, each rack  12  may include the DC bus  16 , or a dedicated DC bus  16  specific to that rack, for example, to maintain and address space within a rack that is distinct from that of other racks and simplify signaling protocols (e.g., by reducing the number of devices contending for a given instance of the network medium) and reduce cost of associated circuitry. 
     In the illustrated embodiment, racks  12  each include a rack control unit  18 , a plurality of rack-mounted computing devices  13 , a plurality of sensors  21 , and a plurality of actuators  23 . The racks  12  may have a plurality of rack computing units  20 , e.g., each being one U and having one or more of the rack-mounted computing device  13  along with device-specific support hardware, like the adapters  30  described below. Two units  20  are illustrated, but embodiments are consistent with substantially more, for example, on the order of 8 or more per rack. Some embodiments may have multiple rack-mounted computing devices  13  per unit  20 , or multiple units  20  per device  13 . 
     In some embodiments, the rack control unit  18  is a type of data center management computing device and, thus, may exercise local control and monitoring (e.g., without directly monitoring or controlling devices in other racks—though embodiments are also consistent with this) over the operation of devices  20 ,  21 , and  23  in the rack  12  (and perform operations distinct from a network switch that routes in-band data), and each rack  12  may include its own independent rack control unit  18 . In other cases, the data center management computing device may be rack-mounted computing device executing a rack controller that exercise local control and monitoring (which is not to imply that monitoring is not an aspect of or form of control). 
     In some embodiments, the rack control units  18  may operate as gateways between an Ethernet out-of-band network  17  and a DC power bus networks  16 , for example, specific to each rack  12 . In some embodiments, the out of band Ethernet network  17  may connect each of the racks  12  via their rack control unit  18 , and the data center may be managed via networks  16  and  17 , with monitoring data being sent back to a data center management computing device  25  via networks  16  and  17  and commands being distributed via network  17  for implementation by controllers  18  and networks  16 . 
     Sensors  21  may be any of a variety of different types of sensors, like those described below as being associated with rack computing units  20 . Examples include temperature, particulate, vibration, humidity, optical, and other sensors. In some cases, the sensors  21  are secured to the rack  12  itself, rather than a computing unit  20  or device  13  (e.g., the device  13  can be removed and the sensor  21  would remain on the rack, and the sensor  21  may be on the rack before any devices  13  are installed). In some cases, the sensors  21  are not specific to an individual computing unit  20  or device  13 . Or some embodiments may include, as part of the rack  12 , one or more sensors for each U in the rack  12 , e.g., a location sensor like those described in U.S. patent application Ser. No. 15/337,732, filed 28 Oct. 2016, titled SENSING LOCATION OF RACK COMPONENTS, the contents of which are incorporated by reference. 
     In some cases, the sensors  20  sense an attribute of the rack and its environment and send signals indicative of measurements via network  16  to controller  24 . In some cases, some sensors  20  are based on microcontrollers rather than full computers (having an operating system executed on a microprocessor) to sense and report values without incurring the cost and thermal load associated with a full computer (though embodiments are also consistent with this approach). 
     Actuators  23  may have features similar to the sensors  21  in the sense that some are microcontroller-based, some are distinct from units  20  and devices  13 , and some draw power from the network  16  for similar reasons. In some cases, the actuators  23  are controlled by the rack control unit  18 , e.g., reporting via network  16  a physical state of the sensor, receiving a command to change that state via network  16 , and effecting the change with power from the network  16 . A variety of different types of actuators may be included. Examples include a fire-suppression actuator operative to release a fire-suppression chemical (e.g., a gas or foam). Other examples include an actuator operative to adjust cooling fluid flow (e.g., a solenoid configured to cause rotation or translation of components of the spatially modulated airflow restrictors described in U.S. patent application Ser. No. 15/065,201, filed 9 Mar. 2016, titled COOLING SYSTEM FOR DATA CENTER RACK, the contents of which are hereby incorporated by reference) in a selected part or all of a rack (like responsive to a fire being detected to remove airflow or responsive to a temperature sensor in a rack indicating higher-local temperatures). For instance, some embodiments may detect a higher temperature in an upper part of one rack with a sensor  21  than a lower part, and with controller  18 , instruct an actuator  23  to adjust a vent to afford greater airflow in the upper part or restrict airflow in the lower part (or other fluids). In another example, some embodiments may include a locking actuator, e.g., a pin driven by a solenoid biased open or closed by a spring into an aperture in an otherwise moveable component, and the lock may lock a given computing unit  20  shelf in place or a rack door closed, thereby providing physical security. In some cases, a sensor on the face of a rack may include a near-field-communication (NFC) sensor by which a technician&#39;s NFC card (or mobile device) is scanned to authenticate access, thereby limiting physical access to those authorized and providing an audit trail for who accessed what when. 
     In some cases, the sensors  21  and actuators  23  may be powered by and communicate with the network  16 . Having distributed DC power and network communication available is expected to facilitate the use of denser and more widely distributed networks of sensors and actuators than is feasible in traditional designs in which each sensor would need its own AC-to-DC power source and an Ethernet network interface, adding cost and thermal load (though not all embodiments afford this benefit, which is not to imply that other features may not also be varied). In some cases, sensors  21  and actuators  23  may be operative without regard to whether a rack computing unit  20  is present or on, thereby providing sensing and control that is robust to crashes or lower-density deployments. 
     In some embodiments, remote terminal sessions, for example, may be maintained between the administrator&#39;s computer  25  connected to network  17  and individual rack computing units  20  via networks  17  and  16 . In some embodiments, rack control units  18  may monitor the operation and presence of rack computing units  20  and, in some cases, components of those rack computing units  20 , via the powerline communication bus  16 . In some embodiments, the rack control unit  18  may be configured to periodically poll existing devices on the network  16  and report back via network  17  the result of the poll to device  25 . In some cases, rack control units  18  may periodically request, from each rack computing unit  20  via the DC power bus  16 , the status of various sensors, such as temperature sensors, vibration sensors, particulate sensors, fan speed sensors, airflow sensors, humidity sensors, air pressure sensors, noise sensors, and the like. In some embodiments, rack control unit  18  may compare the reported values to a threshold and raise or log various alarms, for example, via network  17 , to bring a condition to the attention of an administrator. Similarly, in some cases, rack control unit  18  may implement various changes on rack computing units  20  by a command sent via network  16 . Examples include instructing rack computing units to boot up or turn off, update an EFI, change a setting in persistent flash memory (in some cases bypassing the EFI), update or report a firmware version, change a register value in a peripheral, and initiating and executing a remote terminal session. In some embodiments, rack control unit  18  and network  16  are operative to exercise control over the rack computing units  20  even when the computing devices, such as servers of those rack computing units, are turned off. This is expected to reduce the burden on maintenance personnel, as certain operations can be performed remotely, even in scenarios in which the computing devices are turned off. 
     In some embodiments, rack computing units  20  each occupy a respective shelf or a receptacle, such as a “U,” in the rack. In some embodiments, each rack computing unit includes a distinct computer, having a dedicated processor and memory that operates to execute an operating system and application within a distinct memory address space. Any of a variety of applications may be executed, including web servers, databases, simulations, and the like, in some cases in virtualized computing devices or containers. In some cases, the rack-mounted computing devices  13  in each unit  20  are general purpose computers, or some embodiments may include special purpose computers, such as graphical processing units, bitcoin mining application specific integrated circuits, or low-floating-point precision (e.g., less than 16 bit) ASICS for machine learning. In some embodiments, the applications may communicate with one another and remote users via the in-band network  15  that conveys the data the computing devices operates upon, which stands in contrast to the management data by which the computing devices are managed and monitored via the out-of-band network  17 . 
     In the illustrated embodiment, rack control unit  18  includes a rack controller  24 , and out-of-band network interface  26 , and a powerline modem  28 . In some embodiments, the rack controller  24  may implement the logical functions described above and below, for example, for monitoring the rack computing units  20  and sensors  21 , controlling the rack computing units  20  and actuators  23 , and translating between the networks  16  and  17 . In some embodiments, the rack controller  24  may execute routines that control, engage, and disengage various thermal control units, such as fans or adjustable airflow restrictors, that maintain the temperature of the rack computing units  20 , for example, responsive to temperature sensors on the units  20  indicating an imbalance in airflow or positive pressure in an exhaust region. In some embodiments, the rack controller  24  is an application executing on a distinct computing device having a processor, memory, and an operating system, and such as a computer serving as the rack control unit  18  without hosting in-band applications, e.g., one provided with the rack  12  before in-band computing devices are installed. In some embodiments, the rack controller  24  includes a REST-based web server interface operative to receive instructions and provide responses on the network  17  according to a RESTful API. In some cases, the REST-based API may face the out-of-band network  17 , receiving API requests via this network from other rack control units  18  or the administrator computing device  25 . 
     In some embodiments, the out-of-band network interface  26  is an Ethernet network interface having an associated driver executing in the operating system of the rack controller  24  and configured to move data between buffer memory of the network interface and system memory, e.g., with direct memory access, and provide interrupts indicative of such movements. In some cases, the out-of-band network interface  26  connects to an Ethernet cable, such as a CAT5 (category 5), or CAT6 (category 6) cable connecting to the other racks  12 . 
     In some embodiments, the various devices on the DC power bus  16 , including the rack control unit  18 , include a powerline modem  28 . In some embodiments, the powerline modem  28  is a direct current powerline modem operative to encode data on top of a direct current power source. (Signals readily separated from the DC power, e.g., at higher than a threshold frequency or less than a threshold root-mean-square deviation for the median, do not transform the DC power to AC power.) In some embodiments, the data is transmitted by applying an electrical stimulus to the electrical conductors conveying direct current power. The stimulus may take any of a number of different forms. Examples include selectively connecting a higher or lower voltage to the conductors, thereby pulling the voltage up or down in a manner that may be sensed by other powerline modems. Other examples include selectively connecting a current source or drain to the conductors of the DC power bus  16 , thereby again imparting an electrical signal on top of the DC power that may be sensed by other computing devices. In some embodiments, an impedance may be selectively coupled to the DC power bus, thereby, for example, affecting fluctuations imposed on top of the DC power bus in a manner that may be sensed by other powerline modems. 
     In some embodiments, the electrical stimulus is a time varying electrical stimulus. Data may be encoded by varying the electrical stimulus a number of different ways. In some embodiments, the stimulus may simply be turned on and off according to a clock signal, like with a square wave, and data may be conveyed by determining during each clock cycle whether the stimulus is applied or not, indicating a zero or one. In other examples, the stimulus may be used to adjust an attribute of a wave, like a carrier wave, maintained on the DC power bus. For example, data may be encoded with pulse width modulation, by applying a square wave to the DC power bus and adjusting the time of a falling edge of the square wave according to whether a zero or one is being transmitted. Other examples may adjust a rising edge of the square wave or a duty cycle of the square wave or other waveforms. In some embodiments, multiple attributes may be adjusted, for example varying in amplitude of the wave, a duty cycle of the wave, and times for falling or rising edges of the wave to encode additional data in a more compact form. 
     In some embodiments, at the same time data is being conveyed on the DC power bus, DC power is also being conveyed. In some embodiments, the data signals may be configured such that they do not interfere with the delivery of DC power. For example, the time varying electrical stimulus may change the DC voltage or current by less than a threshold percentage of what is delivered, for example with a RMS value less than 10% of the median, such that filtering can readily remove the data signal from electrical power being delivered to computing devices that are often sensitive to variations in electrical power. In other embodiments, the speed with which the data is conveyed, or a carrier wave, may be at a frequency such that low-pass filters can readily distinguish between the DC power component and the data component. 
     In some embodiments, to facilitate separation of data from power, the data may be encoded with pulse width modulation, such that data-dependent effects are less likely to interfere with power delivery. For example, absent a carrier wave, a relatively long string of ones or zeros that are consecutive may cause power to fluctuate on the downstream side of a low-pass filter, resulting in low-frequency increases or decreases in voltage of the DC powerline that may penetrate a low-pass filter. In contrast, pulse width modulation maintains a relatively uniform average voltage after a low-pass filter is applied, as the frequency of the pulses that are modulated may be selected such that they are readily separated from the underlying DC power signal. 
     In some embodiments, access to the DC power bus as a medium for data transmission may be arbitrated with a variety of techniques. Examples include time division multiplexing, code division multiplexing, frequency division multiplexing, orthogonal frequency-division multiplexing, and the like. In some implementations, it is expected that the bandwidth requirements for the network  16  will be very low (e.g., less than 100 kilobits per second), and an encoding scheme may be selected to reduce the cost of the associated circuitry. For example, in some implementations, the speed and cost of Ethernet connections may be excessive relative to the requirements for signaling. In contrast, relatively low bandwidth time division multiplexing circuitry on a synchronous network is expected to cost substantially less while still providing adequate bandwidth. This is not to suggest that embodiments are inconsistent with higher bandwidth architectures. It should be noted that many in the industry have persistently failed to recognize this opportunity for cost reduction and circuitry simplification. 
