Patent Publication Number: US-11381468-B1

Title: Identifying correlated resource behaviors for resource allocation

Description:
BACKGROUND 
     The recent revolution in technologies for dynamically sharing virtualizations of hardware resources, software, and information storage across networks has increased the reliability, scalability, and cost efficiency of computing. More specifically, the ability to provide on demand virtual computing resources and storage through the advent of virtualization has enabled consumers of processing resources and storage to flexibly structure their computing and storage costs in response to immediately perceived computing and storage needs. Virtualization allows customers to purchase processor cycles and storage at the time of demand, rather than buying or leasing fixed hardware in provisioning cycles that are dictated by the delays and costs of manufacture and deployment of hardware. Rather than depending on the accuracy of predictions of future demand to determine the availability of computing and storage, users are able to purchase the use of computing and storage resources on a relatively instantaneous as-needed basis. 
     Virtualized computing environments may provide various guarantees as to the availability and durability of computing resources. Distributing computing resources amongst multiple resource hosts may provide different availability and durability characteristics. For example, virtual computing resources may provide block-based storage. Such block-based storage provides a storage system that is able to interact with various computing virtualizations through a series of standardized storage calls that render the block-based storage functionally agnostic to the structural and functional details of the volumes that it supports and the operating systems executing on the virtualizations to which it provides storage availability. In order to provide block-based storage, various different placement optimizations and/or constraints may be implemented in order to provide performance guarantees. When placing block-based storage resources amongst resource hosts, selecting from among different placement options that satisfy the optimizations and/or constraints to place storage may prove challenging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a logical block diagram for identifying correlated resource behaviors for resource allocations, according to some embodiments. 
         FIG. 2  is a block diagram illustrating a provider network that includes multiple network-based services such as a block-based storage service that implements identifying correlated resource behaviors for resource allocations, according to some embodiments. 
         FIG. 3  is a logical block diagram illustrating volume placement that implements identifying correlated resource behaviors for resource allocations, according to some embodiments. 
         FIG. 4  is a high-level flowchart illustrating various methods and techniques for identifying correlated resource behaviors for resource allocations, according to some embodiments. 
         FIG. 5  is a high-level flowchart illustrating various methods and techniques for evaluating resource similarity to identify correlated behaviors between resources, according to some embodiments. 
         FIG. 6  is a high-level flowchart illustrating various methods and techniques for applying machine learning to update correlated resource behavior identification, according to some embodiments. 
         FIG. 7  is a high-level flowchart illustrating various methods and techniques for predicting behavioral similarities of resources to predict correlated behavior of new resources, according to some embodiments. 
         FIG. 8  is a block diagram illustrating an example computing system, according to some embodiments. 
     
    
    
     While embodiments are described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that the embodiments are not limited to the embodiments or drawings described. It should be understood, that the drawings and detailed description thereto are not intended to limit embodiments to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. 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). Similarly, the words “include”, “including”, and “includes” mean including, but not limited to. 
     DETAILED DESCRIPTION 
     The systems and methods described herein may implement identifying correlated behaviors for resource allocation. Distributed systems may host various resources for performing or implementing different systems, services, applications and/or functions. Some resources may be part of a larger distributed resource, located at multiple resources amongst different resource hosts. Other resources may be individual or stand-alone. Resources may be one of many different types of resources, such as one of various types of physical or virtualized computing resources, storage resources, or networking resources. For example, a storage service may host different replicas of data across a number of different resource hosts. 
     Allocation decisions may be made to assign, distribute, configure, or otherwise determine the distribution of resources among resource hosts in distributed system. Typically, resources operate in independent or unrelated fashion, allowing for allocation techniques to make allocation decisions based on individual considerations of what is optimal for a particular resource. However, some resources may perform workloads or behaviors that are coordinated across multiple resources. For example, benchmarking multiple deployments of similar resources may operate in a coordinated manner. Such coordinated activities may strain resources hosts in a distributed system which may not be allocated to optimize performance of coordinated activities. Identifying correlated resource behaviors for resource allocation may allow allocation decisions to predict correlated behaviors among resources and make allocation decisions that better optimize and/or distributed resources among infrastructure units, such as resource hosts. 
       FIG. 1  illustrates a logical block diagram for identifying correlated resource behaviors for resource allocations, according to some embodiments. Correlation analysis  100  may be implemented as part of a resource placement or allocation technique to account for coordinated or otherwise correlated behavior amongst resources in a distributed system. As illustrated in  FIG. 1 , correlation analysis  100  may be used as an input to resource allocation  130  which may make allocation decisions based on identified correlated behavior between resources. For example, allocation decisions may be made to avoid locating some resources together at resource hosts, or other infrastructure units. Generally, infrastructure units may be may be defined by devices, such as server racks, networking switches, routers, or other components, power sources (or other resource host suppliers), physical or geographical locations (e.g., locations in a particular row, room, building, data center, fault tolerant zone, etc.), logical groupings of devices (e.g., resource host pools that provide different types of resource), or individual resource hosts. Infrastructure units may vary in scope such that a resource host (and replicas of data volumes implemented on the resource host) may be within multiple different types of infrastructure units, such as a particular network router or brick, a particular room location, a particular site, etc. New resources may be placed at an infrastructure unit, such as a resource host or networking device connected resource hosts, based on resource allocation decision-making that is informed by identified correlated resource behaviors. Similarly, currently placed resources may be moved or migrated from one infrastructure unit to another according to allocation decisions informed by identified correlated resource behaviors. 
     Resource data for resources may be tracked, collected, obtained, stored, and/or monitored, in various embodiments. Resource comparison engine  102 , which may be implemented as part of correlation analysis  100  may analyze resource data to determine behavioral similarities between resources. For example, as illustrate din  FIG. 1 , resource historical behavior data  110  may be utilized by resource comparison engine  102 . Previous actions taken, changes in state, or other behavior data may be collected for particular time periods and compared with other resource historical behavior data to determine similarities between resources. Resource configuration data  120  may be utilized in some embodiments, by resource comparison engine  102  to compare the configuration of different resources. For example, resources with common software applications, platforms, or operating systems may be considered similar. 
     Resource comparison engine  102  may provide correlation recognition  104  with determined behavioral similarities. For instance, similarity scores may be generated and provided which indicate the similarity of two resources for a particular behavior or for behavior of the resources overall (e.g., similarity in number of storage operations, or similarity as a resource as whole). Correlation recognition  104  may identify correlated behaviors according to the determined behavioral similarities, in various embodiments. For example, similarity scores may be compared with a correlation threshold. If the score exceeds the threshold, then the behavior and the associated resources may be identified as having a correlated behavior. In some embodiments, correlation criteria may be implemented to evaluate multiple different similarities between resources. For instance, similarity scores may be weighted according to selectivity and combined, according to the correlation criteria. In at least some embodiments, machine learning  106  may be implemented to improve or adapt correlation criteria over time, such as in response to scarcity requirements from hardware owners. 
