Patent Publication Number: US-9847907-B2

Title: Distributed caching cluster management

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to and incorporates by reference for all purposes the full disclosure of co-pending U.S. patent application Ser. No. 13/685,596, filed Nov. 26, 2012, entitled “DISTRIBUTED CACHING CLUSTER CONFIGURATION”, co-pending U.S. patent application Ser. No. 13/685,607, filed Nov. 26, 2012, entitled “DISTRIBUTED CACHING CLUSTER CLIENT CONFIGURATION” and co-pending U.S. patent application Ser. No. 13/685,620, filed Nov. 26, 2012, entitled “REPLICATION IN DISTRIBUTED CACHING CLUSTER”. 
     BACKGROUND 
     Data centers provide computing resources for use by one or more clients. These services may include computing, storage and networking services. For example, a data center may provide a machine to host an application, storage to store application data, cache to quickly respond to repeated data requests and networking to enable communication between resources. By making use of the data center services, a customer may pay for computing and/or resource use rather than purchasing anticipated hardware needs. This enables a customer to expand and contract use of computing services according to demand. For example, an application may be configured to request more storage as needed rather than a developer or administrator monitoring and anticipating use. 
     On demand systems may be used to reduce the number of times a database must be read by caching data and objects from the database. For example, one implementation uses a client centered architecture where a client knows the servers, but the servers are not known to each other. To read or set a key, the client uses a hash to determine which server to contact and contacts that server. The server then calculates a second hash to determine where to store or read the corresponding value. Additions or subtractions to the group of servers are managed by the client. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments in accordance with the present disclosure will be described with reference to the drawings, in which: 
         FIG. 1  shows an illustrative example of cluster caching management in accordance with at least one embodiment; 
         FIG. 2  shows an illustrative example of a cluster caching management in accordance with at least one embodiment; 
         FIG. 3  shows an illustrative example of an environment in accordance with at least one embodiment; 
         FIG. 4  shows an illustrative example of an embodiment of cluster caching management; 
         FIG. 5  shows an illustrative example of logical connections between components in accordance with at least one embodiment; 
         FIG. 6  shows an illustrative chart of a process in accordance with at least one embodiment; 
         FIG. 7  shows an illustrative example of a process in accordance with at least one embodiment; 
         FIG. 8  shows an illustrative example of a process in accordance with at least one embodiment; 
         FIG. 9  shows an illustrative example of a process in accordance with at least one embodiment; 
         FIG. 10  shows an illustrative example of a process in accordance with at least one embodiment; 
         FIG. 11  shows an illustrative example of hashing in accordance with at least one embodiment; 
         FIG. 12  shows an illustrative example of hashing in accordance with at least one embodiment; and 
         FIG. 13  illustrates an environment in which various embodiments can be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, various embodiments will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the embodiments. However, it will also be apparent to one skilled in the art that the embodiments may be practiced without the specific details. Furthermore, well-known features may be omitted or simplified in order not to obscure the embodiment being described. 
     Techniques described and suggested herein include enabling and monitoring of a cache cluster to make the cache cluster configuration-aware such that initialization and changes to the underlying structure of the cache cluster can be dynamically updated. For example, a cache cluster may comprise a set of one or more memory caching nodes. A configuration may be an indication of, among other things, the number of memory caching nodes in the cache cluster and the location of the memory caching nodes. The configuration may be maintained, in one embodiment, at each of the set of one or more memory caching nodes. As such, each memory caching node may be aware of the configuration of every memory caching node in the cache cluster. A memory caching node storing the configuration may be referenced by an alias in a configuration endpoint. When a client is first attempting to connect to the cache cluster, the client may resolve an alias from a static configuration endpoint to a memory caching node in a cache cluster. The client may request initial configuration from the memory caching node. As additional requests are made to the cache cluster, a first memory caching node may change in one or more ways (such as being added or removed from the cache cluster). As such, the configuration may be updated and sent to each of the set of one or more memory caching nodes in the cache cluster. A management system monitors these changes to the cache cluster, the changes including, for example, provisioning of new memory caching nodes or replacement of failing memory caching nodes. The management system may also update the configuration stored in memory caching nodes. The client may then obtain the updated configuration directly from a memory caching node that has the current configuration of the cache cluster. As such, updated configurations are easily obtained by a client without having to resolve a memory caching node via the configuration endpoint after a node has been resolved. The management system may include one or more computing resources that manage the cache cluster. It should be noted that a plurality of memory caching nodes may grouped together in one or more ways to create a cache cluster. 