     In some embodiments, each powerline modem  28  may select a duration of time over some cycle in which that powerline modem on the network  16  is permitted to transmit, e.g., in the event that the powerline modem does not detect that another device on the network currently has control of the media. In some embodiments, a device has control of the media if it has received a request on the network  16  and has not yet responded. In some embodiments, the network may be a synchronous network. In some embodiments, the duration of time dedicated for each powerline modem on the network  16  to transmit when the media is unclaimed may be selected based on a factory set value, like a media access (MAC) address, initially. 
     In some embodiments, an ad hoc mechanism may be used to deal with collisions, in which multiple devices have selected the same duration of time. In some embodiments, the powerline modem  28  may be operative to detect when another device is transmitting at the same time, and in response, select a different duration of time, for example, randomly (like pseudo-randomly or by seeding a linear shift register with less significant digits of a reading from a temperature sensor). For instance, powerline modem  28  may have reserved as its time to transmit between zero and 100 milliseconds (ms) after some timing signal, while a powerline modem of a first rack control unit may have reserved as its time to transmit 100 ms to 200 ms, and a different rack computing unit may have as its time to transmit 300 ms to 400 ms. Collisions occur when two devices select the same duration of time, and a randomized re-selection may alleviate the conflict without a central authority allocating time slots. Selecting transmission durations in an ad hoc fashion is expected to substantially lower the cost of maintenance and simplify installation, as devices can be installed on the network  16  without additional configuration, in some embodiments. That said, not all embodiments provide this benefit, as several inventions are described that are independently useful. 
     In some embodiments, the modem  28  may encode data and commands in a particular format, for example, in packets having headers with an address of the receiving and transmitting devices. In some embodiments, each powerline modem on the network  16  may receive signals and determine whether the signal includes a packet having a header designated for that device. In some embodiments, the packets may include error correction and detection, for example, with parity bits, Hamming codes, or other redundant lower entropy encoding. 
     A variety of techniques may be used to receive signals. For example, some embodiments may apply the signal on the DC power bus  16  to a low-pass filter and then compare the filtered signal to the signal on the DC power bus  16  to determine a differential signal having, for example, a higher frequency component conveying data. In some cases, the differential may be compared to a threshold to determine whether a zero or one is being transmitted. Or a pulse-width modulated signal may be compared to an unmodulated signal of the same underlying frequency, and changes in edge timing may produce a signal that, when compared to a threshold, indicates a zero or one. 
     In some embodiments, the signals may correspond to those traditionally used in RS232 connections to facilitate re-use of existing hardware and software. Examples include the Data Terminal Ready signal, indicating that data terminal equipment (DTE) is ready to receive, initiate, or continue a call; the Data Carrier Detect signal, indicating a data circuit-terminating equipment (DCE) is receiving a carrier from a remote DCE; Data Set Ready, indicating that DCE is ready to receive commands or data; Request to Send, indicating that a DTE requests the DCE prepare to transmit data; Request to Receive, indicating that a DTE is ready to receive data from a DCE; Transmitted Data, carrying data from the DTE to DCE; and Received Data, carrying data from the DCE to the DTE. 
     In some embodiments, communication may be via request and response, where once a request is sent by one device on the network  16 , the recipient device has the exclusive right to transmit on the network  16  until a response is sent. Or some embodiments may use a master-slave architecture, where, for example, the powerline modem  28  of the rack control unit  18  arbitrates which device communicates on the network  16  and when. Request and response synchronous architectures, however, are expected to allow for relatively simple and inexpensive circuitry, which may be favorable in some implementations. 
     As illustrated, in some embodiments, each rack computing unit  20  may include a network and power adapter  30  and a rack-mounted computing device  13  (a term that is reserved herein for in-band computing devices (which may be hybrid devices that also execute out-of-band applications in some embodiments like those described with reference to  FIGS.  3  and  4   )). In some embodiments, the network and power adapter  30  may separate DC power and data from the DC power bus  16 , provide the power to the rack-mounted computing device  13 , and process the data to implement various routines locally with logic that is independent of the rack-mounted computing device  13  and operates even when the rack-mounted computing device  13  is turned off. 
     In the illustrated embodiment, the network and power adapter  30  includes a low-pass filter  34 , a powerline modem  36 , and a microcontroller  38 . In some embodiments, these components  34 ,  36 , and  38  may be mounted to a printed circuit board that is distinct from a motherboard of the rack-mounted computing device  13  and couples, for example, via a cable, to the motherboard of the device  13 . In some embodiments, the low-pass filter  34  may be operative to receive the DC power from the DC power bus  16 , having the data signals overlaid there on, and remove the data signals to transmit a smooth, high quality DC power source to the rack-mounted computing device  13 . A variety of techniques may be used to implement the low-past DC filter  34 . In some embodiments, an inductor may be placed in series between the bus  16  and the rack-mounted computing device  13  to provide a relatively large impedance therebetween and reduce the power required to drive data signals onto the bus  16  and protect associated driver circuitry. In other embodiments, the low-pass DC filter  34  may also include a resistor placed in series between the bus  16  in the rack-mounted computing device  13 , with a capacitor placed between a ground and high voltage signal of the bus to, again, provide an impedance to reduce the power requirements to drive data signals, while smoothing fluctuations. 
     In some embodiments, the powerline modem  36  is substantially similar to the powerline modem  28  described above and may implement the same protocols. In some embodiments, each rack computing unit  20  contains similar or the same features. 
     In some embodiments, the microcontroller  38  is operative to receive signals from the powerline modem  36  and take responsive action. In some embodiments, the microcontroller  38  monitors addresses in headers on packets received via the powerline modem  36  and determines whether the address corresponds to the rack computing unit  20 . In some embodiments, the address is stored in persistent flash memory of the microcontroller  38 , for example, in flash memory set with a serial number or MAC address set at the factory. In some embodiments, upon initially detecting that the network and power adapter  30  is connected to a DC power bus  16 , the microcontroller  38  may broadcast is address to the other devices, for example, to add the address to a list of addresses maintained by the rack control unit  18  as received via the powerline modem  28 . 
     In some embodiments, the microcontroller  38  may receive commands from the rack control unit  18  and implement those commands, for example, by querying or otherwise polling various sensors, like those described above, to monitor things like resources being used by the rack computing unit  20  (e.g. processor usage or memory usage), or environmental conditions, like temperature, vibrations, airflow, particulates, humidity, electromagnetic radiation, and the like. In some embodiments, the microcontroller  38  may be operative to drive various signals into the rack-mounted computing device  13  that reconfigure the rack-mounted computing device  13 , monitor the rack-mounted computing device  13 , or control the rack-mounted computing device  13 . Examples include sending signals onto a system management bus or other bus of the rack-mounted computing device  13  that cause the rack-mounted computing device  13  to turn on, turn off, change a setting accessible via a BIOS (in some cases without engaging the BIOS and writing directly to flash memory), reconfiguring various settings, like clock speed or register settings for peripheral devices. In some embodiments, the microcontroller  38  is operative to poll various sensors that indicate the location of the rack computing unit  20 , for example, by reading a value with an optical sensor or a radio frequency sensor disposed on a rack that indicates the location of a rack computing unit  20  adjacent that device. 
     In some embodiments, the rack-mounted computing device  13  is a server (e.g., a computer executing a server application), database, or node in a compute cluster that performs operations requested by users of the data center  10 . Examples include serving webpages, servicing queries, processing API requests, performing simulations, and the like. Such computing operations are distinct from those performed to manage and control the operation of computing devices, for example, by changing versions of operating systems, updating or reconfiguring a BIOS, reading sensors, controlling fans, monitoring thermals, and the like. 
     In the illustrated embodiment, each rack-mounted computing device  13  includes persistent memory  40 , a processor  42 , dynamic memory  44 , and an in-band network interface  46 . In some embodiments, these components may be accessed by the microcontroller  38  via a system management bus  48  or various other onboard buses. In some embodiments, the components  40 ,  42 ,  44 , and  46  may reside on a single monolithic motherboard, connected via soldered connections and conductive traces in a printed circuit board. In some embodiments, the persistent memory  40  is flash memory having various values by which the rack-mounted computing device  13  is configured, for example, by changing settings in a BIOS. In some embodiments, the processor  42  is one or more central processing units or graphics processing units. In some embodiments, the dynamic memory  44  contains memory used by the operating system and applications, in some cases having an address space distinct from the computing devices of other rack computing units. 
     In some embodiments, the in-band network interface  46  is an Ethernet network interface operable to communicate on a distinct Ethernet network from the networks  16  and  17 . Separating these networks is expected to make the data center  10  more robust to attacks and facilitate operations even when the in-band network is disabled. Further, in some cases, the in-band network may be substantially higher bandwidth and use more expensive equipment than the out-of-band management networks  17  and  16 . In some embodiments, the network  15  connected to interface  46  may convey the data upon which the applications operate, for example, at the request of users of the data center  10 . 
       FIG.  2    another embodiment of a data center  27  having the features described above, except that the out-of-band network  17  extends to the rack-mounted computing devices  13 . Thus, the rack controller  24  communicates with rack-mounted computing devices  13  directly, via the out-of-band network  17 , rather than via the powerline communication network. In other embodiments, a separate Ethernet network specific to the rack is implemented in place of the power line network described above. In this example, the rack-mounted computing devices  13  may include an out-of-band network interface  29  with which the computing device  13  communicates with the rack controller  24 . In some embodiments, each computing device  13  may include a baseboard management controller that communicates with the rack controller  24  via the out-of-and network interface  29 . In some cases, an Intelligent Platform Management Interface (IPMI) API supported by the BMC may expose various functions by which the rack controller  24  takes inventory of devices on motherboards of the computing devices  13 , read values from sensors (like temperature sensors), reads values of registers and configures and changes these values, in some cases, changing settings including EFI settings. In some embodiments, the BMC is a separate processor from processor  42  executing and in-band application, and the BMC communicates with various devices on the motherboard via the SMbus. 
       FIG.  3    shows another embodiment of a data center  51  in which the above-described rack controller is executed by one of the rack-mounted computing devices  31  (which may otherwise have the features of device  13  described above). In some cases, software instructions to implement the rack controller  33  may be stored in memory  40  and executed by processor  42  of the rack-mounted computing device  31 . In some cases, the rack controller  33  may be executed in a dedicated virtual machine, microkernel, or container of the rack-mounted computing device  31 , with other instances of these computing constructs executing other applications, in some cases executing in-band applications. In some embodiments, one rack controller  33  may be executed by one (e.g., one and only one) rack-mounted computing device on a given rack to monitor other rack-mounted computing devices  13  on that rack  12 . Or multiple instances per rack may be executed. Again, in this example, the out-of-band network  17  may extend to the rack-mounted computing devices  13  and  31 , in some cases without passing through a power line communication network. 
       FIG.  4    shows another embodiment of a data center  35  in which the present techniques may be implemented. In this example, the rack controller  37  (e.g., having the features of the rack controllers described above) may be executed by one of the rack-mounted computing devices  41  (otherwise having the features of device  13  above), by executing corresponding code in persistent memory  40  with a processor  42 , in some cases within a dedicated virtual machine, container, or microkernel, e.g., in the arrangement described above for  FIG.  3   . In this example, the rack controller  37  may communicate with rack controllers on other racks  12  via the out-of-band network interface  39  of the rack-mounted computing device  41  via the out-of-band network  12 . Further, the rack controller  37  executed by the rack-mounted computing device  41  may control other rack-mounted computing devices  13  of the rack  12  via the rack-specific power line communication network  16  and corresponding instances of the network and power adapter  30  described above. 
       FIG.  5    illustrates an example of a process  50  that may be performed by some embodiments of the above-described network and power adapter  30  in rack computing units  20 . In some embodiments, steps for performing the process  50 , or the other functionality described herein, are encoded as instructions on a tangible, non-transitory, machine-readable media, such that when the instructions are read and executed by one or more processors, the associated functionality occurs. 
     In some embodiments, the process  50  includes receiving, with a given rack computing unit, direct current power via a DC bus connected to a plurality of rack computing units of a rack and configured to deliver DC power to the plurality of rack computing units, as indicated by block  52 . 
