Abstract:
Techniques for facilitating and accelerating log data processing are disclosed herein. The front-end clusters generate a large amount of log data in real time and transfer the log data to an aggregating cluster. When the aggregating cluster is not available, the front-clusters write the log data to local filers and send the data when the aggregating cluster recovers. The aggregating cluster is designed to aggregate incoming log data streams from different front-end servers and clusters. The aggregating cluster further sends the aggregated log data stream to centralized NFS filers or a data warehouse cluster. The local filers and the aggregating cluster stage the log data for access by applications, so that the applications do not wait until the data reach the centralized NFS filers or data warehouse cluster.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to data staging systems, and in particular to an multi-level data staging system for low latency data access. 
       BACKGROUND 
       [0002]    Developments in computer and networking technology have given rise to applications that require massive amounts of data storage. For example, tens of millions of users can create web pages and upload images and text to a social media website. Consequently, a social media website can accumulate massive amounts of data each day and therefore need a highly scalable system for storing and processing data. Various tools exist to facilitate such mass data storage. 
         [0003]    Front end clusters of these social media website monitor user activities and produce log data based the activities of social media users. The front end clusters transmit the log data to a centralized storage filer or a data warehouse. The centralized storage filer or data warehouse organizes the received log data and responds to requests from data processing applications. In order to accommodate the massive amounts of log data, large-scale data warehouses are commonly used to store the log data and service the data-intensive inquiries from the data processing applications. 
         [0004]    Frameworks exist that support large-scale data-intensive distributed applications, by enabling applications to interact with a cluster of thousands of computers (also referred to as nodes) and petabytes of data. For instance, a framework called Hadoop utilizes a distributed, scalable, portable file system, called Hadoop Distributed File System (HDFS), to distribute a massive amount of data among data nodes (also referred to as slave nodes) in a Hadoop cluster. In order to reduce the adverse impact of a data node power outage or network failure (including switch failure), data in an HDFS is typically replicated on different data nodes. 
         [0005]    Hive, an open source data warehouse system, was developed to run on top of Hadoop clusters. Hive supports data queries expressed in a scripted query language (SQL)-like declarative language called HiveQL. The Hive system then compiles the queries expressed in HiveQL into map-reduce jobs that can be executed on the Hadoop cluster, in a mathematical form of directed acyclic graph. The HiveQL language includes a type system that supports tables containing primitive types, collections such as arrays and maps, and nested compositions of types. In addition, the Hive system includes a system catalog; called Hive-Metastore, containing schemes and statistics, which is useful in data exploration and query optimization. 
         [0006]    Coupled with the Hadoop cluster, the Hive system can store and analyze large amounts of data for a social networking system. For example, the Hive system can analyze the degree of connection between users to rank stories that users follow on the social networking system. The Hive system can analyze activity logs to gain insights into how services of the social networking system are being used to help application developers, page administrators and advertisers make development and business decisions. The Hive system can run complex data mining programs to optimize the advertisements shown to the users of the social networking system. The Hive system can further analyze the usage logs to identify spam and abuse of the social networking system. 
         [0007]    The Hive system includes web-based tools for people without programming ability to author and execute Hive queries, for authoring, debugging and scheduling complex data pipelines, and for generating reports based on data stored in the Hive system and other relational databases like MySQL and Oracle. 
         [0008]    However, the front end clusters sends the captured log data to the centralized data warehouse periodically, instead of in real time. Furthermore, it takes time for the data warehouse to organize the received log data before the data warehouse is able to respond to data inquiries for these log data. Therefore, the log data in the data warehouse is only available after a time period since the log data was captured. The time period can be an hour or even a day. The data processing and consuming applications can only access the log data with a significant latency. 
         [0009]    Furthermore, the centralized data warehouse needs to maintain connections with the front end servers for continuously receiving the log data. In a modern social network, the number of front end servers can be thousands or even more. The data warehouse carries a significant burden of maintaining the connections. Such a burden of maintaining the connections impacts the overall performance of the data warehouse. 