     Please note that previous descriptions are not intended to be limiting, but are merely provided as an example of identifying correlated resource behavior for resource allocation. Various components may perform resource allocation. Different numbers or types of data may be analyzed to identify correlated resource behaviors. 
     This specification begins with a general description of a provider network, which may implement identifying correlated resource behavior for resource allocation for resources offered via one or more network-based services in the provider network, such as identifying correlated behaviors of data volumes offered via a block-based storage service. Then various examples of a block-based storage service are discussed, including different components/modules, or arrangements of components/module that may be employed as part of volume placement for data volumes in the block-based storage service which may perform identifying correlated resource behavior for resource allocation. A number of different methods and techniques to implement identifying correlated resource behavior for resource allocation are then discussed, some of which are illustrated in accompanying flowcharts. Finally, a description of an example computing system upon which the various components, modules, systems, devices, and/or nodes may be implemented is provided. Various examples are provided throughout the specification. 
       FIG. 2  is a block diagram illustrating a provider network implementing multiple network-based services including a block-based storage service that implements identifying correlated resource behavior for resource allocation, according to some embodiments. Provider network  200  may be set up by an entity such as a company or a public sector organization to provide one or more services (such as various types of cloud-based computing or storage) accessible via the Internet and/or other networks to clients  210 . Provider network  200  may include numerous data centers hosting various resource pools, such as collections of physical and/or virtualized computer servers, storage devices, networking equipment and the like (e.g., computing system  1000  described below with regard to  FIG. 8 ), needed to implement and distribute the infrastructure and services offered by the provider network  200 . In some embodiments, provider network  200  may provide computing resources, such as virtual compute service  230 , storage services, such as block-based storage service  220  and other storage service  240  (which may include various storage types such as object/key-value based data stores or various types of database systems), and/or any other type of network-based services  250 . Clients  210  may access these various services offered by provider network  200  via network  260 . Likewise network-based services may themselves communicate and/or make use of one another to provide different services. For example, computing resources offered to clients  210  in units called “instances,” such as virtual or physical compute instances or storage instances, may make use of particular data volumes  226 , providing virtual block storage for the compute instances. 
     As noted above, virtual compute service  230  may offer various compute instances to clients  210 . A virtual compute instance may, for example, comprise one or more servers with a specified computational capacity (which may be specified by indicating the type and number of CPUs, the main memory size, and so on) and a specified software stack (e.g., a particular version of an operating system, which may in turn run on top of a hypervisor). A number of different types of computing devices may be used singly or in combination to implement the compute instances of virtual compute service  230  in different embodiments, including special purpose computer servers, storage devices, network devices and the like. In some embodiments instance clients  210  or other any other user may be configured (and/or authorized) to direct network traffic to a compute instance. In various embodiments, compute instances may attach or map to one or more data volumes  226  provided by block-based storage service  220  in order to obtain persistent block-based storage for performing various operations. 
     Compute instances may operate or implement a variety of different platforms, such as application server instances, Java™ virtual machines (JVMs), special-purpose operating systems, platforms that support various interpreted or compiled programming languages such as Ruby, Perl, Python, C, C++ and the like, or high-performance computing platforms) suitable for performing client applications, without for example requiring the client  210  to access an instance. In some embodiments, compute instances have different types or configurations based on expected uptime ratios. The uptime ratio of a particular compute instance may be defined as the ratio of the amount of time the instance is activated, to the total amount of time for which the instance is reserved. Uptime ratios may also be referred to as utilizations in some implementations. If a client expects to use a compute instance for a relatively small fraction of the time for which the instance is reserved (e.g., 30%-35% of a year-long reservation), the client may decide to reserve the instance as a Low Uptime Ratio instance, and pay a discounted hourly usage fee in accordance with the associated pricing policy. If the client expects to have a steady-state workload that requires an instance to be up most of the time, the client may reserve a High Uptime Ratio instance and potentially pay an even lower hourly usage fee, although in some embodiments the hourly fee may be charged for the entire duration of the reservation, regardless of the actual number of hours of use, in accordance with pricing policy. An option for Medium Uptime Ratio instances, with a corresponding pricing policy, may be supported in some embodiments as well, where the upfront costs and the per-hour costs fall between the corresponding High Uptime Ratio and Low Uptime Ratio costs. 
     Compute instance configurations may also include compute instances with a general or specific purpose, such as computational workloads for compute intensive applications (e.g., high-traffic web applications, ad serving, batch processing, video encoding, distributed analytics, high-energy physics, genome analysis, and computational fluid dynamics), graphics intensive workloads (e.g., game streaming, 3D application streaming, server-side graphics workloads, rendering, financial modeling, and engineering design), memory intensive workloads (e.g., high performance databases, distributed memory caches, in-memory analytics, genome assembly and analysis), and storage optimized workloads (e.g., data warehousing and cluster file systems). Size of compute instances, such as a particular number of virtual CPU cores, memory, cache, storage, as well as any other performance characteristic. Configurations of compute instances may also include their location, in a particular data center, availability zone, geographic, location, etc. . . . and (in the case of reserved compute instances) reservation term length. 
     In various embodiments, provider network  200  may also implement block-based storage service  220  for performing storage operations. Block-based storage service  220  is a storage system, composed of a pool of multiple independent resource hosts  224   a ,  224   b ,  224   c  through  224   n (e.g., server block data storage systems), which provide block level storage for storing one or more sets of data volumes data volume(s)  226   a ,  226   b ,  226   c , through  226   n . Data volumes  226  may be mapped to particular client(s) (e.g., a virtual compute instance of virtual compute service  230 ), providing virtual block-based storage (e.g., hard disk storage or other persistent storage) as a contiguous set of logical blocks. In some embodiments, a data volume  226  may be divided up into multiple data chunks or partitions (including one or more data blocks) for performing other block storage operations, such as snapshot operations or replication operations. A volume snapshot of a data volume  226  may be a fixed point-in-time representation of the state of the data volume  226 . In some embodiments, volume snapshots may be stored remotely from a resource host  224  maintaining a data volume, such as in another storage service  240 . Snapshot operations may be performed to send, copy, and/or otherwise preserve the snapshot of a given data volume in another storage location, such as a remote snapshot data store in other storage service  240 . 