     In some embodiments, the process  50  further includes determining that a duration of time designated for the given rack computing unit to access the DC power bus for transmission is occurring, as indicated by block  54 . Next, in response to the determination, some embodiments may apply a time-varying electrical stimulus to the DC power bus, as indicated by block  56 . In some cases, the time-varying electrical stimulus encodes an address on the DC power bus of a rack controller and a sensor measurement indicative of operation of the given rack computing unit. In other cases, the stimulus may encode control signals rather than data signals. Next, concurrent with applying the time-varying electrical stimulus, some embodiments include filtering voltage fluctuations of the DC power bus resulting from the time-varying electrical stimulus to produce electric power used by the given rack computing unit, as indicated by block  58 . Producing electrical power does not require that the power be generated, merely that power obtained from some source be conditioned properly for usage by the rack computing unit. 
       FIG.  6    is a flowchart of an example of a process  60  that may be executed by one of the above-described rack controllers  24  (executed in various forms of data center management devices, e.g., rack control units  18  or rack-mounted computing devices  13  executing a rack controller  24 ) in order to monitor and otherwise control a subset of a data center (e.g. a rack) responsive to commands from other subsets of the data center or the above-described administrator computing device  25 , e.g. being operated by an administrator. In some embodiments, the process  60  may implement the northbound and southbound APIs described above, for instance, with the northbound API facing the network  17  and the southbound API facing the network  16 . In some embodiments, an instance of the process  60  may be executed by a different computing device associated with each rack  12  shown in  FIGS.  1 - 4   . In some embodiments, API requests may be sent from one rack controller  24  to another rack controller  24  or from the administrator computing device  25 , for instance. In some embodiments, the process  60  may be executed ongoing, for instance, listening to a port on the out-of-band network interface  26  on the out-of-band network  17  and responding to API requests as they are received. In some embodiments, the process  60  may be part of an event processing loop in which API requests are handled, in some cases with a nonblocking server using deferreds. 
     In some embodiments, the process  60  includes receiving an API request via a first out-of-band network, as indicated by block  62 . In some embodiments, this API request may be one of the above-described northbound API requests received by the rack controllers  18 . In some embodiments, the API request is received via an Ethernet network that is distinct from the in-band network described above, or some embodiments may receive the API requests from the in-band network in cases in which the networks have been consolidated. In some embodiments, the API request is a REST-based request encoded in hypertext transport protocol (HTTP), for instance as a POST or GET request received by a server executed by the rack controller  24  described above. Some embodiments may parse received requests and take responsive action, for instance, via a common Gateway interface (CGI) routine. In some cases, requests may contain both commands and parameters of those commands, for instance, separated from the command with the delimiter like “?” and having key-value pairs. In some cases, these parameters may specify a particular device, such as a particular rack-mounted computing device on a rack, or in some cases, these parameters may specify various other attributes by which actions are taken. Using a REST-based API, with HTTP formatted exchanges, over an Ethernet-implemented IP network, is expected to facilitate reuse of other tools built for the data center ecosystem, thereby lowering costs and providing a relatively feature-rich implementation, though it should be noted that embodiments are not limited to systems providing these benefits or implementing these protocols, which is not to imply that any other feature is limiting in all embodiments. 
     Next, some embodiments may select, based on the API request, a routine to control rack-mounted computing devices, as indicated by block  64 . Control may include reading data from such devices or associated sensors or actuators either on the rack-computing devices or on the rack itself, whether associated with a specific rack-mounted computing device or with the rack generally. Control may also include sending commands to write to, reconfigure, or otherwise actuate such devices, for instance, in accordance with the routines described herein, and including instructing computing devices to power cycle, updating firmware in rack-mounted computing devices or the various sensors or actuators, reconfiguring firmware or EFI settings, actuating fans, solenoids, electromagnets, lights, and the like. In some embodiments, the routine is selected based on text parsed from the API request, and in some embodiments, the routine is a script selected by a server that received the API request executed by a rack-controller  24 . In some embodiments, selecting the routine includes calling the routine with parameters parsed from the API request as arguments in the function call. 
     In some embodiments, selecting a routine may include selecting a routine that takes action via a different network from the network with which the API request was received. In some cases, this different network may be a powerline communication network like those described above with reference to  FIGS.  1 ,  4 , and  5   , but embodiments are consistent with other implementations, e.g., like those in  FIGS.  2  and  3   . For instance, in some cases, the other network is a network implemented with Ethernet, RS-232, USB, and the like. Or in some embodiments, the other network is the same, for instance, a branch of the network with which the API request is received. In some cases, this other network connects to each of a plurality of computing devices on the rack and various sensors and actuators associated with the rack, for instance, in accordance with the techniques described above with reference to network  16  in  FIG.  1   . 
     In some embodiments, selecting a routine includes selecting a routine that reads a sensor via the other network on one or more of the rack-mounted computing devices. In some cases, this may include sending a command to the above-described network and power adapters  30  that cause the above-described microcontroller  38  to query sensor data via an SMbus  48  (as shown in  FIG.  1   ). In some cases, the sensor is on a motherboard or a chassis of the rack computing unit  20  described above, for instance sharing an output of the low-pass DC filter  34  described above with the rack-mounted computing unit  13  (as shown in  FIG.  1   ). 
     In some embodiments, the routine selected is a routine that reads a sensor via the other network on the rack. In some embodiments, the sensor is not itself mounted to a rack control unit or powered by an output of a specific low-pass DC filter of a rack computing unit  20 . For instance, some embodiments may read a value from a sensor on a rack that measures temperature, humidity, airflow, vibration, or particles, and open or close a lock state of a lock for a door or drawer, or the like. In some cases, the sensor is a sensor that reads and identifier indicative of the location of a given computing device in the rack, in accordance with the techniques described above. 
     In some embodiments, reading a value from a sensor may include processing that value before sending a response to the API request on the network  17  described above. Processing may take various forms, depending upon the embodiment and may include, in some cases, converting an electrical property, like resistance, capacitance, inductance, current, frequency, or voltage, to some other physical property correlated with the electrical property. For example, some embodiments may convert one or more of these values into units of temperature (like degrees Celsius or Fahrenheit), into units of humidity, into units of vibration (e.g. RPMs), into Boolean values indicating whether doors are open or closed locked or unlocked, and the like. In some cases, processing may include combining readings from multiple sensors or combining readings from a given sensor over time, for instance, selecting a largest or smallest value, calculating statistics on the sensor output like standard deviation or mean, and comparing sensor readings (such as these statistics) to various thresholds, for instance, to determine whether to respond with an alarm or emit an alarm even in the absence of a recent API request when a reading is above or below a threshold. 
     In some embodiments, the routines may be selected based on a scheduled API request (including an internal API request obtained via a loopback IP address) rather than a user-driven API request, for instance according to a cron process run by the rack controller to periodically read values from sensors and compare those values thresholds for monitoring purposes or logging those values and reporting those values to the data center management computing device  25 . Or in some cases, these periodic requests may be received from a corresponding process that periodically automatically sends API requests from the administrator computing device  25  for monitoring purposes, for instance to update a dashboard. In either case, the request initiating action may still be an API request, such as one sent to a loopback address on a network interface coupled to the network  17  described above. 
     In some embodiments, the routine may be a routine that scans electronic devices on the second network and produces an inventory of the electronic devices on that network, such as computing devices, sensors, and actuators on the powerline communication network  16  described above or computing devices on an SMbus  48  described above. 
     In some embodiments, the routine may be a routine that changes a configuration of an EFI of a given one of the rack-mounted computing devices, for instance one that changes the same configuration on each of the rack-mounted computing devices. For example, some embodiments may change a boot target of the EFI, such that when the rack-mounted computing device is power cycled (or otherwise rebooted), the corresponding processor may look to a different media indicated by the new boot target when loading an operating system. 
     Some embodiments may send operating system updates to the rack controller, which may store those operating up system updates on an instance of this alternate media (such as a different disk drive, solid-state drive, or the like) on each of the rack-mounted computing devices before changing a boot target on each of those computing devices and commanding the computing devices to reboot to implement the new operating system with a reboot. In another example, some embodiments may update applications in this fashion, for instance by downloading an image of a new operating system or container with the new version of an application to alternate media and then changing the boot target to that alternate media. In some cases, these changes may be effectuated without interfacing with an operating system of the computing device  13  receiving the change. Similar techniques may be used to update firmware of peripheral devices. 
     Next, some embodiments may execute the selected routine, as indicated by block  66 . In some embodiments, the routine may be executed by the rack controller described above. In some embodiments, this may include sending a sequence of commands via the second network, such as network  16  described above, and in some cases, these commands may be received by the network and power adapters  30 , which may respond by executing their own routines with microcontroller  38  to effectuate various actions via the SMbus  48  or other interfaces. In other examples, the above-described sensors  21  or actuators  23  may receive the corresponding commands via the network  16  (which again, maybe a powerline communication network or other form of network), and the functionality described above may be implemented at the direction of a rack controller  24 . In the course of executing the routine, some embodiments may send commands via a second out-of-band network to effectuate an action indicated by the API request, as indicated by block  68 . For instance, the routine may include a query for values in registers of computing devices (or components thereof) or an inventory of such devices or components. Or the routine may include instructions to change a configuration of these devices, or read a sensor, or actuate an actuator. Other examples are described above. The routine may include operations that cause these commands to be sent and responses to be processed, consistent with the approaches described above. 
     In some embodiments, the rack controller  24  may perform agentless monitoring of the rack-mounted computing devices  13  via the networks  16  and  48  described above. For example, some embodiments may read various values indicative of the performance of the computing device, like temperature, processor utilization, fan speed, memory utilization, bandwidth utilization on the in-band network  15 , packet loss on the in-band network  15 , storage utilization, power utilization, and the like. In some cases, these values may be read from registers associated with the corresponding electronic devices without interfacing with an operating system of the corresponding rack-mounted computing device  13 , in some cases via a BMC using the IPMI protocol. Or, in some cases, these values, and the other values read and actions taken, may be effectuated without using a BMC of the rack-mounted computing devices. 
       FIGS.  7  and  8    depict techniques implemented in a system referred to as “Vapor Crate” for container, configuration, and file management for server/workload orchestration/automation. 
     In some embodiments of a collection of racks, such as a Vapor chamber described by the documents incorporated by reference, there are six wedges (each being a rack), where each wedge contains a Vapor Edge Controller (VECs) (or other type of rack control unit, like those described above, which may be dedicated computers coupled to a rack executing software to manage and monitor rack-mounted devices, like servers, or may be executed by the rack-mounted computing devices), used to host Vapor software such as Vapor CORE. The problem of how to configure and manage a large number of VECs (e.g., the rack controllers above) while reducing the amount of human intervention is worth addressing, as it may be difficult to manage all of the rack control units in a larger data center. In some cases, centralized management may present challenges. Relatively expensive, powerful computing equipment may be required for management tasks as the number of devices in a data center scales. Thus, at commercially relevant scales, computational resources may impose constraints, in addition to or independent of constraints imposed by out-of-band management networks, particularly when relatively large machine images, OS updates, and container images are distributed to a relatively large number of computing devices. That said, some embodiments may have a centralized control architecture, as the various other inventions described herein may also benefit such systems. 
     To mitigate some or all of these issues and others, in some embodiments, an autonomous management system called “Vapor Crate” (or Crate) is provided. Vapor Crate may carry out some or all of the following tasks:
         Service discovery, file and configuration synchronization (e.g., with a system referred to as “Forklift”)   Physical/logical mapping of management components based on role (e.g., with a system referred to as “Manifest”)   Device (VEC) bootstrap (Forklift/bootstrap)   Configuration management (Manifest)   File management (Manifest)   Container management (e.g., with a system referred to as “Dockworker”)   OS update management (e.g., with a system referred to as “Hoist”)   UI Management (e.g., with Control Panel)       