     A cache cluster may cache data and object to reduce accesses to a data source. The cache cluster may include one or more memory caching nodes. Each node may store a portion of a set of cached data. The cached data may be split between memory caching nodes based on keys that are used to retrieve an element of data from the cache cluster. Data sources may include a database, application programming interface (API) or other data store or data source. The memory caching nodes may use computer memory as storage for cache, such as RAM, to speed up responses to requests. 
     In one illustrative example, a cache cluster is managed by a management system. The management system may determine, store, and/or maintain configurations of one or more cache clusters. In addition, the management system may be logically connected to distributed computing resources via a network. The management system may be configured to provision the computing resources as memory caching nodes. The management system may also be configured to associate the provisioned memory caching nodes with any of the one or more cache clusters. In an embodiment, the management system may also be configured to wind down a memory caching node and to remove the computing resources from the cache cluster to a general pool of distributed computing resources. When a cache cluster is updated, the configuration of the cache cluster may be updated in the management system and sent to one or more of the memory caching nodes. As such, each memory caching node may comprise a current version of the configuration of the cache cluster. 
     As one example, a cache cluster may have four memory caching nodes servicing a web application making requests to a database. Because of heavy traffic, a fifth memory caching node may be brought on line to aid in the servicing of requests between the web application and the database. The memory caching node may be brought online by, as one example, a management system which may provision computing resources for the fifth node and cause the fifth node to spin up as a portion of the cache cluster. After bringing the fifth memory caching node online, the management system may update the configuration for the cache cluster to incorporate the fifth memory caching node. The updated configuration may be pushed to one or more of the memory caching nodes for storage and retrieval. In one embodiment, the clients may receive the updated configuration from a memory caching node without contacting the configuration endpoint. In another embodiment, a static configuration endpoint may receive an update to an alias referencing a memory caching node that comprises the updated configuration. Clients of the cache cluster may receive the alias from the configuration endpoint and receive the updated configuration to minimize caching disturbances. 
     Configurations may be delivered to clients in a plurality of ways. In one embodiment, a pre-defined configuration endpoint can be queried by a client driver to resolve an address for a memory caching node from which to request a current configuration. The configuration endpoint may be statically declared such that support software, including a client driver for the client system may be able to resolve an alias provided by the configuration endpoint to a memory caching node from which a configuration may be obtained. The configuration can describe how to access memory caching resources of the cache cluster, including the memory caching nodes. This static declaration of an alias allows for new clients to start, resolve a memory caching node, retrieve a configuration and self-configure to current cache cluster conditions rather than manually configure a new client before starting the new client. In another embodiment, the memory caching nodes can also contain reserved space for storing a configuration which describes the structure of the cache cluster. While normal caching of data can cause data to expire, a section of storage within control of the memory caching node may be partitioned such that configuration may be stored in such reserved configuration space without worry of deletion. 
     A client already in communication with a memory caching node to may also request a current configuration, if newer than the current configuration from a memory caching node. In one embodiment, the request is made if the configuration stored in the memory caching node has been updated from the current configuration that the client has. In another embodiment, the client may request configurations on a periodic basis or random time basis from one or more memory caching nodes. In another embodiment, instead of responding to a request from a client, new configurations may be pushed to the client by the memory caching node upon the event of a configuration change or any other change to the cache cluster. Further still, the configuration of the cache cluster may be sent out to the client by the memory caching node based on time intervals, such as a periodic, heartbeat, expiration, or random time basis. In an additional embodiment, configurations may be pushed to a client when a client makes a request to access the cache cluster. By providing the current configuration, memory caching nodes may be altered with the expectation that clients will update upon access of a memory caching node. 