     Block-based storage service  220  may implement block-based storage service control plane  222  to assist in the operation of block-based storage service  220 . In various embodiments, block-based storage service control plane  222  assists in managing the availability of block data storage to clients, such as programs executing on compute instances provided by virtual compute service  230  and/or other network-based services located within provider network  200  and/or optionally computing systems (not shown) located within one or more other data centers, or other computing systems external to provider network  200  available over a network  260 . Access to data volumes  226  may be provided over an internal network within provider network  200  or externally via network  260 , in response to block data transaction instructions. 
     Block-based storage service control plane  222  may provide a variety of services related to providing block level storage functionality, including the management of user accounts (e.g., creation, deletion, billing, collection of payment, etc.). Block-based storage service control plane  222  may further provide services related to the creation, usage and deletion of data volumes  226  in response to configuration requests. In at least some embodiments, block-based storage service control plane may implement volume placement  228 , such as described in further detail below with regard to  FIG. 3 . Block-based storage service control plane  222  may also provide services related to the creation, usage and deletion of volume snapshots on other storage service  240 . Block-based storage service control plane  222  may also provide services related to the collection and processing of performance and auditing data related to the use of data volumes  226  and snapshots of those volumes. 
     Provider network  200  may also implement another storage service  240 , as noted above. Other storage service  240  may provide a same or different type of storage as provided by block-based storage service  220 . For example, in some embodiments other storage service  240  may provide an object-based storage service, which may store and manage data as data objects. For example, volume snapshots of various data volumes  226  may be stored as snapshot objects for a particular data volume  226 . In addition to other storage service  240 , provider network  200  may implement other network-based services  250 , which may include various different types of analytical, computational, storage, or other network-based system allowing clients  210 , as well as other services of provider network  200  (e.g., block-based storage service  220 , virtual compute service  230  and/or other storage service  240 ) to perform or request various tasks. 
     Clients  210  may encompass any type of client configurable to submit requests to network provider  200 . For example, a given client  210  may include a suitable version of a web browser, or may include a plug-in module or other type of code module configured to execute as an extension to or within an execution environment provided by a web browser. Alternatively, a client  210  may encompass an application such as a database application (or user interface thereof), a media application, an office application or any other application that may make use of compute instances, a data volume  226 , or other network-based service in provider network  200  to perform various operations. In some embodiments, such an application may include sufficient protocol support (e.g., for a suitable version of Hypertext Transfer Protocol (HTTP)) for generating and processing network-based services requests without necessarily implementing full browser support for all types of network-based data. In some embodiments, clients  210  may be configured to generate network-based services requests according to a Representational State Transfer (REST)-style network-based services architecture, a document- or message-based network-based services architecture, or another suitable network-based services architecture. In some embodiments, a client  210  (e.g., a computational client) may be configured to provide access to a compute instance or data volume  226  in a manner that is transparent to applications implement on the client  210  utilizing computational resources provided by the compute instance or block storage provided by the data volume  226 . 
     Clients  210  may convey network-based services requests to provider network  200  via external network  260 . In various embodiments, external network  260  may encompass any suitable combination of networking hardware and protocols necessary to establish network-based communications between clients  210  and provider network  200 . For example, a network  260  may generally encompass the various telecommunications networks and service providers that collectively implement the Internet. A network  260  may also include private networks such as local area networks (LANs) or wide area networks (WANs) as well as public or private wireless networks. For example, both a given client  210  and provider network  200  may be respectively provisioned within enterprises having their own internal networks. In such an embodiment, a network  260  may include the hardware (e.g., modems, routers, switches, load balancers, proxy servers, etc.) and software (e.g., protocol stacks, accounting software, firewall/security software, etc.) necessary to establish a networking link between given client  210  and the Internet as well as between the Internet and provider network  200 . It is noted that in some embodiments, clients  210  may communicate with provider network  200  using a private network rather than the public Internet. 
       FIG. 3  is a logical block diagram illustrating volume placement that implements identifying correlated resource behavior for resource allocation, according to some embodiments. As noted above, multiple resource hosts, such as resource hosts  300 , may be implemented in order to provide block-based storage services. A resource host may be one or more computing systems or devices, such as a storage server or other computing system (e.g., computing system  1000  described below with regard to  FIG. 8 ). Each resource host may maintain respective replicas of data volumes. Some data volumes may differ in size from other data volumes, in some embodiments. Resource hosts may also provide multi-tenant storage. For example, in some embodiments, one resource host may maintain a data volume for one account of block-based storage service  220 , while another data volume maintained at the same resource host may be maintained for a different account. Resource hosts may persist their respective data volumes in one or more block-based storage devices (e.g., hard disk drives, solid state drives, etc.) that may be directly attached to a computing system or device implementing the respective resource host. Resource hosts may implement different persistent storage devices. For example, some resource hosts may implement solid state drives (SSDs) for persistent block storage, while other resource hosts may implement hard disk drives (HDDs) or other magnetic-based persistent storage devices. In this way different volume types, specifications, and other performance characteristics may be provided according to the persistent storage devices implemented at the resource host. 
     Block-based storage service  220  may manage and maintain data volumes in a variety of different ways. Different durability schemes may be implemented for some data volumes among two or more resource hosts as a distributed resource maintaining a same replica of a data volume at different partitions of the data volume. For example, different types of mirroring and/or replication techniques may be implemented (e.g., RAID 1) to increase the durability of a data volume, such as by eliminating a single point of failure for a data volume. In order to provide access to a data volume, resource hosts may then coordinate I/O requests, such as write requests, among the two or more resource hosts maintaining a replica of a data volume. For example, for a given data volume, one resource host may serve as a master resource host. A master resource host may, in various embodiments, receive and process requests (e.g., I/O requests) from clients of the data volume. Thus, the master resource host may then coordinate replication of I/O requests, such as write requests, or any other changes or modifications to the data volume to one or more other resource hosts serving as slave resource hosts. Thus, when a write request is received for the data volume at a master resource host, the master resource host may forward the write request to the slave resource host(s) and wait until the slave resource host(s) acknowledges the write request as complete before completing the write request at the master resource host. Master resource hosts may direct other operations for data volumes, like snapshot operations or other I/O operations (e.g., serving a read request). 