     Crate may be implemented as a set of microservices, distributed across the rack controllers or VECs in a data center. The architecture parallels best practices in the data center industry surrounding fault-expectancy, redundancy, replication, security, configuration management, and decentralization. (Though embodiments are not limited to systems that implement best practices, which is not to imply that any other feature is limiting in all cases.) A feature of some implementations of Crate is to remain fully available to surviving equipment in the event of a large-sale system failure (e.g. power outage in part of a data center in which a device previously having a leadership role in Crate is taken offline). Another feature of some implementations is the ability to contain network traffic to a local branch of the data center management network, as the transfer of data involved with such an endeavor can be non-trivial. Finally, a feature of some implementations is a self-managing system that lowered operational cost by reducing or eliminating the need for operator intervention in managing hardware-management related software in a data center. 
     Vapor Crate, in some cases, includes service discovery and the ability to synchronize files and configuration. These capabilities may be provided by “Forklift”, a service that is included with rack controllers or VECs (e.g., as part of the program code) that discovers or is configured with the optimal (e.g., a local or global optimum) “leader”, which serves as the “master” for a given chamber (or other collection or racks). In some embodiments, the “leader” runs a redundant copy of all of the services listed above, typically within the scope of a single Vapor chamber or group of racks—the leader&#39;s services are synchronized to a “primary” rack controller or VEC, designated by consensus in the data center by the population of leaders. Updates may be made through the primary rack controller or VEC, and may be replicated to the leaders in the management unit. In case of failure of the ‘primary’, an election may be held by the consensus layer to choose a new primary, and the results of the election may be replicated out to all leaders. 
     In some cases, Forklift discovers the existing services (including in bootstrap scenarios where a rack controller or VEC is the first to be brought online in a data center), and discovers its own service profile, which specifies the services the rack controller or VEC is to run (for example, to serve as a leader, or as an instance running Vapor CORE). The Crate API may be used by Forklift to communicate with Manifest™ to retrieve file and configuration information, and the appropriate containers (services) are retrieved from Dockworker™, and are spun up using docker-compose or similar orchestration tool. When a container is started, in some embodiments, the Crate API may also be used within the container to retrieve from Manifest files or configuration information needed that is specialized to the VEC itself, allowing for a single image to be used, but configured to the needs of each individual rack controller or VEC where appropriate. 
     In some embodiments, OS updates may be provided by Hoist™, a reprepro Debian™ repository that may also contain specific package updates (e.g. to Forklift) as well. Hoist may provide a secure Debian™ repository for software, as many data center management networks (e.g., out-of-band networks) do not have outside network access. Additionally, Hoist™ may allow for auditing and curation of package updates to suit user needs. In some embodiments, Dockworker™ is included for a similar reason—access to Docker Hub™ is typically not possible for similar reasons in some implementations, and Dockworker™ therefore may fill that void, and allows for curation and auditing of packages. That said, other embodiments may have out-of-band management networks with Internet access. 
     Gatekeeper™ may be used to provide authentication services used to secure Crate, and may also be used as the authentication service for Vapor CORE or other products to provide fine-grained security from a single secure location. 
     Manifest may use MongoDB™ as the data store and MongoDB&#39;s GridFS for schematized file storage. In some cases, MongoDB also includes a consensus-based replication back-end that Crate piggybacks for its own consensus purposes—providing dual purpose consensus protocol and elections, as well as notification of election results. 
     The Crate API, in some embodiments, includes a lightweight object mapping that schematizes and validates object data exchanged between the API endpoint and MongoDB™. The API itself is may be implemented as a Python™ library that uses the object mapping to allow for ease of use by API consumers by providing a set of standard Python objects that may be exchanged between the API client and the endpoint. The API endpoint may be a RESTful API backed by Nginx™ and uWSGI, with Flask™ as the microdevelopment framework. Data may be exchanged in JSON format, and validated prior to interacting with the Manifest data store. 
     Some variations include a hosted management component to Crate, which is a service used to remotely manage Crate and Vapor services remotely, without the need for any customer intervention whatever. Other variations include interaction with OpenSwitch™ or other Network Operating Systems (NOSes), where Crate is used on a switch or as part of a NOS to perform management capabilities. 
     The form factor on which Crate runs may vary. In some embodiments, Crate may be implemented for rack controller or VEC management, or it may be applied to broader devops or automated systems management tasks. 
     Thus, some embodiments may implement control and updates at the rack-level, in a distributed, concurrent fashion to facilitate relatively large-scale operations. In some cases, activities specific to a rack can be handled locally at the rack level. To facilitate fan-out, some embodiments may distribute commands (which may include data by which the commands are implemented) through a hierarchical tree structure network graph, which in some cases may be established in part on an ad hoc, peer-to-peer, distributed basis. For instance, the rack-level may be at a lower level than a Vapor chamber, and some embodiments may distribute one request per chamber, and then that request may be replicated among the racks in that chamber. Examples may include instructions for file and configuration management (e.g., profiles), which may dictate how containers behave in each rack control unit. Embodiments may push out something like a service profile that says what each controller should be running, e.g., via REST-based exchange. To this end, for example, embodiments may send that file via an API to an endpoint, which may be an arbitrarily chosen controller (e.g., with a consensus algorithm). The designated rack control unit may then internally route that request to the known primary for the replicated database. The data may then be sent to the primary, which then replicates it out to the replicas, e.g., one rack control unit per chamber. Often updates entail applying the same thing to every chamber and every rack, or in some cases updates may designate the set of configuration and file data that is relevant to a particular class of machines or servers or racks. The determination to apply the received data may be made at replica or at the primary level. 
     An example of a profile change is a change in a setting in a configuration for a container. To effect the change, some embodiments may send instructions to a replica to change the setting and that setting may be committed to the database. In some embodiments, other rack control units may periodically check the designated master for updates, e.g., with a pull operation every minute, making an API request to the master requesting needed files. Some embodiments may have one master per chamber, and in some of these examples, anything pushed out to the primary gets pushed out to the masters/replicas. In some embodiments, if the primary fails, a new primary may be elected by consensus among the devices. 
     When initializing a rack or a data center, files may be distributed in a similar fashion to updates, with an arbitrarily chosen rack control unit acting as a seed. Upon boot, an initial master controller client that runs on rack control units may periodically query for updates. A laptop may be connected and initially be set to be the provisioning unit via the endpoint running on that particular machine, and other rack control unit may thereby obtain the files and settings from the laptop acting as a master. Then, some embodiments may change the settings of the chosen rack control unit to act as a master and seed distribution to other devices. 
     In some embodiments, a hierarchy of rack control units may be maintained to limit the number of participants in a distributed database. For instance, some may be designated management units, which may then replicate to devices lower in the hierarchy. 
     In other embodiments, other techniques may be used for configuring rack control units in a data center. For instance, some embodiments may use peer-to-peer file sharing protocols, like BitTorrent. In some embodiments, device discovery and data routing may be achieved with a distributed hash table algorithm executing on the participating rack control units (or other computers executing a rack controller). Such techniques, and those described above, are expected to make distributed computing systems run better than centralized management architectures, particularly as the number of nodes in a network scales and the amount of data to be distributed to each node expands. (This should not be taken as a disclaimer, though, of centralized architectures.) These techniques are also applicable to query routing and distributed processing. For instance, the commands may take the form of queries or MapReduce functions. 
     As noted above, in some embodiments, the rack controllers may communicate with one another. In some embodiments, the rack controllers may be updated, configured, monitored, queried, and otherwise accessed via a peer-to-peer data center management system  70 . The illustrated system  70  has topology shown in  FIG.  7   , indicating the way in which information (such as commands, peer-to-peer networking overhead, and query responses) flows through the management system  70  to manage the rack controllers. In some cases, the topology represents connections at the application layer, which may be built over the network layer topology described above with reference to  FIGS.  1 - 4   . As discussed above, in some cases, the rack controllers are executed by dedicated computing devices associated with racks, or in some cases by rack-mounted computing devices to execute in band applications. In some embodiments, the illustrated topology may be formed by the rack controllers, using a peer-to-peer consensus protocol described below. In some embodiments, the topology may be a structured topology, such as a tree topology like that shown in  FIG.  7   , or in other embodiments, the topology may be an unstructured topology, e.g., in other forms of mesh topologies. 
     In some embodiments, the illustrated system may be relatively robust to failure by one member of the system, for instance, by one of the rack controllers. In some embodiments, for certain operations, remaining rack controllers may detect the failure of a given rack controller and continue operation, designating other rack controllers fill a role previously performed by a failed rack controller if that rack controller had a role significant to other parts of the system  70 . In some cases, this may be accomplished with various consensus algorithms executed in a distributed fashion as described below with reference to  FIG.  8   , such as a leader-based consensus algorithm or a consensus algorithm that does not rely on a leader. Examples include the Raft consensus algorithm described in  Ongaro, Diego; Ousterhout, John  (2013). “In Search of an Understandable Consensus Algorithm” (the contents of which are hereby incorporated by reference), and the Paxos consensus algorithm described in  Lamport, Leslie  (May 1998). “The Part-Time Parliament” (PDF). ACM Transactions on Computer Systems 16, 2 (May 1998), 133-169 (the contents of which are hereby incorporated by reference), among others. In contrast, systems with rigid, predefined, unchangeable roles may be relatively sensitive to failure by any one computing device, as often those systems require human intervention to replace that one computing device or otherwise reconfigure the system. In contrast, some embodiments may be fault tolerant and resilient to failures by computing devices, applications therein, and network. That said, embodiments are not limited to systems that afford these benefits, as there are various independently useful techniques described herein, some of which are not based on consensus algorithms. 
     In the illustrated example, the rack controllers are arranged in a hierarchical tree in the topology of the management system  70 . In  FIG.  7   , the differing modifiers of “primary” and “lead” should not be taken to indicate that, at least in some embodiments, the devices have a different architecture. Rather, in some embodiments, each of the devices illustrated in  FIG.  7   , in some embodiments, may be a (e.g., identical) instance of a peer rack controller, each controlling a rack in the fashion described above. The lead and primary controllers may be simply designated rack controllers that perform additional tasks based on their role. The topology may be determined by the rack controllers themselves, dynamically, by executing the routines described below with reference to  FIG.  8   , in some cases, without a human assigning the roles and arrangement shown in  FIG.  7   , and with the topology self-evolving to heal from the failure of devices. In this example, there are three levels to the topology. At the highest level is a primary rack controller  72 . At a next lower level, adjacent the primary rack controller, and therefore in direct communication with the primary rack  72  are lead rack controller  74 . Three rack controllers  74  are illustrated, but embodiments are consistent with substantially more, for instance on the order of more than 50 or more than 500. At the next level of the hierarchy, there are a plurality of rack controller  76 . Each lead rack controller  74  may communicate directly with a plurality of rack controllers  76 , in some cases with those rack controllers  76  communicating exclusively with the rack controller  74  through the management system  70  or purposes of management performed by the system  70 . In some embodiments, each of the rack controllers  76  may control a plurality of rack-mounted computing devices  78  in the fashion described above. In some embodiments, the illustrated management systems  70  may be implemented in one or more of the above-described out-of-band networks. In some embodiments, management system may pass through the illustrated spanning tree, with replication chaining, thereby distributing communication load across the network and mitigating bottlenecks communication by which rack-mounted computing devices, racks, or rack controllers are controlled. 