     The term provisioning is meant to be read broadly to include the assignment of a computing resource to a use. In some embodiments, this includes preparing the computing resource for use. In one embodiment, a provisioning of a memory caching node would include the assignment of a server, installation of an operating system, installation and configuration of the memory caching node and enabling the memory caching node for use. For example, a provisioning system may select a server to use as a memory caching node. The provisioning system may then create a workflow that prepares the server for use as a memory caching node. As part of the workflow, a machine image may be loaded on the server. The machine image may include operation system, memory caching software and/or settings. After loading the machine image, the server may be caused to boot into the operating system and receive any further software and/or settings. Such settings may include cache cluster configuration. After provisioning is complete, the server may be turned over to a management system for use as a memory caching node. 
     Turning now to  FIG. 1 , an illustrative example of a distributed memory caching system  100  in accordance with at least one embodiment is shown. A management system  114  may include one or more computing resources that manage a cache cluster  113  that provides cached information to clients from cache  112 . The management system  114  may monitor and adjust the cache cluster  113  based at least in part on service needs and/or failures. A caching service  100  may include a configuration endpoint  106  that comprises an alias for one or more memory caching nodes  110  that form a cache cluster  113  and each memory caching node  110  may serve cached data from a cache  112  and configuration  108 . A cache cluster  113  may comprise a plurality of caching nodes  110 . In one embodiment, a cache cluster  113  may include multiple memory caching nodes  110 . The memory caching nodes  110  may each comprise a virtual machine executing on a computing device such as a server. In another embodiment, a cache cluster  113  may include multiple memory caching nodes  110  executing directly on hardware. As such, it should be recognized that the programs and processes referenced herein may be run on actual hardware or virtual machines. 
     In one embodiment, the management system  114  may include a monitoring system that monitors the cache cluster  113  and a provisioning system that adjusts the cache cluster  113 . The monitoring may include recording measurements and/or determining indicators relating to the input, output and status of components of the cache cluster  113  and/or the caching service  100  as a whole. The provisioning system may provide such services as provisioning new memory caching nodes, de-provisioning current memory caching nodes and determining new configuration  108  based on the changes performed on the caching cluster  113 . For example, a monitoring system may detect that a memory caching node  110  is failing and communicate the problem to the provisioning system. The provisioning system may cause a workflow to be created that provisions a new memory caching node  110 , removes the failing memory caching node  110  and updates configuration  108 . The provisioning system may then cause the workflow to be executed such that a new memory caching node takes the place of the failing memory caching node, the configuration endpoint is updated with a valid alias and one or more of the memory caching nodes  110  receive the new configuration  108 . In one embodiment, the provisioning system need only communicate the configuration to one of the memory caching nodes  110 , as the memory caching nodes  110  will then propagate the information between them, such as in a peer-to-peer fashion. 
     The memory caching node  110  may also provide storage for a configuration  108 , the configuration  108  may detail communication parameters with the cache cluster  113 . In some embodiments, this configuration  108  may be stored in a reserved configuration space that is not subject to cache expiration rules. In one embodiment, the configuration is stored in a cache  112 , and a client or management system  114  ensures frequent enough requests and/or updates to ensure the configuration  108  is available. The configuration  108  may be overwritten and/or updated to keep current with changes to the cache cluster  113 . 
     A configuration endpoint  106  may also be provided to aid in configuration  108  retrieval. In some embodiments, a configuration endpoint  106  is a static resource that may be directly referenced as an alias to a memory caching node for configuration  108  retrieval. For example, new clients may be initialized with a hostname of the configuration endpoint  106 . Upon instantiation of a new client, the client may connect with the configuration endpoint  106 , resolve an alias to a memory caching node and retrieve the configuration  108  from the memory caching node. By providing the configuration endpoint  106 , clients may self-configure rather than start with a list of memory caching nodes that may become obsolete and require maintenance. In some embodiments, a client may continue to retrieve a more current configuration  108  by requesting the configuration  108  from a memory caching node referenced in the alias provided by the configuration endpoint  108  or a memory caching node  110 . In one embodiment the configuration endpoint is implemented by domain name system (DNS) services. A client may request a static hostname from the domain name server and receive an IP address that resolves to a memory caching node. 