     Please note, that in some embodiments, the role of master and slave resource hosts may be assigned per data volume. For example, for a data volume maintained at one resource host, the resource host may serve as a master resource host. While for another data volume maintained at the same resource host, the resource host may serve as a slave resource host. Resource hosts may implement respective I/O managers. The I/O managers may handle I/O requests directed toward data volumes maintained at a particular resource host. Thus, I/O managers may process and handle a write request to volume at resource host, for example. I/O managers may be configured to process I/O requests according to block-based storage service application programming interface (API) and/or other communication protocols, such as such as internet small computer system interface (iSCSI). 
     Resource hosts may be located within and/or considered different infrastructure units. Infrastructure units may be defined by devices, such as server racks, networking switches, routers, or other components, power sources (or other resource host suppliers), physical or geographical locations (e.g., locations in a particular row, room, building, data center, fault tolerant zone, etc.), or logical groupings of devices (e.g., resource host pools that provide different types of resource). Infrastructure units may vary in scope such that a resource host (and replicas of data volumes implemented on the resource host) may be within multiple different types of infrastructure units, such as a particular network router or brick, a particular room location, a particular site, etc. 
     Block-based storage service control plane  222  may implement volume placement  228 , in various embodiments. Volume placement  228  may be implemented at one or more computing nodes, systems, or devices (e.g., system  1000  in  FIG. 8 ). In at least some embodiments, volume placement  228  may implement placement data collection  320  to collect information, metrics, metadata, or any other information for performing volume placement (e.g., collecting historical behavioral data and/or configuration data for resources). Placement data collection  320  may periodically sweep resource host(s)  300  with a query for the information, metrics, or metadata. For example, resource hosts may provide current utilization metrics, ongoing tasks or operations (e.g., such as migration or remirror tasks), and any other state information for the resource host, including resource specific information for resources residing at the resource hosts. In some embodiments, placement data collection  320  may aggregate the data according to infrastructure units, partitions, resource hosts, or other granularities for block-based storage service  220 . Placement data collection  320  may store the data at volume/service state store  322 , which may persistently maintain the collected data. In some embodiments volume/service state store  322  may be implemented as a database or otherwise searchable/query-able storage system to provide access to other components of volume placement  228  or block-based storage service control plane  226 . 
     Volume placement  228  may implement placement engine  310 , in various embodiments. Placement engine  310  may perform various kinds of analysis to identify placement locations for partitions of new data volumes or migrating data volumes. In response to receiving a placement request at placement engine  310 , placement engine  310  may determine prospective placements by accessing volume/service state  322 . Those resource hosts which are available, and which do not violate any placement constraints may be evaluated (e.g., two partitions of a data volume cannot be hosted by the same resource host, resource hosts with enough capacity, or resource hosts that implement particular hardware and/or software). In some embodiments, a subset of available resource hosts may be evaluated for placement decisions (as evaluating a very large pool of available resource hosts may be too computationally expensive). 
     Placement requests may be received to place a new resource, in some embodiments. Placement requests may also be received to move or migrate already hosted resources, such as discussed below with regard to opportunistic placement manager  330 . In at least some embodiments, placement engine  310  may provide placement or reconfiguration recommendations for currently placed resources (e.g., to a user associated with resources). 
     Placement engine  310  may implement correlation analysis  312  to identify correlated resource behaviors. For example, correlation analysis  312  may access volume/service state  322  to obtain historical behavioral data for resources at resource host(s)  300  and perform comparisons. Similarly, correlation analysis  312  may access volume/service state  322  to compare configuration data (e.g., resource tags, or hardware or software configurations). Correlated behaviors of resources may be identified according to behavioral similarities and placement decisions may be made (possibly utilizing other additional analysis techniques in combination), such as the various techniques described below with respect to  FIGS. 4-7 . For example, placing resources on resource hosts with correlated behaviors may be avoided. 
     Placement engine  310  may implement other analysis  314  to determine resource placements. For example, scores may be generated for placements based on the last time a particular resource host was contacted or heard from. Analysis may be performed to prevent multiple master-slave replica pairs from being placed on the same two resource hosts. In some embodiments, resource host fragmentation analysis may be performed, to optimize placement of resources on resource hosts that can host the resource and leave the least amount of space underutilized. Other analysis  314  may implement configuration analysis to evaluate prospective placement configurations of all of the resources in a distributed resource, such as the placement of master, slave(s) of a data volume. In some embodiments, a client or other user of a distributed resource (or resource of the distributed resource) may be considered in the configuration analysis (e.g., evaluating the placement configuration including a virtual instance attached to a data volume). Prospective placement configurations may be generated or identified based on the available resource hosts for the resource. Other replicas of the data volume may be evaluated based on actual or hypothetical placement locations. One or more infrastructure zone localities may be determined for the different prospective placement configurations, in various embodiments, based on volume/service state  332 . 
     In some embodiments, volume placement  228  may implement opportunistic placement manager  330 . Opportunistic placement management  330  may dynamically or proactively migrate currently placed resources from one resource host to another resource host so that the placement for the data volume is more optimal. For example, opportunistic placement manager  330  may implement migration operation scheduling  332  to request placements for resources from placement engine  310  that are determined to be placed sub-optimally (e.g., a lower scoring infrastructure zone category). Migration operation scheduling  332  may then determine which placements if performed would exceed a migration optimization threshold (e.g., a difference between a current placement score and new placement score). For those resources with possible placements that would exceed the placement optimization threshold, migration operation scheduling  332  may place a migration operation for the partition in migration operation queue  336 . In some embodiments, migration operation scheduling  332  may assign a priority to migration operations, so that more beneficial migration operations are performed sooner. 
     Migration operation scheduling  332  may also remove migration operations from queue  336 , such as those migration operations identified as complete or failed. Those migration operations that have not yet been performed may have update priorities stored in the queue (e.g., raising or lowing the priority value). Time of last update may indicate when an update to the migration operation in the queue was last made. For example, a migration operation that has a later update time than other migration operations, may be considered to have more recent/relevant data. Priority values may be assigned to migration operations in order to schedule the migration operations opportunistically. In at least some embodiments, migration operation queue  336  may be implemented as a priority queue, and thus the highest priority migration operation may be selected for performance. 
     Migration worker(s)  340  may be implemented to perform migration operations. Migration worker(s)  340  may send a request to opportunistic placement manger  330  for a migration operation to perform. Opportunistic placement manger  330  may pull a migration operation from migration operation queue  336  and assign the migration operation to a migration worker  340  to direct. Alternatively, migration workers may directly access migration operation queue  336  to identify migration operations to perform, in some embodiments. Migration worker(s)  340  may, in some embodiments, update metadata for a migration operation in migration operation queue  336  (e.g., to change state from “ready” to “in progress”). 