     Control may take a variety of different forms. In some embodiments, a command may be sent by the primary rack controller to update an operating system of the rack controller or a rack-mounted computing device. In some embodiments, command may include an image of an operating system, in some cases, an image of an operating system, an application executed within the operating system, dependencies of that application may be included in the image. In another example, a container or microkernel may be configured or provisioned, for instance with a corresponding disk image stored in, and distributed through, the topology. In some cases, the command may be sent in sequence of messages, some including content by which the command is actuated and other messages including instructions to apply that content. 
     Other embodiments may include more or fewer levels to the hierarchy illustrated. For example, some embodiments may omit the primary rack controller  72 , and commands may be distributed via chains spanning trees of the rack controllers  74  to the rack controller  76 . Or some embodiments may include additional levels of hierarchy, for instance with a plurality of primary rack controllers that are adjacent a higher level “super-primary” rack controller. 
     In some embodiments, updates, settings, and other management content applied to rack controllers or rack-mounted computing devices, like operating systems, applications, microkernel, containers, configuration files, and the like they be stored in a distributed repository, such as a distributed file system, or a distributed document database. In some cases, a distributed repository may have a topology that mirrors that of the illustrated management system  70 . For example, some embodiments may implement the MongoDB™ document database, in some cases with the illustrated topology within the database and content be replicated across multiple instances illustrated containers, thereby providing redundancy, fault tolerance, data storage, as well as management capabilities. Other examples may implement a clustered file system, such as the the InterPlanetary File System as a distributed file system. In some embodiments, the same consensus algorithm by which the management system determines, may be used to determine roles and authoritative copies of data in the distributed file system. In some cases, like in leaderless systems, roles may correspond to addresses within the topology of management content. 
     In some embodiments, the illustrated roles of the different rack controllers shown in  FIG.  7    implement a distributed consensus protocol executed by the rack controllers. In some embodiments, the rack controllers may monitor the out-of-band network for a heartbeat signal, for instance in every few seconds, like every two seconds, sent by a leader among a group of rack controllers. Some embodiments may determine that that heartbeat signal has not been received within a threshold duration of time and in response initiates an election for a new leader. In some embodiments, each group of rack controllers, for instance, a plurality of 2 to 50, may have one designated leader for the group through which commands are distributed to the group, and through which information about the group is returned up through the illustrated tree of  FIG.  7   . 
     Upon determining that no leader heartbeat was received in time, and action is warranted, a given rack controller making this determination may send a message to other rack controllers in the group indicating that the given rack controller requests their vote in an election. In some embodiments, each rack controller may receive this message and to determine whether to vote for that given rack controller in response. This determination may be based on a variety of different considerations. For instance, each receiving rack controller may determine whether the group already has a leader and, in response, send a no vote (or decline to respond). In some embodiments, each receiving rack controller may determine whether the rack controller previously voted an election within less than a threshold duration of time, in which case the rack controller may vote no. In some embodiments, each rack controller may determine whether the rack controller already voted in the current election, in which case the rack controller may vote no. To this end, in some embodiments, when a given rack controller requests a vote, that rack controller may increment a count that serves as a unique identifier for an election attempt within the group, and other rack controllers may use this identifier sent with the vote request to determine whether they have already voted within a given election by logging their responses in memory and accessing this log to determine whether they already have a logged vote associated with the election attempt identifier. In another example, the receiving rack controller may determine whether the request is the first request received with the given election attempt and, in response, return with a yes vote to the first request received and a no vote to other requests. In another example, the vote requests may include a value indicative of a version of data stored in a distributed file system by the requesting rack controller, and receiving rack controllers may determine whether to vote for that rack controller based on whether they store a more up-to-date version or based on whether another rack controller has requested a vote with a more up-to-date version. 
     In some embodiments, each rack controller may send a heartbeat signal periodically, like every two seconds, to every rack controller in the respective group, and every rack controller in the group may receive these heartbeat signals and maintain a list of rack controller in the group. Based on this list, the rack controller requesting a vote may receive votes in its favor and determine whether more than a majority of votes in favor have been received by counting votes received and comparing the count to a threshold that is half the number of unique heartbeat signals received from members of the group. Upon receiving a majority of votes, and determining this to be the case, a given rack controller may determine that it has taken the role of a leader, communicate this to the group, and other rack controllers in the group may look to that rack controller as filling the role of leader. 
     Once leaders for each group are determined, in some cases, those leaders may determine the primary rack controller with a similar technique, with the leaders serving as the group in electing a member of the group to operate as a primary rack controller. 
     In some embodiments, the time thresholds for determining whether a given rack controller has failed may be adjusted according to a random value. Randomizing the threshold is expected to reduce the likelihood that different rack controllers call for elections concurrently with the group, thereby reducing the likelihood of tie elections delayed election results causing an election to be re-run. 
     At various times, various rack controllers filling various roles may fail. Failure does not require that the computing device cease operation, merely that the rack controller be perceived by other rack controllers to not perform at least part of the function corresponding to the role held by the rack controller. Examples include failure to send a heartbeat signal, or failure to send a command or other information through the topology  70  shown in  FIG.  7    within a (e.g., randomized, like pseudorandomized) threshold duration of time. In some cases, durations of time since the last signal was received may serve as a health score for each rack controller, and these health scores may be propagated through the management system  70  according to the illustrated topology, with a given rack controller reporting health scores for those below it, and advancing those health scores upward, while distributing health scores for those upward to those below. This is expected to scale better relative to systems that implement a fully connected graph, though embodiments are also consistent with this approach. 
       FIG.  8    shows an example of a process  80  by which roles may be determined and management may be effectuated within a data center management system, such as that described above with reference to  FIG.  7   . In some embodiments, the process  80  includes determining a leader rack controller among a group a rack controllers, as indicated by block  82 . In some cases, this operation may be performed when initializing a data center or in response to determining that a given previous leader rack controller is not responding. In some embodiments, determining a leader may include determining a leader through the techniques described above with a distributed consensus protocol, with a plurality of rack controllers with a group communicating with one another in order to select a leader (or primary) and arriving at a consensus regarding the selection. 
     Next, some embodiments may determine whether threshold duration has elapsed to receive a periodic signal (or other signal) from the leader, as indicated by block  84 . In some cases, this may be receiving a heartbeat signal, or in some cases this may include a poor request, such as a request for a response indicating the help of the leader. In some cases, block  82  may be done in a distributed fashion by the group, while block  84  may be performed by each member of the group. Similarly, each member of the group may determine in response to the time elapsing in block  84  whether the leader is still operative, as indicated by block  86 . As noted above, an inoperative leader may still be functioning, but not filling a portion of the role in the system, for instance in the event of a network failure. If the leader is not operative, some embodiments may determine a new leader rack controller and return to block  82 , using the techniques described above. 
     If the leader is operative, some embodiments may determine whether there is a new command from the leader, as indicated by block  88 . In some cases, the command may be a series of messages, such as a message instructing rack mounting computing device to retrieve information stored in a distributed file system, like an application update, descriptions of changing configurations, a container image, a disk image, and the like, and apply that management content to a respective rack controller with the command. In another example, the command may be a query to downstream rack controllers or a query to be translated and sent to downstream rack-mounted computing devices, for instance, with the techniques described above. 
     Upon determining that there are no commands, some embodiments may return to before block  84  and continue to wait for when for a periodic check that the leader is operative. In some cases, messages may be received between periodic heartbeats, for instance, with commands. 
     Upon determining that there is a new command, some embodiments may distribute the commands to the rack controllers under the respective leader. In some embodiments, these operations  82  through  90  may be executed concurrently, asynchronously, by a plurality of different groups within the data center, in some cases, with different groups selecting the leaders at different times. 
     In another example of a concurrent operation in some embodiments, upon determining a leader for the rack controller among a group rack controllers, each rack controller may determine whether it is itself a leader, as indicated by block  92 . This concurrent branch may end when the answer is no. Those rack controllers that determine that they have been elected the leader, may proceed to block  94 , in which the leader rack controllers may each determine whether there is a primary rack controller that is operative, as indicated by block  94 . This determination may be similar to that of block  86  described above, with the group of leaders serving as the relevant group. Upon determining that the leader is not operative, some embodiments may determine a primary rack controller among the leader rack controllers, as indicated by block  96 . This operation may include executing one or more of the above-described consensus protocols on the various leader rack controllers to elect a primary rack controller. Next, some embodiments may determine whether the time has arrived for a periodic check, as indicated by block  98 . If the answer is no, some embodiments may continue waiting, or if the answer is yes, some embodiments may return to block  94  and determine whether the primary rack controllers still operative. If the primary rack controller is determined to be operative, some embodiments may determine whether there is new command from the primary rack, as indicated by block  100 . Again, this operation may include the operations described above with respect to block  88 , except from the perspective of a leader. The operations of blocks  98 ,  94 , and  100  may be performed by each leader rack controller concurrently and asynchronously, and the operation of block  96  may be performed collectively by the leader rack controllers. Upon determining that there is no new command, some embodiments may return to block  98  and continue waiting for a periodic check or new command. 
     In some cases, new commands may be pushed, without waiting to make a determination whether a new command is available. Upon determining that a new command is available, some embodiments may distribute the command to the leader rack controllers, as indicated by block  102 . This operation may include the operations described above with reference to block  90 , except from the perspective of leader rack controller. The distribution of block  102  may cause a positive response in the determination of block  88  above. Thus, new commands may be fanned out throughout a topology of rack controllers without forming a bottleneck at any one rack controller, and the distribution may be accomplished in a way that is relatively fault-tolerant to the failure of any one rack controller of the data center management system  70 . In some embodiments, the content by which the commands are implemented, which may include relatively large files including operating systems, containers, applications, dependencies, and configuration settings, may be distributed in the same or a similar fashion, thereby also distributing the relatively bandwidth heavy load throughout the data center and avoiding bottlenecks, while also remaining resilient to failures. Again, it should be noted that several inventions are described, and those inventions may be viewed independently, so these benefits may not be afforded by all embodiments consistent with the present description. 
       FIG.  9    is a diagram that illustrates an exemplary computing system  1000  in accordance with embodiments of the present technique. In some cases, each rack of the above-described racks may house one or more of these systems  1000 . Various portions of systems and methods described herein, may include or be executed on one or more computer systems similar to computing system  1000 . Further, processes and modules described herein may be executed by one or more processing systems similar to that of computing system  1000 . 