       SUMMARY 
       [0010]    The technology introduced here provides the benefits of log data access and processing in real time or near real time. In particular, the technology introduced here includes front-end clusters that generate a large amount of log data in real time and transfer the log data to a aggregating cluster. When the aggregating cluster is not available, the front-clusters write the log data to local filers and send the data when the aggregating cluster recovers. The aggregating cluster is designed to aggregate incoming log data streams from different front-end servers and clusters. The aggregating cluster further sends the aggregated log data stream to centralized NFS filers or a data warehouse cluster. The local filers and the aggregating cluster stage the log data for access by applications, so that the applications do not wait until the data reach the centralized NFS filers or data warehouse cluster. 
         [0011]    In accordance with the techniques introduced here, therefore, a method for staging log data is provided. The method produces log data based on real-time user activities at a plurality of front end servers. The front end servers transmit the log data to a aggregating server. The aggregating server aggregates the log data at the aggregating server. The aggregating server further stages the log data so that a back end server can access the log data in real time. The aggregating server periodically sends the log data to a data warehouse. The data warehouse processes the log data so that the data warehouse can respond to a data query based on the processed log data. 
         [0012]    In accordance with the techniques introduced here, therefore, a computer-implemented system for staging log data is also provided. The computer-implemented system includes a plurality of front end servers and at least one aggregating server. The front end servers are configured for producing log data based on real-time user activities. The aggregating server is configured for aggregating the log data received from at least some of the front end servers. The aggregating server is connected with at least some of the front end servers via a network. The aggregating server includes a data staging area configured for staging the log data so that a back end server can access the log data in real time. 
         [0013]    In accordance with the techniques introduced here, therefore, a aggregating server for staging log data is also provided. The aggregating server includes a processor, a network interface, a data storage and a memory. The network interface is coupled to the process, through which the aggregating server can communicate with a plurality of front end servers. The front end servers produce the log data based on real-time user activities. The data storage includes a data staging area. The memory stores instructions which, when executed by the processor, cause the aggregating server to perform a process including receiving log data from the front end servers, aggregating the log data, and staging the log data at the data staging area so that a back end server can access the log data in real time. 
         [0014]    Other aspects of the technology introduced here will be apparent from the accompanying figures and from the detailed description, which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    These and other objects, features and characteristics of the present invention will become more apparent to those skilled in the art from a study of the following detailed description in conjunction with the appended claims and drawings, all of which form a part of this specification. In the drawings: 
           [0016]      FIG. 1  illustrates an example of online data collection system. 
           [0017]      FIG. 2  illustrates an example of a aggregating cluster capable of splitting log data streams. 
           [0018]      FIG. 3  illustrates an example structure of a log data entry. 
           [0019]      FIG. 4  illustrates another example of online data collection system having multiple layers of aggregating servers. 
           [0020]      FIG. 5  illustrates an example process for aggregating and staging log data. 
           [0021]      FIG. 6  illustrates an example process for staging and splitting log data. 
           [0022]      FIG. 7  is a high-level block diagram showing an example of the architecture of a computer server, which may represent any node or server described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    References in this description to “an embodiment”, “one embodiment”, or the like, mean that the particular feature, function, or characteristic being described is included in at least one embodiment of the present invention. Occurrences of such phrases in this description do not necessarily all refer to the same embodiment, nor are they necessarily mutually exclusive. 
         [0024]      FIG. 1  illustrates an example of online data collection system. The online data collection system  100  includes a plurality of front end clusters  110 . Each front end cluster  110  includes multiple interconnected front end servers  112 . In one embodiment, a front end cluster  110  can includes from 5000 to 30000 front end servers  112 . The front end cluster handles web traffic and produces log data in real time based on the user activities monitored by the online data collection system  100 . In one embodiment, the front end clusters  110  is further responsible for providing user interface to the users of the system  100 , including providing HTTP services. 