     Turning now to  FIG. 2 , an illustrative example of a distributed memory caching system  200  in accordance with at least one embodiment is shown. A client system  203  uses a client driver  204  to retrieve information from a cache cluster  213  managed by a management system  214 . In the embodiment shown, a client system  203  includes an application  202  that retrieves data from the cache cluster  213 . The client system  203  uses a client driver  204  to manage the interface between the application  202  and the cache cluster  213 . For example, the application  202  may be a shopping website and the client driver  204  may be a library that exposes the caching functionality through function calls and/or an application programming interface (API). 
     The client driver  204  may manage the communication with the cache cluster  213 . In one embodiment, the client driver  204  supports automatic configuration. An initial configuration of the client driver  204  may be small, such as a hostname of a configuration endpoint  206  that provides an alias at which configuration  208  may be obtained. In one embodiment, the alias is provided as part of the configuration endpoint acting as a domain name server. The configuration  208  may include information needed for the client driver  204  to connect to and use the cache cluster  213 . For example, an application  202  may provide a hostname and/or address of a configuration endpoint  206  to the client driver  204  as part of initializing the client driver  204 . Using the given hostname and/or address, the client driver  204  contacts the configuration endpoint  206  to resolve an alias to a memory caching node  210  and requests a configuration  208  from the memory caching node  210 . In one embodiment, this configuration is stored in a reserved memory space of the memory caching node  210  that is not subject to certain cache rules of a protocol implemented by the cache node (such as the memcached protocol and variations thereof), such as eviction. Information in the reserved memory space may be accessed according to an extension to a standard caching protocol, such as memcached. Upon receiving the configuration  208 , the client driver  204  may load the configuration  208 . Once loaded, the client driver  204  may verify the configuration  208 . In one embodiment, the client driver  204  contacts one or more memory caching nodes  210  and verifies the version of the configuration  208  against a configuration version contained in a second memory caching node  210 . The client driver  204  may use the most recent configuration  208  discovered. The client driver  204  may then act upon requests for data from the application  202  by requesting the data from one or more memory caching nodes  210  that store the data in their cache  212 . Periodically, the client driver  204  may check the loaded configuration version against configuration versions stored by the memory caching nodes  210 . The client driver  204  may elect to use the newest configuration discovered, which may be the current version loaded in the client driver  204 . By loading the configuration  208 , the client driver can react to dynamic changes in the cache cluster  213 . For example, the configuration  208  may identify memory caching nodes  210  that are added or removed from the cache cluster  213 . By loading the configuration, the client driver  204  may react to any changes in the cache cluster  213  infrastructure without instruction from the application  202 . 
     Loading the configuration may synchronize, with other computing systems, a client driver&#39;s information about the cache cluster. In one embodiment, several client drivers  204  exist at the same time to serve multiple instances of a web application, each on its own server. Synchronizing a configuration of the cache cluster  213  allows each client driver  204  to properly populate and request information from memory caching node  210  that form the cache cluster  213 . For examples of populating and cache requests of memory caching nodes, see  FIGS. 11-12  and the associated description. 
     The client driver  204  and caching nodes  210  may communicate using a standard protocol, such as a memcached protocol, and extensions to the protocol. For example, caching operations may use the standard protocol, while configuration operations may use extensions to the protocol, such as additions to a command set of the protocol. In some embodiments, the extension operations operable on the reserved configuration storage may include create, retrieve, update and destroy operations. Other extension operations may include a get configuration version operation, other metadata manipulation operations and a propagate configuration request. 
     A management system  214  may be one or more computing resources responsible for management of other systems. In  FIG. 2 , the management system  214  is responsible for the distributed memory caching system  200 , including the provisioning and monitoring of memory caching nodes  210  in the cache cluster  213 . The management system  214  may also receive instructions from customers and/or administrators such that the management of the management system  214  fits the needs of the customer and/or administrator. For example, a management system  214  may be responsible for a set of memory caching nodes  210  that form the cache cluster  213 . The management system  214 , through a provisioning system as an example, may cause new memory caching nodes  210  to be instantiated or current memory caching nodes  210  to be stopped. The management system  214  may also be responsible for monitoring the cache cluster  213 , which may include monitoring the set of memory caching nodes  210  for indicators. The indicators may include usage, failure or other information about the use and/or underlying systems. A configuration endpoint  206  may also be maintained by the management system  214  to ensure that an alias to an active memory caching node  210  that can provide configuration  208  is always available. 