     Computing system  1000  may include one or more processors (e.g., processors  1010   a - 1010   n ) coupled to system memory  1020 , an input/output I/O device interface  1030 , and a network interface  1040  via an input/output (I/O) interface  1050 . A processor may include a single processor or a plurality of processors (e.g., distributed processors). A processor may be any suitable processor capable of executing or otherwise performing instructions. A processor may include a central processing unit (CPU) that carries out program instructions to perform the arithmetical, logical, and input/output operations of computing system  1000 . A processor may execute code (e.g., processor firmware, a protocol stack, a database management system, an operating system, or a combination thereof) that creates an execution environment for program instructions. A processor may include a programmable processor. A processor may include general or special purpose microprocessors. A processor may receive instructions and data from a memory (e.g., system memory  1020 ). Computing system  1000  may be a uni-processor system including one processor (e.g., processor  1010   a ), or a multi-processor system including any number of suitable processors (e.g.,  1010   a - 1010   n ). Multiple processors may be employed to provide for parallel or sequential execution of one or more portions of the techniques described herein. Processes, such as logic flows, described herein may be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating corresponding output. Processes described herein may be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). Computing system  1000  may include a plurality of computing devices (e.g., distributed computer systems) to implement various processing functions. 
     I/O device interface  1030  may provide an interface for connection of one or more I/O devices  1060  to computer system  1000 . I/O devices may include devices that receive input (e.g., from a user) or output information (e.g., to a user). I/O devices  1060  may include, for example, graphical user interface presented on displays (e.g., a cathode ray tube (CRT) or liquid crystal display (LCD) monitor), pointing devices (e.g., a computer mouse or trackball), keyboards, keypads, touchpads, scanning devices, voice recognition devices, gesture recognition devices, printers, audio speakers, microphones, cameras, or the like. I/O devices  1060  may be connected to computer system  1000  through a wired or wireless connection. I/O devices  1060  may be connected to computer system  1000  from a remote location. I/O devices  1060  located on remote computer system, for example, may be connected to computer system  1000  via a network and network interface  1040 . 
     Network interface  1040  may include a network adapter that provides for connection of computer system  1000  to a network. Network interface may  1040  may facilitate data exchange between computer system  1000  and other devices connected to the network. Network interface  1040  may support wired or wireless communication. The network may include an electronic communication network, such as the Internet, a local area network (LAN), a wide area network (WAN), a cellular communications network, or the like. 
     System memory  1020  may be configured to store program instructions  1100  or data  1110 . Program instructions  1100  may be executable by a processor (e.g., one or more of processors  1010   a - 1010   n ) to implement one or more embodiments of the present techniques. Instructions  1100  may include modules of computer program instructions for implementing one or more techniques described herein with regard to various processing modules. Program instructions may include a computer program (which in certain forms is known as a program, software, software application, script, or code). A computer program may be written in a programming language, including compiled or interpreted languages, or declarative or procedural languages. A computer program may include a unit suitable for use in a computing environment, including as a stand-alone program, a module, a component, or a subroutine. A computer program may or may not correspond to a file in a file system. A program may be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program may be deployed to be executed on one or more computer processors located locally at one site or distributed across multiple remote sites and interconnected by a communication network. 
     System memory  1020  may include a tangible program carrier having program instructions stored thereon. A tangible program carrier may include a non-transitory computer readable storage medium. A non-transitory computer readable storage medium may include a machine readable storage device, a machine readable storage substrate, a memory device, or any combination thereof. Non-transitory computer readable storage medium may include non-volatile memory (e.g., flash memory, ROM, PROM, EPROM, EEPROM memory), volatile memory (e.g., random access memory (RAM), static random access memory (SRAM), synchronous dynamic RAM (SDRAM)), bulk storage memory (e.g., CD-ROM or DVD-ROM, hard-drives), or the like. System memory  1020  may include a non-transitory computer readable storage medium that may have program instructions stored thereon that are executable by a computer processor (e.g., one or more of processors  1010   a - 1010   n ) to cause the subject matter and the functional operations described herein. A memory (e.g., system memory  1020 ) may include a single memory device or a plurality of memory devices (e.g., distributed memory devices). 
     I/O interface  1050  may be configured to coordinate I/O traffic between processors  1010   a - 1010   n , system memory  1020 , network interface  1040 , I/O devices  1060 , or other peripheral devices. I/O interface  1050  may perform protocol, timing, or other data transformations to convert data signals from one component (e.g., system memory  1020 ) into a format suitable for use by another component (e.g., processors  1010   a - 1010   n ). I/O interface  1050  may include support for devices attached through various types of peripheral buses, such as a variant of the Peripheral Component Interconnect (PCI) bus standard or the Universal Serial Bus (USB) standard. 
     Embodiments of the techniques described herein may be implemented using a single instance of computer system  1000  or multiple computer systems  1000  configured to host different portions or instances of embodiments. Multiple computer systems  1000  may provide for parallel or sequential processing/execution of one or more portions of the techniques described herein. 
     Those skilled in the art will appreciate that computer system  1000  is merely illustrative and is not intended to limit the scope of the techniques described herein. Computer system  1000  may include any combination of devices or software that may perform or otherwise provide for the performance of the techniques described herein. For example, computer system  1000  may include or be a combination of a cloud-computing system, a data center, a server rack, a server, a virtual server, a desktop computer, a laptop computer, a tablet computer, a server device, a client device, a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a vehicle-mounted computer, or a Global Positioning System (GPS), or the like. Computer system  1000  may also be connected to other devices that are not illustrated, or may operate as a stand-alone system. In addition, the functionality provided by the illustrated components may in some embodiments be combined in fewer components or distributed in additional components. Similarly, in some embodiments, the functionality of some of the illustrated components may not be provided or other additional functionality may be available. 
     Those skilled in the art will also appreciate that while various items are illustrated as being stored in memory or on storage while being used, these items or portions of them may be transferred between memory and other storage devices for purposes of memory management and data integrity. Alternatively, in other embodiments some or all of the software components may execute in memory on another device and communicate with the illustrated computer system via inter-computer communication. Some or all of the system components or data structures may also be stored (e.g., as instructions or structured data) on a computer-accessible medium or a portable article to be read by an appropriate drive, various examples of which are described above. In some embodiments, instructions stored on a computer-accessible medium separate from computer system  1000  may be transmitted to computer system  1000  via transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a network or a wireless link. Various embodiments may further include receiving, sending, or storing instructions or data implemented in accordance with the foregoing description upon a computer-accessible medium. Accordingly, the present invention may be practiced with other computer system configurations. 
     The reader should appreciate that the present application describes several inventions. Rather than separating those inventions into multiple isolated patent applications, applicants have grouped these inventions into a single document because their related subject matter lends itself to economies in the application process. But the distinct advantages and aspects of such inventions should not be conflated. In some cases, embodiments address all of the deficiencies noted herein, but it should be understood that the inventions are independently useful, and some embodiments address only a subset of such problems or offer other, unmentioned benefits that will be apparent to those of skill in the art reviewing the present disclosure. Due to cost constraints, some inventions disclosed herein may not be presently claimed and may be claimed in later filings, such as continuation applications or by amending the present claims. Similarly, due to space constraints, neither the Abstract nor the Summary of the Invention sections of the present document should be taken as containing a comprehensive listing of all such inventions or all aspects of such inventions. 
     It should be understood that the description and the drawings are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description and the drawings are to be construed as illustrative only and are for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed or omitted, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. Headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description. 
     As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). The words “include”, “including”, and “includes” and the like mean including, but not limited to. As used throughout this application, the singular forms “a,” “an,” and “the” include plural referents unless the content explicitly indicates otherwise. Thus, for example, reference to “an element” or “a element” includes a combination of two or more elements, notwithstanding use of other terms and phrases for one or more elements, such as “one or more.” The term “or” is, unless indicated otherwise, non-exclusive, i.e., encompassing both “and” and “or.” Terms describing conditional relationships, e.g., “in response to X, Y,” “upon X, Y,”, “if X, Y,” “when X, Y,” and the like, encompass causal relationships in which the antecedent is a necessary causal condition, the antecedent is a sufficient causal condition, or the antecedent is a contributory causal condition of the consequent, e.g., “state X occurs upon condition Y obtaining” is generic to “X occurs solely upon Y” and “X occurs upon Y and Z.” Such conditional relationships are not limited to consequences that instantly follow the antecedent obtaining, as some consequences may be delayed, and in conditional statements, antecedents are connected to their consequents, e.g., the antecedent is relevant to the likelihood of the consequent occurring. Statements in which a plurality of attributes or functions are mapped to a plurality of objects (e.g., one or more processors performing steps A, B, C, and D) encompasses both all such attributes or functions being mapped to all such objects and subsets of the attributes or functions being mapped to subsets of the attributes or functions (e.g., both all processors each performing steps A-D, and a case in which processor 1 performs step A, processor 2 performs step B and part of step C, and processor 3 performs part of step C and step D), unless otherwise indicated. Further, unless otherwise indicated, statements that one value or action is “based on” another condition or value encompass both instances in which the condition or value is the sole factor and instances in which the condition or value is one factor among a plurality of factors. Unless specifically stated otherwise, as apparent from the discussion, it is appreciated that throughout this specification discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining” or the like refer to actions or processes of a specific apparatus, such as a special purpose computer or a similar special purpose electronic processing/computing device. 
     The present techniques will be better understood with reference to the following enumerated embodiments: 
     1. A method of managing rack-mounted computing devices in a data center with a distributed peer-to-peer management system, the method comprising: determining roles of data-center management computing devices in a distributed peer-to-peer data-center management system; receiving, via an out-of-band network, a data-center management command at a given data-center management computing device; distributing, based on at least some of the roles, via the out-of-band network, the data-center management command from the given data-center management computing device to a first plurality of other data-center management computing devices; and for each of the first plurality of other data-center management computing devices, distributing, based at least some of the roles, via the out-of-band network, the data-center management command to a respective second plurality of other data-center management computing devices, wherein: the given data-center management computing device, the first plurality of other data-center management computing devices, and the respective second pluralities of other data-center management computing devices are each configured to manage a respective plurality of rack-mounted computing devices; each of the pluralities of respective rack-mounted computing devices executes an in-band application that communicates on an in-band network different from the out-of-band network; and the data-center management command is a command to update or otherwise configure a microkernel, a virtual machine, a container, an operating system, or an application executing on one or more data-center management computing devices.
 