         [0025]    The online data collection system  100  further includes a plurality of aggregating clusters  120  responsible for aggregating the log data, i.e. collecting and buffering log data for efficient network data transport. Each of the front end clusters  110  sends a request to at least one of the aggregating clusters  120  to determine whether the aggregating cluster  120  is available. If the aggregating cluster  120  is available, the front end cluster  110  streams the captured log data to the aggregating cluster  120  in real time. Each of the aggregating clusters  120  receives the streaming log data from multiple servers and aggregates the log data. In one embodiment, the aggregating of log data includes reorganizing the log data in terms of combining files and directories. The aggregated log data files do not necessarily have one to one mapping with incoming log data files. The aggregating clusters  120  are designed to scale to a very large number of server nodes and be robust to network and node failure. In one embodiment, the aggregating cluster  120  performs additional processing on the aggregated log data, including compressing the aggregated log data. In another embodiment, the aggregating cluster  120  performs no additional processing on the aggregated log data. 
         [0026]    The aggregating clusters  120  then periodically copy the aggregated log data to a data warehouse  130 , such as a Hive data warehouse implemented on a Hadoop cluster. In some embodiments, the Hive data warehouse can store petabytes of data. In some other embodiments, the aggregating clusters  120  send the aggregated log data to one or more NFS filers. 
         [0027]    In some embodiments, the aggregating clusters  120  can be implemented as a two-level structure. One of the aggregating clusters is designated as a master aggregating cluster. The rest of the aggregating clusters are level  2  aggregating clusters. The master aggregating cluster is responsible for receiving log data from the front end clusters  110 , and distributing the received log data to the level  2  aggregating clusters. The data consuming applications running on the back end servers retrieve the log data in real time from the level  2  aggregating clusters. This two-level structure of aggregating clusters provides a large bandwidth for the back end servers to pull down the log data from them. In some other embodiments, the data consuming applications running on the back end servers retrieve the log data in real time directly from the master aggregating cluster. 
         [0028]    Back end servers  140  can send query requests to the data warehouse  130  for offline data analysis once the data warehouse  130  has received and processed the relevant log data. For faster log data access, each of the aggregating cluster  120  includes a data staging area  122 . The data staging area  122  is an intermediate storage area within the aggregating cluster  120  for temporarily storing (also referred to as parking) the aggregated log data before processing. In one embodiment, the data staging area  122  can expand across servers of the aggregating cluster  120 . The back end servers  140  can directly access the data staging area  122  for real-time or near real-time log data feed. The log data stored in the data staging area  122  may be removed after a predetermined time period. For instance, in one embodiment, the log data is removed 3 days after the log data was captured. In one embodiment, the back end servers  140  are part of the online data collection system  100 . In another embodiment, the back end servers  140  are separated from the online data collection system  100 . 
         [0029]    In some embodiments, the servers  112  in the front end clusters  110  can further include a level one plus staging area  115 . For instance, a front end server  112  in a front end cluster  110  is notified that its corresponding aggregating cluster  120  is unavailable. Instead of streaming the log data to the currently unavailable aggregating cluster  120 , the front end server  112  temporarily stores the log data in the level one plus staging area  115 . The level one plus staging area  115  can be directly accessed by back end servers. In other words, the back end servers  140  can directly stream the log data from the level one plus staging area  115 , without the need of accessing the aggregating clusters  120  or the data warehouse  130 . In some other embodiments, the frond end clusters  110  can further include a level zero staging area that can be directly accessed by servers from any levels within the system, with or without level one plus staging area. The multi-level staging system allows accessing data on any staging level in the system. 
         [0030]    In one embodiment, the online data collection system can prioritize the log data by dividing the data into multiple priority tiers. For instance, the top tier log data will have higher priority to be transmitted to data warehouse and back end servers. The top tier log data can also have longer retention time when the data is parking in the staging area. Lower tier log data will be deleted earlier after a shorter retention time period than the top tier log data. In some embodiments, the instructions on setting priority can be implemented in a policy system. When a data consuming application is submitted, a policy can be specified on the bandwidth as well as the priority tier for the category of data that it requests. The policy can further specify whether the requested log data is to be transmitted directly to the back end servers that the application runs in real time from a staged area, whether the requested log data is to be transmitted from the data warehouse, or whether the requested log data is to be staged on level one plus staging area or a staging area in the aggregating servers. 