     In some embodiments, migration operation throttling  342  may be implemented to control the number of ongoing migration operations. Placement data collection  320  may track, maintain, or monitor current migration operations that are ongoing at resource host(s)  310 , along with other data, such as network utilization, resource host utilization, or any other operational metrics and update volume/service state  322 . Migration worker(s)  340  may access volume/service state  322  to determine whether a migration operation should be throttled according to some migration limit. For example, in some embodiments, network localities, which may include one or more resource host(s)  310 , networking device(s), router(s), switches, power source(s), or other component or device of a virtual block-based storage service may be evaluated with respect to the effect of performing the identified resource migration operation. Different migration limits (e.g., number of migration operations, network utilization, resource host utilization, etc.) may be enforced with respect to the network localities. If the migration operation exceeds the limit for one of the different network localities, then the migration worker may throttle performance of the migration operation (e.g., the migration operation may be denied or delayed). In some embodiments, migration operation throttling may be limited to specific infrastructure zones or network localities (e.g., to the infrastructure zones or network localities which would be involved with perform a migration, such as zones that include the current and destination resource hosts of a migration operation). In some embodiments, opportunistic placement management  330  may perform migration operation throttling in addition to, or in place of migration worker(s)  340 . 
     In various embodiments, migration worker  340  may request an updated placement for a resource that is to be migrated from placement engine  310 , which may perform the various techniques discussed above and below to provide a new placement location for the resource. 
     Resources may be one of many different types of resources, such as one of various types of physical or virtualized computing resources, storage resources, or networking resources. Some resources may be part of a group of resources that make up a distributed resource. For example, a data volume of the block-based storage service described above with regard to  FIGS. 2 and 3  may be a distributed resource that is implemented as a master replica and one or more replica slaves. 
     In some instances, the operations, functions, tasks, or otherwise behaviors of the resources of a distributed system may be implemented as part of coordinated, related, directed, or predetermined activities, resulting in an impact on a distributed system that is greater than the individual effects of behaviors exhibited by resources. For instance, different resources may be similarly configured to perform a backup operation at same or similar time of day, increasing the burden on those portions of the distributed system involved in the backup operation (e.g., back up storage servers). By predicting such correlated behaviors, allocation decisions for resources in a distributed system may be made so as to compensate for the impact of correlated behaviors.  FIG. 4  is a high-level flowchart illustrating various methods and techniques for identifying correlated resource behaviors for resource allocations, according to some embodiments. 
     As indicated at  410 , respective resource data for resources allocated amongst infrastructure units of a distributed system may be analyzed to determine behavioral similarities between the resources. Respective resource data may be resource data which is indicative of past, current, and/or future behavior(s) of a resource. For example, in a least some embodiments, actions, operations, performance, or utilization of a resource at a resource host or other infrastructure unit may be tracked and organized according to time (e.g., as a time series). In some embodiments, descriptive information about resources, such as configuration data may be collected or obtained, either from system generated sources (e.g., network configuration information) or user supplied sources (e.g., resource tags or attributes or hardware/software configurations). Resource data may be periodically (or aperiodically) updated, such as discussed above with regard to  FIG. 3 , in order to obtain a more current version or state of resources in the distributed system. 
     Analysis may be performed to determine behavioral similarities between different resources. Behavioral similarities may be a certain type of action, operation, or other behavior performed by (or with regard to) multiple resources in similar manner and/or time period. Behavioral similarity may be determined by recognizing common patterns, frequencies, or effects as a result of analyzing the resource data between different resources (e.g., common actions or common configuration which may determine actions).  FIG. 5 , discussed below, provides further examples of determining behavioral similarities. For example, similarity scores, or other indicators of similarity, may be calculated for different resources with respect to other resources. In some embodiments, as discussed below with regard to  FIG. 7 , a prediction of behavioral similarities (e.g., for a new resource which has not historical behavioral data) may be performed. 
     As indicated at  420 , behaviors of resources may be identified as correlated according to the determined behavioral similarities of the resources, in some embodiments. For example, if two different resources, such as two different compute instances, perform the same computational task at the same or similar time once in a given time period (e.g., a hour, day, or week), then the behavioral similarity may be identified as correlated. Consider the scenario where multiple different compute instances perform analysis on purchase data for the previous 24 hour period at similar time (e.g., daily at 01:00:00). The different compute instances may exhibit similar processor demands to perform the various computational actions associated with the analysis and make similar storage demands to access the data under analysis. These determined behavioral similarities may be recognized (as discussed above at element  410 ) as sufficient to identify the similar behavior of the compute instances as correlated based on the similarities between processing load and storage access load. Note, the term correlated as used herein may be a quantitative term, and thus behaviors may be more or less correlated than other behaviors. Behaviors that are identified as “correlated” may be correlated with respect to particular degree, amount, or threshold of correlation. In at least some embodiments, correlation criteria may be implemented or applied to identify correlated behaviors. For example, correlation criteria may implement a weighting schema which provides maps different behavioral similarities to different correlation weights. If, for instance, two resources perform a similar task at similar times, but have different hardware configurations, then the weighting schema may determine that correlation between the two resources is lower than if the hardware configurations were the same (e.g., valuing common hardware configurations). As discussed below with regard to  FIG. 6 , machine learning techniques may be applied to identify correlated behaviors, for instance, by updating, modifying, or changing correlation criteria. 
     As indicated at  430 , based, at least in part, on the identified correlated behavior(s), an allocation of the infrastructure units for resource(s) may be determined. For example, in some embodiments, it may be desirable to move or migrate a resource from one infrastructure unit to another (e.g., as discussed above with regard to  FIG. 3  in order to optimize placement or allocation of resources amongst resource hosts). Moving resources from one networking device to another, for instance, may be performed to allocate resource hosts amongst networking devices of a distributed system. In some embodiments, migration of the resource may be performed by directing a current resource host to transfer the resource to the destination resource host. An intermediary, such as worker(s)  340  in  FIG. 3  may direct and/or receive the resource before sending the resource to the destination resource host, in at least some embodiments. A failure scenario may trigger the migration of currently placed resources from one resource host to another, for instance. Allocation of resources may be placement decisions made to determine new or destination infrastructure unit, such as a resource host, for migrated resources. In some embodiments, multiple infrastructure units may be selected for placing a resource host, such as a particular network router and a particular resource host connected to the particular network router. In some instances, it may be optimal to ensure that resources with a correlated behavior are located at separate infrastructure units. Placement decisions may eliminate from possible destination infrastructure units those infrastructure units that already include a resource with an identified correlated behavior. Migration of resources may occur as a result of a manual or user generated request to migrate a resource, an automated or dynamic migration technique, as discussed above, and/or in response to particular events, such as certain failure scenarios. 