2. The method of embodiment 1, wherein: determining roles of data-center management computing devices in a distributed peer-to-peer data-center management system comprises: selecting a primary data-center management computing device; selecting a plurality of leader data-center management computing devices by concurrently executing a plurality of respective instances of a distributed algorithm by which the respective leader is determined by consensus, each instance being executed by one of the second pluralities of other data-center management computing devices to select a respective leader for the respective one of the second pluralities of other data-center management computing devices.
 
3. The method of embodiment 3, wherein selecting a plurality of leader data-center management computing devices by concurrently executing a plurality of respective instances of a distributed algorithm by which the respective leader is determined by consensus comprises: detecting with a first one of the data-center management computing devices that a signal has not been received from a leader within a threshold duration of time; incrementing or decrementing a count with the with first one of the data-center management computing devices; sending, with the first one of the data-center management computing devices, a request to vote for a new leader along with the incremented or decremented count to other data-center management computing devices in the same second plurality of other data-center management computing devices; receiving the request with a second one of the data-center management computing devices; determining, with the second one of the data-center management computing devices, to vote for the first one of the data-center management computing devices, at least in part, by: determining that the second one of the data-center management computing devices has not yet submitted a vote associated with the incremented or decremented count; and determining that the request is the first request received by the second one of the data-center management computing devices in a given round of voting; sending, with the second one of the data-center management computing devices, the vote to other data-center management computing devices in the same second plurality of other data-center management computing devices; selecting a leader based on amounts of votes received by candidate data-center management computing devices.
 