         [0031]    To accelerate the processing of the log data, the aggregating cluster can split the log data into multiple log data streams so that the log data streams are sent in parallel to a data consuming application running on back end servers.  FIG. 2  illustrates an example of an aggregating cluster capable of splitting log data streams. The aggregating cluster  210  splits the log data by examining the entries of the log data to ensure the log data evenly and randomly distributed in the split log data streams  212  (also referred to as buckets).  FIG. 3  illustrates an example structure of a log data entry. The entry  300  of the log data includes an application ID  310 , which identifies the data consuming application that is going to consume and process that log data entry. In one embodiment, the application ID  310  is a developer ID. The entry  300  further includes a category  320 . The category  320  is a high level description of the intended destination of the message. The entry  300  further includes a message  330 . The message  330  includes actual content of the log data entry. 
         [0032]    In one embodiment, the aggregating server reads the application ID and category, and then calculate a hash function of the application ID and category. For instance, the hash function can be MurmurHash3 hash function which yields a 32-bit or 128-bit hash value. The aggregating server further mod the value of the hash function by the total number of buckets. The result is the bucket number that the log data entry is assigned. In other words, the log data entry is going to be included in the log data stream having that bucket number. The hash function reshuffles the entries of the log data such that the entries are evenly and randomly distributed among the buckets (i.e. the split log data streams). The assignment of the bucket numbers to the entries has no correlation in time when the entries are created or with application IDs of the entries. In some embodiments, the bucket number can be a function of data fields of the log data entries besides the application ID. For instance, the hash function can use the application ID and another data field of the log data entries to generate the hash value for determining bucket numbers. 
         [0033]    The total number of buckets can be determined in various ways. In one embodiment, the total number of back end servers is determined by the estimated total data volume and data bandwidth that each back end server can handle. The number of buckets is determined as a number larger than the number of back end servers so that the system can scale up to include more back end servers. For instance, if the number of buckets is 1024, that means the system can scale up to 1024 back end servers without the need of making changes in the stream splitting. A plurality of back end servers can be utilized to process the split data streams in parallel in real time. The back end servers are equally loaded because the streams are evenly split. In another embodiment, when submitting the data consuming application, a policy can be specified for the data consuming application including the number of buckets for parallel processing the log data streams. 
         [0034]    In another embodiment, the bucket number assigned to each log data entry is determined by random numbers. The aggregating server generates a random integer within the range of the total number of bucket and assigns that generated integer to a log data entry as the assigned bucket number. 
         [0035]    In yet another embodiment, a data consuming application can specify the function or the way for assigning the bucket number. For instance, a data consuming application can assign the bucket numbers based on the certain IDs of the entries. The data consuming application can assign a first range of user IDs of the entries to a first bucket number; and assign a second range of user IDs of the entries to a second bucket number, etc. 
         [0036]    In one embodiment, the online data collection system can have more than one layer of middle servers similar to the aggregating servers in  FIG. 1 .  FIG. 4  illustrates another example of online data collection system having multiple layers of aggregating servers. The online data collection system  400  includes a plurality of front end clusters  410 . Each front end cluster  410  includes multiple interconnected front end servers  412 . The front end cluster s 410  produce log data in real time based on the user activities monitored by the online data collection system  100 . In one embodiment, the servers  412  in the front end clusters  110  can further include a level one plus staging area  415  that can be directly accessed by back end servers  450 . 
         [0037]    The online data collection system  400  further includes a plurality of first level aggregating clusters  420  responsible for aggregating the log data transmitted from the front end clusters  410 . Furthermore, the online data collection system  400  includes one or more second level aggregating clusters  430  responsible for aggregating the log data transmitted from the first level aggregating cluster  420 . The second level aggregating clusters  430  then periodically copy the aggregated log data to a data warehouse  440 , such as a Hive data warehouse implemented on a Hadoop cluster. 