     In one embodiment, the management system  214  may use a monitoring system and react to perceived problems with the caching service  200 . For example, if a failure occurs in a failed memory caching node  210 , the failed memory caching node may be de-provisioned and removed from the cache cluster  213 . A new memory caching node  210  may be provisioned to replace the failed memory caching node and recover from loss of the failed memory caching node. In other examples, the failed memory caching node may be repaired by replacing, reinitializing and recovering the memory caching node. Using the changes made to the cache cluster  213 , the management system  214  may update the configuration  208  and cause the updated configuration  208  to be stored in each memory caching node  210 . If needed, the alias provided by the configuration endpoint  206  may also be updated. In another example, the management system  214  provisions a new memory caching node  210  due to an increase in demand for cached data. The management system  214  may update the configuration  208  with the connection information to the new memory caching node  210  and cause the configuration  208  to be stored in memory caching nodes  210 . 
     A configuration may include information necessary to connect to the cache cluster  213 . In some embodiments that use a direct connecting configuration, this may include information to directly connect to each memory caching node  210 . In other embodiments using a request forwarding configuration, the configuration  208  may identify a memory caching node  210  responsible for the forwarding of requests to a memory caching node  210  that holds the data in cache  212 . In one embodiment, a hybrid approach may be taken where direct connection and request forwarding are both available. 
     Turning now to  FIG. 3 , an illustrative example of a distributed memory caching environment  300  in accordance with at least one embodiment is shown. The client application  309  ( 202  in  FIG. 2 ), management system  312  ( 214  in  FIG. 2 ) and configuration endpoint  308  ( 206  in  FIG. 2 ) may exist in the context of a data center. The computers may be divided into trusted computing resources within the data center  304  and untrusted external computing systems  316 ,  318 ,  320 , sometimes referred to as application clients  322 , outside the data center  304 . Inside the data center  304 , computing resources and networking  306  may be under the domain and control of known administrators and thus have trusted internal connections. Outside of the data center  304  may be beyond the control of administrators, and therefore untrusted, such as the Internet  305 . 
     Inside the data center  304  may be memory caching nodes  302 , internal networking  306 , a management system  312 , a gateway  310 , a configuration endpoint  308  and a client application  309 . An memory caching node  302  may be connected to other memory caching nodes  302  through internal networking  306 . The memory caching nodes  302  may also be connected with a management system  312 . The management system  312  may receive requests to manipulate computing resources, including provisioning resources and changing routing. The memory caching nodes  302  and management system  312  may also be connected with a gateway  310 . The gateway  310  may filter and route external traffic to a client application  309 , such as HTTP traffic to Web servers. For example, a client application  309  may communicate with external systems  316 ,  318 ,  320 , but memory caching nodes  302  are not allowed external communications. 
     Outside the data center  304  may be any of a number of different components or environments, and may include the Internet  305  and various external computing systems such as desktops  316 , laptops  318  and mobile devices  320 , such as electronic book readers, mobile phones, tablet computing devices, etc. The systems  316 ,  318 ,  320  may be viewed as untrusted because the systems  316 ,  318 ,  320  may not be administered by a trusted administrator. Further, the communication channels, such as the Internet, are not controlled by a trusted administrator. Thus, a message from an external computing system  316 ,  318 ,  320  may be intercepted, counterfeited and/or exploited. 
     In some cases, and for protective reasons, client applications  309  on a secure internal network  306  may only be given the Internet  305  access required to operate, if any at all. For example, a Web server in a data center  304  may only receive outside traffic on port  80  because a gateway  310  provides access controls to the secure internal network that prevent all other Internet  305  traffic from directly reaching the Web server. In another example, a memory caching node  302  on a secure internal network  306  may not be connected to the Internet  305  because it is only queried by a local Web server over the secure internal network. In other embodiments, a client application  309  may be behind a load balancer, which may occasionally direct Internet  305  requests to the client application  309 . 
     Turning now to  FIG. 4 , an illustrative example of a distributed memory caching management environment  400  in accordance with at least one embodiment is shown. A management system  416  may monitor and/or manage memory caching node  402 . Memory caching node  402  may manage cached key-value pairs  410 , respond to requests to provide cached values (from the key-value pairs  410 ) and provide a configuration  412  identifying how to communicate with the cache cluster  413  and/or each memory caching node  402 . Key value pairs  410  may be inserted into a cache of the memory caching node  402  when read and/or changed from a data store  408 . The cache cluster  413  allows potentially quicker responses to frequently accessed and/or high access cost data than requesting data directly from the data store  408 . 