     Allocation of infrastructure units for resource(s) may also be performed to allocate infrastructure units for a new resource. For example, a request to place a new resource may be received. Placement of the new resource may be made so as to avoid placing the new resource within infrastructure units that are impacted by correlated behaviors of other resources. In some embodiments, as discussed below with regard to  FIG. 7 , a prediction may be made as to possible correlated behaviors of the new resource with respect to other resources. 
     In at least some embodiments, allocation decisions for resources may provide a recommendation to reconfigure resources with correlated behavior as a different resource or resources. For example, if multiple resources have correlated data processing behaviors, a recommendation may be made to reconfigure the multiple resources as a single resource on a different resource host with greater processing capability. Similarly, multiple resources can be further divided into more resources. In some embodiments, recommendations (and operations to implement the recommendation) may modify the behavior of one or more of the multiple resources so that the behaviors are less (or no longer) correlated. Please note that the previous examples illustrate some of the many different ways in which the identity of correlated resource behaviors may be used to allocate resources amongst resource hosts, and thus the previous examples are not intended to be limiting. 
     As noted above, behavioral similarities may be determined by analyzing resource data specific to the resources of a distributed system. Such analyses may be performed as part of a placement engine, workflow or other technique to allocate resources as the need to allocate resources is detected (e.g., via a request), or may be performed to generate a model or other data structure which can be later used to identify correlated behaviors of resources, such as discussed above with regard to element  420  of  FIG. 4 .  FIG. 5  is a high-level flowchart illustrating various methods and techniques for evaluating resource similarity to identify correlated behaviors between resources, according to some embodiments. 
     As indicated, at  510 , historical behavior data of a resource may be compared with other resource(s) in a distributed system, in some embodiments. Historical behavior data may indicate any action, change, or utilization to implement, operate, or perform the resource. For instance, utilization of a processor by a resource, such as a data volume (e.g., for providing read or write access to data) or virtual compute instance (e.g., for performing different calculations as part of an application), may be tracked or collected over time as historical behavior data for a resource. Network utilization (e.g., number of data packets sent or received), storage utilization (e.g., number of bytes stored), or any other utilization (e.g., system memory utilization) may indicate the behavior of a resource. Behavior data may also be tracked based on the type or classification of actions performed. For example, the number and/or types of different API requests made by a resource or received at a resource may be tracked. 
     Historical behavior data may be represented and/or evaluated in different ways. For example, time series data for a resource may be obtained with respect to a particular action, state, or other information (e.g., on/off or active/inactive). The time series data of the resource and other resources may be compared. For instance, the number of times at which the action, state, or other information match or agree may be calculated. Processor utilization, for instance, may be compared for similar workload patterns. In another example, actions, state, or other information in historical behavior data may be plotted as a vector for the different resources, and vector comparison techniques (e.g., cosine similarity) may be performed to compare the resource with other resources. Please note, that in some embodiments, a subset of the larger number of resources may be selected for this and other evaluations discussed below, in some embodiments, in order to reduce the workload (e.g., computational cost) to make the respective comparisons. 
     As indicated at  520 , configuration data of the resource may be compared with the other resource(s) in the distributed system, in some embodiments. For example, resource attributes or tags, hardware/software configuration information (e.g., processor type, amount of system memory, graphics processor, magnetic or solid state storage technology, operating system, software applications, device drivers, etc.), or other description information of a resource (or resource host) may be compared. Some configuration data may always be known for a resource, as it may constitute a hard constraint for placing the resource that must be satisfied. Such configuration data may, in some embodiments, be given more weight in a similarity analysis. Other resource configuration data may be optional, or only possibly known. User-defined resource attributes, such as resource tags, may provide optional or additional data or sets of data which may be used in a comparison analysis. Such configuration data may, in some embodiments, be given less weight than hard constraint configuration data. Configuration data may be used to identify behavioral similarities. Consider the scenario where two resources are implementing the same analytics application. A comparison of the two resources may recognize this common configuration. In another example, two (or more) resources tagged as “backup” or “production” may be recognized by comparing configuration data. 
     Similarity score(s) for the resource with respect to the other resource(s) may be calculated, as indicated at  530 , in various embodiments. For example, separate similarity scores may be generated with respect to one or more different portions of historical behavior data (e.g., score for write operations, score for read operations, score for requests processed, or score for requests sent) and configuration data (e.g., software similarity score, hardware similarity score, or resource tag similarity score). In some examples, historical behavior data and configuration data may both either increase or decrease the similarity score, depending upon the data. These scores may then be transformed or combined to create an overall similarity score for a resource with respect to another resource (e.g., resource A is 47/100 similar to resource B, 95/100 similar to resource C, and so on). In at least some embodiments, however, similarity scores may be considered separately. 
     If the similarity score(s) with other resource(s) exceed a correlation threshold, as indicated by the positive exit from  540 , then correlated behavior may be identified for allocation determinations (as discussed above with regard to element  430  of  FIG. 4 ). Correlated behavior between two or more resources may be specific or general, in various embodiments. For example, I/O throughput behavior for different resources may be identified as correlated, whereas other behaviors of the same resources may not be considered correlated. Correlation thresholds, in such a scenario, may be different for the specific behaviors for which correlation is determined (e.g., correlation threshold for I/O throughput and a separate correlation threshold for configuration data, such as matching 3 of 4 tags). Alternatively, resources may be generally identified as correlated (or not correlated) with one another. For instance, in at least some embodiments, correlation criteria may be implemented to provide selection rules, weightings, or some other schema which may be used to identify correlated behavior between resources (and behavior that is not correlated). 
     Once correlated behavior is identified, or if no correlated behavior is identified as indicated by the negative exit from  540 , then another resource may be selected to evaluate, as indicated at  560 , in some embodiments. For instance, the technique illustrated in  FIG. 5  may be used to construct and/or update a correlation model, which indicates the identified correlated behaviors of resources in a distributed system. 
     As identification of correlated behavior is performed over time, improvements may be made to identification techniques, such as those discussed above. Some behavioral similarities may become more selective or indicative of correlated behavior, while other behavioral similarities may become less so. For systems that perform identifying correlated resource behavior for resource allocation, adaptations to account for these changes in selectivity may increase the effectiveness of allocation decisions made.  FIG. 6  is a high-level flowchart illustrating various methods and techniques for applying machine learning to update correlated resource behavior identification, according to some embodiments. 