         [0038]    The first level aggregating cluster  420  includes first level data staging area  422  and the second level aggregating cluster  430  includes second level data staging area  432 . Back end servers  450  can retrieve log data directly from the first level data staging area  422  or the second level data staging area  432 . In one embodiment, a back end server  450  can decide whether it retrieves log data from the first level staging area or the second level staging area based on the network topology. For instance, if the back end server  450  is closer to the second level aggregating cluster  430  than the first level aggregating cluster  420  in terms of network topology, the back end server  450  decides to retrieve log data from the second level data staging area  432 . 
         [0039]    For example, in one example, the log data are user activity data and the data consuming application is a newsfeed application. Instead of waiting for the offline log data available on the data warehouse after a time period, the newsfeed application can request to retrieve the relevant user activity log data in real time from either the first or second level staging area, depending which aggregating cluster is closer to the newsfeed application&#39;s back end servers in the network topology. The newsfeed application pulls the user activity log data with very low latency and is able to process the log data and populate the newsfeed result as soon as possible. 
         [0040]    In one embodiment, an online data collection system with multiple levels of aggregating clusters include front end clusters with level one plus staging capabilities. When an aggregating cluster is unavailable, the front end clusters can use its level one plus staging capabilities to continue feed log data to back end servers. In another embodiment, an online data collection system can even include more than two levels of aggregating clusters with data staging capabilities. 
         [0041]    Such an online data collection system having multi-level staging capability can provide log data in real time at each stage of log data aggregation. The system can supply low latency data feed in response to data consuming queries in each stage, when the log data is not available from the central data repository (e.g. data warehouse) yet. The back end servers can choose a closer cluster for retrieving the stage log data in terms of the network topology. The back end servers&#39; data streams travel through less nodes and systems in the network; thus, there is less chance of losing log data or corrupting the log data during the transmission. Therefore, the multi-level staging system enhances the durability and reliability of the log data transmission. 
         [0042]      FIG. 5  illustrates an example process for aggregating and staging log data. At step  510 , front end servers produce log data based on real-time user activities. In one embodiment, the front end servers further provide web content to users. In one embodiment, the front end servers divide the log data a plurality of priority tiers, and only transmit the log data in a top tier of the priority tiers to an aggregating server. 
         [0043]    At step  514 , the front end servers check whether an aggregating server is available. If the aggregating server is unavailable, the front end servers stage the log data at a level one plus staging area in at least one of the plurality of front end servers at step  516 . At step  518 , the front end servers feed the log data from the level one plus staging area to a back end server in real time if the back end server requests. If the aggregating server is available, at step  520 , the front end servers transmit the log data to the aggregating server. 
         [0044]    At step  530 , the aggregating server aggregates the log data. In one embodiment, the aggregating includes reorganizing the log data by combining streams of the log data at the aggregating server. In another embodiment, the aggregating server further compresses the log data at the aggregating server. 
         [0045]    At step  540 , the aggregating server stages the log data so that a back end server can access the log data in real time. In one embodiment, one back end server sends an instruction to the aggregating server and the front end servers regarding the data staging location for the log data. Then the aggregating server and the front end servers determines whether to stage the log data according to the instruction. At step  550 , the aggregating server feeds the log data from the aggregating server directly to the back end server in real time. 
         [0046]    At step  560 , the aggregating server sends the log data to a data warehouse. At step  570 , the data warehouse processes the log data so that the data warehouse can respond to a data query based on the processed log data. 
         [0047]    At step  580 , the aggregating server removes the log data from the aggregating server after a predetermined time period. If the front end servers stages the log data at the level one plus staging area, the front end servers also remove the log data from the level one plus staging are after a predetermined time period. 
         [0048]      FIG. 6  illustrates an example process for staging and splitting log data. At step  610 , front end servers produce log data based on real-time user activities. In one embodiment, the front end servers further provide web content to users. 