     A memory caching node  402  may be provided that includes cache space  404  and reserved memory space  406 . The memory caching node  402  may be serviced by virtual and/or physical hardware, including a virtual machine. The memory caching node may receive key/values pairs  410  to store within the cache space  404 . The key-values  410  may have an expiration time, as well as early expiration depending on whether the memory caching node  402  runs out of cache space  404 . The memory caching node  402  may use an algorithm to determine which key-value pairs  410  may be expired early. In some embodiments, a least-frequently used algorithm is used to determine which items are expired early when a cache is full. In other embodiments, a cost of querying the data store may be factored in. In one embodiment, the expiration may be based on which key-value pairs  410  are not expected to be frequently accessed in the future. The memory caching node  402  may also provide storage for a configuration  412  detailing communication parameters with the cache cluster  413 . In some embodiments, this configuration  412  may be stored in a reserved memory space  406  that is not subject to expiration. In one embodiment, the configuration  412  is stored in the cache space  404 , but a client or management system  416  ensures frequent enough requests and/or updates to ensure the configuration  412  is available. The configuration  412  may be overwritten and/or updated to keep current with changes to the cache cluster  413 . 
     A configuration endpoint  414  may also be provided to aid in configuration  412  retrieval. In some embodiments, a configuration endpoint  414  is a static resource that may be directly referenced as an alias to a memory caching node for configuration  412  retrieval. For example, new clients may be initialized with a hostname of the configuration endpoint  414 . Upon instantiation of a new client, the client may connect with the configuration endpoint  414  resolve an alias to a memory caching node  402  and retrieve the configuration  412  from the memory caching node  402 . By providing the configuration endpoint  414 , clients may self-configure rather than start with a list of memory caching nodes  402  that may become obsolete and require maintenance. In some embodiments, a client may continue to retrieve a more current configuration  412  by requesting the configuration  412  from a memory caching node  402  referenced in the alias provided by the configuration endpoint  414  or a memory caching node  402  directly. 
     In one embodiment, the management system  416  assumes responsibility for the configuration  412 . In another embodiment, memory caching nodes  402  may be cluster-aware such that, as new memory caching nodes  402  are detected, they may be added to the configuration  412 . In another embodiment, the management system  416  may store an updated configuration  412  in an identified memory caching node  402 , such as a memory caching node  402  identified by an alias maintained by the configuration endpoint  414 . Each memory caching nodes  402  may then monitor the identified memory caching node  402  for changes and download the configuration  412  when it is determined that the configuration  412  has changed. In some embodiments, the identified memory caching node  402  may distribute and/or notify other memory caching node  402  in the cache cluster  413  of changes to configuration  412 . By obtaining an updated configuration  412 , a client may adapt to dynamically changing memory caching nodes  402  within the caching cluster  413 . 
     A memory caching node  413  may follow a protocol that includes rules governing cached data. In one embodiment, the rules specify cache eviction upon a last recently used basis when the cache space  404  is full. In another embodiment, the rules allow cached data, such as the key-value pair  410  to be associated with a time to live after which the data will no longer be available. In some embodiments, the protocol governing cached data has been extended such that configuration  412  stored in the reserved configuration space  402  is not subject to the rules governing cache eviction and/or time to live. 
     Turning now to  FIG. 5 , an illustrative example  500  of logical connections between components in accordance with at least one embodiment is shown. A purpose of the cache clusters  513  using memory caching nodes  510  may be to prevent a load on and/or slow response from an API or data store  514 , such as a relational database, NoSQL database and key-value store. In the embodiment shown, an application  502  may cause a client driver  504 , such as through a library API call, to retrieve a configuration  508  from a memory caching node  510  identified by an alias retrieved from a predetermined configuration endpoint  506 . The configuration  508  may include information to enable communication with data store  514  and memory caching nodes  510 . Upon configuring the communication, the client driver  504  may field requests from the application  502  for data within the data store  514 . The client driver  504  may determine a memory caching node  510  to contact to see if the data is in the cache  512  of the memory caching node  510 . If so, the client driver  504  may return the data to the application  502 . If not, the client driver may request the information from the data store  514  directly. Because of the request, the data store  514  and/or the client driver  504  may cause the data to be stored in a cache  512  of a memory caching node  510  for future retrieval. In some embodiments, during a request to a memory caching node  510 , the client driver  504  may check and/or be notified that a configuration change has occurred. 