     As indicated at  610 , in at least some embodiments, based, at least in part, on determined similarities with other resources, similarity scores may be generated between resources according to correlation criteria—such as the similarity scores generated with respect to historical behavior data and configuration data. As indicated at  620 , results of the correlation identifications made according to these similarity scores may be stored, in some embodiments. For instance, a database or other structured data store may be used to persistently store generated scores and identified behaviors based on the scores. 
     As indicated at  630 , machine learning technique(s) may be applied to evaluate the stored results of correlation identification between the resources, in some embodiments. A feature selection algorithm may be implemented to determine if current correlation criteria should be modified, updated, and/or replaced which would increase the accuracy of correlated behavior identifications (e.g., minimize the error of false negatives). Pattern recognition, such as different classification or clustering techniques may be implemented to determine appropriate weights for different correlation criteria (e.g., increase or decrease weight of tag similarity vs. action similarity). As many other machine learning techniques may be implemented, the previous examples are not intended to be limiting. 
     The results of machine learning evaluations may be used to alter how correlated behaviors are identified. For example, as indicated at  640 , the correlation criteria may be updated according to the machine learning evaluation of the results of correlation identification between the resources, in some embodiments. Weights may be changed, new similarities evaluated, or other changes to the way in which correlated behaviors are identified may be made. Applying the techniques discussed above with regard to  FIG. 6  may allow correlated behavior identification to keep up with changing patterns or new correlated behaviors that would not be recognized without applying machine learning techniques. 
     In some scenarios, a new resource may be placed for which no specific historical behavioral data is available. Other resource data, such as configuration data, may be available, however. Thus, in some scenarios, correlated behavior may be predicted for resources that have not yet been allocated in a distributed system.  FIG. 7  is a high-level flowchart illustrating various methods and techniques for predicting behavioral similarities of resources to predict correlated behavior of new resources, according to some embodiments. 
     As indicated at  710 , a request to place a new resource at a resource host in a distributed system may be received. The request may include data that describes the resource to be placed, such as desired hardware, software, location or association with other resources (e.g., a virtual compute instance to which the data volume resource is attached), and/or resource tag(s). In other words, configuration data for the new resource may be obtained. 
     As indicated at  720 , in some embodiments, behavioral similarit(ies) of the new resource may be predicted with respect to currently hosted resources in the distributed system, in some embodiments. For example, configuration data comparisons may be made with other resources (e.g., the new resource may be tagged as “production” and “user data”, and other currently placed resources tagged as “production” and “user data” may be recognized). In some embodiments, the source of the request (e.g., user or client) may be used to predict behavioral similarities. As indicated at  730 , the new resource may be identified as correlated with behaviors of different one(s) of the currently hosted resource(s), in some embodiments. Similar to the identification techniques discussed above with regard to  FIGS. 4 and 5 , similarity scores may be generated, thresholds evaluated, and/or correlation criteria applied to identify correlated behavior(s). 
     Based, at least in part, on the identified correlated behavior(s), a selection of the resource host to place the new resource may be made, in some embodiments. For instance, resource hosts that host a resource with a correlated behavior may not be considered and/or selected to place the new resource (or alternatively on those resource hosts with resources that have correlated behavior(s) may be selected). As illustrated at  740 , for example, if a resource host without correlated behavior(s) is available, then the resource may be placed at that resource host. On the other hand, if no correlated behavior(s) are determined, then there may be more options for resource hosts at which to place the new resource. Once identified, as indicated at  750 , placement of the new resource at the resource host may be performed. For example, data may be written or copied to the selected resource host, an operation to configure and/or instantiate the new resource at the resource host may be performed, and/or other operations, such as migration of a resource off of the selected resource host, may be performed. If no resource hosts without correlated behavior(s) are available, as indicated at  760 , then the placement request may be denied. Alternatively, in some embodiments, the placement of the resource may be made at a resource host with correlated behavior(s) based on other placement criteria. Please note, that various different schemes to place resources with or apart from other resources with correlated behaviors may be implemented, and thus the previous example is not intended to be limiting. 
     The methods described herein may in various embodiments be implemented by any combination of hardware and software. For example, in one embodiment, the methods may be implemented by a computer system (e.g., a computer system as in FIG.  8 ) that includes one or more processors executing program instructions stored on a computer-readable storage medium coupled to the processors. The program instructions may be configured to implement the functionality described herein (e.g., the functionality of various servers, resource hosts, control planes, managers and/or other components, such as those that implement the block-based storage service described herein). The various methods as illustrated in the figures and described herein represent example embodiments of methods. The order of any method may be changed, and various elements may be added, reordered, combined, omitted, modified, etc. 
     Embodiments of identifying correlated resource behaviors for resource placement as described herein may be executed on one or more computer systems, which may interact with various other devices.  FIG. 8  is a block diagram illustrating an example computer system, according to various embodiments. For example, computer system  1000  may be configured to implement storage and/or compute nodes of a compute cluster, a data stores, and/or a client, in different embodiments. Computer system  1000  may be any of various types of devices, including, but not limited to, a personal computer system, desktop computer, laptop or notebook computer, mainframe computer system, handheld computer, workstation, network computer, a consumer device, application server, storage device, telephone, mobile telephone, or in general any type of computing device. 
     Computer system  1000  includes one or more processors  1010  (any of which may include multiple cores, which may be single or multi-threaded) coupled to a system memory  1020  via an input/output (I/O) interface  1030 . Computer system  1000  further includes a network interface  1040  coupled to I/O interface  1030 . In various embodiments, computer system  1000  may be a uniprocessor system including one processor  1010 , or a multiprocessor system including several processors  1010  (e.g., two, four, eight, or another suitable number). Processors  1010  may be any suitable processors capable of executing instructions. For example, in various embodiments, processors  1010  may be general-purpose or embedded processors implementing any of a variety of instruction set architectures (ISAs), such as the x86, PowerPC, SPARC, or MIPS ISAs, or any other suitable ISA. In multiprocessor systems, each of processors  1010  may commonly, but not necessarily, implement the same ISA. The computer system  1000  also includes one or more network communication devices (e.g., network interface  1040 ) for communicating with other systems and/or components over a communications network (e.g. Internet, LAN, etc.). 
     In the illustrated embodiment, computer system  1000  also includes one or more persistent storage devices  1060  and/or one or more I/O devices  1080 . In various embodiments, persistent storage devices  1060  may correspond to disk drives, tape drives, solid state memory, other mass storage devices, block-based storage devices, or any other persistent storage device. Computer system  1000  (or a distributed application or operating system operating thereon) may store instructions and/or data in persistent storage devices  1060 , as desired, and may retrieve the stored instruction and/or data as needed. For example, in some embodiments, computer system  1000  may host a storage system server node, and persistent storage  1060  may include the SSDs attached to that server node. 