         [0049]    At step  614 , the front end servers check whether an aggregating server is available. If the aggregating server is unavailable, the front end servers stage the log data at a level one plus staging area in at least one of the plurality of front end servers at step  616 . 
         [0050]    At step  618 , the front end servers feed the log data from the level one plus staging area to a back end server in real time if the back end server requests. If the aggregating server is available, at step  620 , the front end servers transmit the log data to the aggregating server. 
         [0051]    At step  630 , the aggregating server aggregates the log data. In one embodiment, the aggregating includes reorganizing the log data by combining streams of the log data at the aggregating server. In another embodiment, the aggregating server further compresses the log data at the aggregating server. 
         [0052]    At step  640 , the aggregating server stages the log data so that a back end server can access the log data in real time. 
         [0053]    In one embodiment, the log data includes a plurality of log data entries. Each log data entry includes an application identification and a category field. The category field includes a high level description of an intended destination of the log data entry; the application identification identifies a data consuming application for processing the log data entry. The aggregate server can examines prefixes of entries of the log data to recognize the application identification and category field. 
         [0054]    In one embodiment, at step  650 , for each entry of the log data, the aggregating server calculates a bucket number by a hash function of the application identification and the category field modulo a total number of buckets. The total number of buckets is a total number of the plurality of log data streams. In one embodiment, the total number of buckets is determined by a number of back end servers that are available to receive the log data streams and a number of connections that each back end server is capable of handling. In another embodiment, the total number of buckets is instructed by a data consuming application running on at least one back end server. The purpose is that the back end servers are equally loaded when the back end servers receive and process the log data streams. At step  652 , the aggregating server assigns that entry of the log data to a log data stream identified by the bucket number. 
         [0055]    In another embodiment, for each log data entry, the aggregating server randomly generates an integer from 1 to a total number of buckets. Then the aggregating server assigns that log data entry to a log data stream identified by the bucket number. 
         [0056]    At step  654 , the aggregating server split the log data into a plurality of log data streams. In one embodiment, the aggregating server receives an instruction from a back end server regarding how to split the log data into the plurality of log data streams. In one embodiment, the front end servers can also split the log data into a plurality of log data streams, in ways similar to the aggregating server splitting the log data. At step  660 , the aggregating server feeds the log data streams to one or more back end servers in parallel. 
         [0057]    At step  670 , the aggregating server sends the log data to a data warehouse. At step  680 , the data warehouse processes the log data so that the data warehouse can respond to a data query based on the processed log data. 
         [0058]    At step  690 , the aggregating server removes the log data from the aggregating server after a predetermined time period. If the front end servers stages the log data at the level one plus staging area, the front end servers also remove the log data from the level one plus staging are after a predetermined time period. 
         [0059]    In one embodiment, after the back end servers receive the split log data streams, the back end servers consolidate the split log data streams. Assuming the log data before splitting belong to a plurality of streams, each of the streams is assigned to one of the back end servers. When any of the back end servers receives a message of a split log data streams from the aggregating servers, it determines which back end server is assigned for the stream that the message belongs to. Then the back end server forwards the message to the assigned back end server (also referred to as the owner). In one embodiment, the back end servers consolidate the log data for one category into a queue. The queue can be implemented in various ways. For instance, the queue can be implemented in a naming convention analogous to files in a directory wherein the last file is the only potentially open file. Another way to implement the queue is to utilize a 2-layer system. 
         [0060]    In some embodiments, in order to record the stream-to-server assignment information, the back end servers maintain a distribution map for the assignment information. The distribution map is updated when there is change on the assignment information, for example, when a server is unavailable, or when a new stream is added. The back end servers gets notification of these changes in a low latency manner. 
         [0061]    This stream-to-server assignment mechanism provides an automatic and dynamic way for managing the relationship between the streams and back end servers. When one of the back end servers fails, the remaining back end servers can redistribute the ownership of the streams that were assigned to the failed back end server. By transferring the ownership of some streams from some back end servers to other back end servers, this mechanism can balance the work loads of the back end servers. 