4. The method of any one of embodiments 1-3, wherein: determining roles comprises determining roles in a peer-to-peer distributed application having a structured topology.
 
5. The method of embodiment 4, wherein: the structured topology is a tree topology having at least three levels, wherein: the given data-center management computing device is at a first level of the tree topology; the first plurality of other data-center management computing devices is at a second level of the tree topology, each being adjacent to the given data-center management computing device in the tree topology; the second pluralities of other data-center management computing devices are at a third level of the tree topology, each respective plurality of other data-center management computing devices being adjacent to a respective one of the first plurality of other data-center management computing devices in the tree topology.
 
6. The method of any one of embodiments 1-5, wherein: determining roles comprises determining roles in a peer-to-peer distributed application having an unstructured topology.
 
7. The method of any one of embodiments 1-6, comprising: determining, with at least one of the first plurality of other data-center management computing devices, that the given data-center management computing device has failed; electing, with at least one of the first plurality of other data-center management computing devices, one of the first plurality of other data-center management computing devices to serve as a replacement to the given data-center management computing device in at least some roles filled by the given data-center management computing device; and distributing a result of the election to each the first plurality of other data-center management computing devices.
 
8. The method of any one of embodiments 1-7, comprising: obtaining a file with configuration information from a distributed file system stored with cross-computing-device redundancy by at least a plurality of the first plurality of other data-center management computing devices, the second pluralities of other data-center management computing devices, or both.
 
9. The method of embodiment 8, wherein: the file is stored with consensus-based replication based on a consensus protocol, and wherein the consensus protocol is used to determine at least some roles of the data center management computing devices.
 
10. The method of any one of embodiments 1-9, wherein: at least some of the data-center management computing devices are a rack controller executed in an out-of-band computer having a processor and memory storing instructions that when executed effectuate at least some of the operations of c at least some of the data-center management computing devices manage rack-mounted computing devices via respective ones of a plurality of different power-line communication networks; and the out-of-band network is a different network from the power-line communication networks.
 
11. The method of any one of embodiments 1-9, wherein: at least some of the data-center management computing devices are rack-mounted computing devices executing a rack controller; and at least some of the data-center management computing devices manage rack-mounted computing devices via an Ethernet network, without a power-line communication network connecting the at least some of the data-center management computing devices to the rack-mounted computing devices.
 
12. The method of any one of embodiments 1-9, wherein: at least some of the data-center management computing devices are a rack-mounted computing device executing a rack controller; and at least some of the data-center management computing devices manage rack-mounted computing devices via a power-line communication network.
 
13. The method of any one of embodiments 1-9, wherein: at least some of the data-center management computing devices are a rack controller executed in an out-of-band computer having a processor and memory storing instructions that when executed effectuate at least some of the operations of embodiment 1; and at least some of the data-center management computing devices manage rack-mounted computing devices via a non-power-line communication network.
 
14. The method of any one of embodiments 1-13, wherein: at least some of the data-center management computing devices execute rack controllers, each configured to manage a plurality of rack-mounted computing devices on a respective rack, a sensor on the respective rack, and an actuator on the respective rack, wherein the sensor and actuator are not on a chassis of one of the rack-mounted computing devices on the respective rack, wherein controlling the rack-mounted computing devices comprises: changing a boot target in an extensible firmware interface of a given one of the rack-mounted computing devices on the respective rack; reading a value indicative of temperature from the sensor or a sensor of a given one of the rack-mounted computing devices on the respective rack; and actuating the actuator.
 
15. The method of any one of embodiments 1-14, wherein: the command includes a disk image containing an operating system, an application, and dependencies of the application to be installed on each of the first data-center management computing device, the first plurality of other data-center management computing devices, and the respective pluralities of other data-center management computing devices.
 
16. The method of any one of embodiments 1-15, wherein: the command is a command to orchestrate containers executing on at least some of the data-center management computing devices.
 
17. The method of any one of embodiments 1-16, wherein: the command is a command to configure a network operating system.
 
18. The method of any one of embodiments 1-17, comprising implementing control and updates of rack controllers at the rack-level, in a distributed, concurrent routine.
 
19. The method of any one of embodiments 1-18, wherein distributing, based at least some of the roles, via the out-of-band network, the data-center management command to a respective plurality of other data-center management computing devices comprises: periodically, by each of the plurality of other data-center management computing devices, sending a request to one of the plurality of other data-center management computing devices for an update; and receiving an update, from the one of the plurality of other data-center management computing devices, with each of the plurality of other data-center management computing devices.
 
20. The method of any one of embodiments 1-19, comprising: initializing a data center with a computer designated to act as a seed unit to distribute configuration files via the given data-center management computing device and the first plurality of other data-center management computing devices.
 
21. The method of any one of embodiments 1-20, wherein determining roles of data-center management computing devices in a distributed peer-to-peer data-center management system comprises: steps for device discovery and data routing.
 
22. The method of any one of embodiments 1-21, comprising: performing steps for configuring rack control units in a data center.
 
23. The method of any one of embodiments 1-22, wherein determining roles of data-center management computing devices in a distributed peer-to-peer data-center management system comprises: steps for arriving at a consensus among data-center management computing devices.
 
24. The method of any one of embodiments 1-23, comprising: executing the in-band application to provide resources to remote computing devices over the Internet with rack-mounted computing devices controlled by at least some of the given data-center management computing device, the first plurality of other data-center management computing devices, and the second pluralities of other data-center management computing devices.
 
25. A system, comprising: a plurality of racks, each rack comprising: a plurality of rack-mounted computing devices; and a medium storing instructions configured to instantiate a rack controller coupled to each of the rack-mounted computing devices via a network, wherein each rack controller is configured to effectuate operations comprising: determining roles of the rack controllers in a distributed peer-to-peer data-center management system; receiving a data-center management command at a first rack controller; distributing, based on at least some of the roles, the data-center management command from the first rack controller to a first plurality of other rack controller; and for each of the first plurality of other rack controllers, distributing, based at least some of the roles, the data-center management command to a respective plurality of other rack controllers.
 
26. A tangible, non-transitory, machine-readable medium storing instructions that when executed by one or more processors effectuate operations comprising: the operations of any one of embodiments 1-25.