     Computer system  1000  includes one or more system memories  1020  that are configured to store instructions and data accessible by processor(s)  1010 . In various embodiments, system memories  1020  may be implemented using any suitable memory technology, (e.g., one or more of cache, static random access memory (SRAM), DRAM, RDRAM, EDO RAM, DDR 10 RAM, synchronous dynamic RAM (SDRAM), Rambus RAM, EEPROM, non-volatile/Flash-type memory, or any other type of memory). System memory  1020  may contain program instructions  1025  that are executable by processor(s)  1010  to implement the methods and techniques described herein. In various embodiments, program instructions  1025  may be encoded in platform native binary, any interpreted language such as Java™ byte-code, or in any other language such as C/C++, Java™, etc., or in any combination thereof. For example, in the illustrated embodiment, program instructions  1025  include program instructions executable to implement the functionality of a resource host, in different embodiments. In some embodiments, program instructions  1025  may implement multiple separate clients, nodes, and/or other components. 
     In some embodiments, program instructions  1025  may include instructions executable to implement an operating system (not shown), which may be any of various operating systems, such as UNIX, LINUX, Solaris™, MacOS™, Windows™, etc. Any or all of program instructions  1025  may be provided as a computer program product, or software, that may include a non-transitory computer-readable storage medium having stored thereon instructions, which may be used to program a computer system (or other electronic devices) to perform a process according to various embodiments. A non-transitory computer-readable storage medium may include any mechanism for storing information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). Generally speaking, a non-transitory computer-accessible medium may include computer-readable storage media or memory media such as magnetic or optical media, e.g., disk or DVD/CD-ROM coupled to computer system  1000  via I/O interface  1030 . A non-transitory computer-readable storage medium may also include any volatile or non-volatile media such as RAM (e.g. SDRAM, DDR SDRAM, RDRAM, SRAM, etc.), ROM, etc., that may be included in some embodiments of computer system  1000  as system memory  1020  or another type of memory. In other embodiments, program instructions may be communicated using optical, acoustical or other form of propagated signal (e.g., carrier waves, infrared signals, digital signals, etc.) conveyed via a communication medium such as a network and/or a wireless link, such as may be implemented via network interface  1040 . 
     In some embodiments, system memory  1020  may include data store  1045 , which may be configured as described herein. In general, system memory  1020  (e.g., data store  1045  within system memory  1020 ), persistent storage  1060 , and/or remote storage  1070  may store data blocks, replicas of data blocks, metadata associated with data blocks and/or their state, configuration information, and/or any other information usable in implementing the methods and techniques described herein. 
     In one embodiment, I/O interface  1030  may be configured to coordinate I/O traffic between processor  1010 , system memory  1020  and any peripheral devices in the system, including through network interface  1040  or other peripheral interfaces. In some embodiments, I/O interface  1030  may perform any necessary 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., processor  1010 ). In some embodiments, I/O interface  1030  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, for example. In some embodiments, the function of I/O interface  1030  may be split into two or more separate components, such as a north bridge and a south bridge, for example. Also, in some embodiments, some or all of the functionality of I/O interface  1030 , such as an interface to system memory  1020 , may be incorporated directly into processor  1010 . 
     Network interface  1040  may be configured to allow data to be exchanged between computer system  1000  and other devices attached to a network, such as other computer systems  1090 , for example. In addition, network interface  1040  may be configured to allow communication between computer system  1000  and various I/O devices  1050  and/or remote storage  1070 . Input/output devices  1050  may, in some embodiments, include one or more display terminals, keyboards, keypads, touchpads, scanning devices, voice or optical recognition devices, or any other devices suitable for entering or retrieving data by one or more computer systems  1000 . Multiple input/output devices  1050  may be present in computer system  1000  or may be distributed on various nodes of a distributed system that includes computer system  1000 . In some embodiments, similar input/output devices may be separate from computer system  1000  and may interact with one or more nodes of a distributed system that includes computer system  1000  through a wired or wireless connection, such as over network interface  1040 . Network interface  1040  may commonly support one or more wireless networking protocols (e.g., Wi-Fi/IEEE 802.11, or another wireless networking standard). However, in various embodiments, network interface  1040  may support communication via any suitable wired or wireless general data networks, such as other types of Ethernet networks, for example. Additionally, network interface  1040  may support communication via telecommunications/telephony networks such as analog voice networks or digital fiber communications networks, via storage area networks such as Fibre Channel SANs, or via any other suitable type of network and/or protocol. In various embodiments, computer system  1000  may include more, fewer, or different components than those illustrated in  FIG. 8  (e.g., displays, video cards, audio cards, peripheral devices, other network interfaces such as an ATM interface, an Ethernet interface, a Frame Relay interface, etc.) 
     It is noted that any of the distributed system embodiments described herein, or any of their components, may be implemented as one or more network-based services. For example, a compute cluster within a computing service may present computing and/or storage services and/or other types of services that employ the distributed computing systems described herein to clients as network-based services. In some embodiments, a network-based service may be implemented by a software and/or hardware system designed to support interoperable machine-to-machine interaction over a network. A network-based service may have an interface described in a machine-processable format, such as the Web Services Description Language (WSDL). Other systems may interact with the network-based service in a manner prescribed by the description of the network-based service&#39;s interface. For example, the network-based service may define various operations that other systems may invoke, and may define a particular application programming interface (API) to which other systems may be expected to conform when requesting the various operations. 
     In various embodiments, a network-based service may be requested or invoked through the use of a message that includes parameters and/or data associated with the network-based services request. Such a message may be formatted according to a particular markup language such as Extensible Markup Language (XML), and/or may be encapsulated using a protocol such as Simple Object Access Protocol (SOAP). To perform a network-based services request, a network-based services client may assemble a message including the request and convey the message to an addressable endpoint (e.g., a Uniform Resource Locator (URL)) corresponding to the network-based service, using an Internet-based application layer transfer protocol such as Hypertext Transfer Protocol (HTTP). 
     In some embodiments, network-based services may be implemented using Representational State Transfer (“RESTful”) techniques rather than message-based techniques. For example, a network-based service implemented according to a RESTful technique may be invoked through parameters included within an HTTP method such as PUT, GET, or DELETE, rather than encapsulated within a SOAP message. 
     Although the embodiments above have been described in considerable detail, numerous variations and modifications may be made as would become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such modifications and changes and, accordingly, the above description to be regarded in an illustrative rather than a restrictive sense.