         [0062]      FIG. 7  is a high-level block diagram showing an example of the architecture of a server  700 , which may represent any of the front end servers, aggregating servers, data warehouse servers, and back end servers. The server  700  includes one or more processors  710  and memory  720  coupled to an interconnect  730 . The interconnect  730  shown in  FIG. 7  is an abstraction that represents any one or more separate physical buses, point to point connections, or both connected by appropriate bridges, adapters, or controllers. The interconnect  730 , therefore, may include, for example, a system bus, a Peripheral Component Interconnect (PCI) bus or PCI-Express bus, a HyperTransport or industry standard architecture (ISA) bus, a small computer system interface (SCSI) bus, a universal serial bus (USB), IIC (I2C) bus, or an Institute of Electrical and Electronics Engineers (IEEE) standard 1394 bus, also called “Firewire”. 
         [0063]    The processor(s)  710  is/are the central processing unit (CPU) of the server  700  and, thus, control the overall operation of the server  700 . In certain embodiments, the processor(s)  710  accomplish this by executing software or firmware stored in memory  720 . The processor(s)  710  may be, or may include, one or more programmable general-purpose or special-purpose microprocessors, digital signal processors (DSPs), programmable controllers, application specific integrated circuits (ASICs), programmable logic devices (PLDs), trusted platform modules (TPMs), or the like, or a combination of such devices. 
         [0064]    The memory  720  is or includes the main memory of the server  700 . The memory  720  represents any form of random access memory (RAM), read-only memory (ROM), flash memory, or the like, or a combination of such devices. In use, the memory  720  may contain, among other things, code  770  embodying at least a portion of an operating system of the server  700 . Code  770  may also include instructions for executing the techniques disclosed herein. 
         [0065]    Also connected to the processor(s)  710  through the interconnect  730  are a network adapter  740  and a storage adapter  750 . The network adapter  740  provides the server  700  with the ability to communicate with devices, such as other front end servers, consolidating servers, data warehouse servers, or back end servers, over a network and may be, for example, an Ethernet adapter or Fibre Channel adapter. In some embodiments, a server may use more than one network adapter to deal with the communications within and outside of the data storage cluster separately. The storage adapter  750  allows the server  700  to access a persistent storage, and may be, for example, a Fibre Channel adapter or SCSI adapter. 
         [0066]    The code  770  stored in memory  720  may be implemented as software and/or firmware to program the processor(s)  710  to carry out actions described below. In certain embodiments, such software or firmware may be initially provided to the server  700  by downloading it from a system through the server  700  (e.g., via network adapter  740 ). 
         [0067]    The techniques introduced herein can be implemented by, for example, programmable circuitry (e.g., one or more microprocessors) programmed with software and/or firmware, or entirely in special-purpose hardwired circuitry, or in a combination of such forms. Special-purpose hardwired circuitry may be in the form of, for example, one or more application-specific integrated circuits (ASICs), programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), etc. 
         [0068]    Software or firmware for use in implementing the techniques introduced here may be stored on a machine-readable storage medium and may be executed by one or more general-purpose or special-purpose programmable microprocessors. A “machine-readable storage medium”, as the term is used herein, includes any mechanism that can store information in a form accessible by a machine (a machine may be, for example, a computer, network device, cellular phone, personal digital assistant (PDA), manufacturing tool, any device with one or more processors, etc.). For example, a machine-accessible storage medium includes recordable/non-recordable media (e.g., read-only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; etc.), etc. 
         [0069]    The term “logic”, as used herein, can include, for example, programmable circuitry programmed with specific software and/or firmware, special-purpose hardwired circuitry, or a combination thereof. 
         [0070]    In addition to the above mentioned examples, various other modifications and alterations of the invention may be made without departing from the invention. Accordingly, the above disclosure is not to be considered as limiting and the appended claims are to be interpreted as encompassing the true spirit and the entire scope of the invention.