     In some embodiments, request forwarding may occur. For example, a client driver  504  may make a request for data to a first memory caching node  510  that may forward the request to a second memory caching node  510 . If the second memory caching node  510  does not have the data requested in cache  512 , the second memory caching node  510  may forward the request to the data store  514 . The data store may return the requested data, either through the same path or directly to the client driver  504 . An advantage of request forwarding is that the client driver  504  need not have a current configuration  508 . However, the delays may be more significant than direct communication with a memory caching node  510 . 
     Turning now to  FIG. 6 , an illustrative chart of distributed memory caching configuration process in accordance with at least one embodiment is shown. This process may be accomplished, in one embodiment, by computing resources such as those seen in  FIG. 2  including application  202 , client driver  204 , configuration endpoint  206  and memory caching nodes  210 . The configuration process may include three phases: initialization  600 , use  614  and reconfiguration  622 . During initialization, a client driver  604  prepares to receive data from a cache on behalf of an application  602  by obtaining a configuration  612 . In  FIG. 6 , the application  602  gives the client driver  604  a configuration endpoint identifier  606  that identifies a configuration endpoint  610 , such as by hostname, address or other identifying information. The client driver  604  uses this configuration endpoint identifier  606  to resolve an alias identifying a memory caching node  620  comprising the configuration  612 . The client driver requests  608  the configuration  612  from the memory caching node  620 . The memory caching node  620  may send the configuration  612  to the client driver  604 . The client driver  604  may then load the configuration  612  to enable communications with a cache cluster  613  having memory caching nodes  620 . 
     In some embodiments, a configuration endpoint  610  is ensured for high availability, as new applications  602  rely on the availability of the configuration endpoint alias. The configuration endpoint  610  may be access restricted based on the request or an identifier associated with the request, such as requesting IP address, destination IP address and/or credentials. 
     In the use phase  614 , the client driver  604  may act as an interface between the application  602  and the cache cluster  613 . In some embodiments, this interface may be done with an API and/or code library. The application  602  may send a request for data  616  that is analyzed by the client driver  604  to determine which memory caching node  620  may have the requested data in its cache. The client driver  604  may then send the request  616  for data in a format recognized by the memory caching node  620 . If the data is found within the memory caching node  620 , the memory caching node  620  returns the data  618  to the client driver  604 . The client driver  604  may then return the data  618  to the application  602 . However, if the data is not found within the memory caching node  620 , the client driver&#39;s request may fail and/or be redirected to the data store. 
     In some embodiments, the data in a request  616  may be serviced by more than one memory caching node  620  in a cache cluster  613 . In one embodiment, this redundancy may be due to cached data that is expensive to recreate. In other embodiments, this redundancy may be due to reducing a server load due to a collection of frequently accessed data. The client driver  604  may use configuration information  612 , information from a management system regarding cache cluster  613 , request latency from a memory caching node  620  and/or other information or indicators to determine which memory caching node  620  should be contacted for redundant information. In another embodiment, a memory caching node  620  is randomly selected if the data is available from two or more memory caching nodes  620 . 
     In the reconfiguration phase  622 , the client driver  604  ensures that its configuration  612  is up to date by comparing its version with a version known to one or more of the memory caching nodes  620 . In one embodiment, a client driver  604  may periodically send a request for configuration  608  to one or more memory caching nodes  620 . The contacted memory caching nodes  620  may return a stored configuration  612  which may be compared against a configuration used by the client driver  604 . In another embodiment, the client driver may request version information of the configuration  612  from the memory caching node  620 . The client driver  604  may compare the version information retrieved against version information of a local configuration. If the retrieved information is a newer version, the client driver  604  may request the new version of the configuration  612 . For example, version information may be a combination of a serially incremented number and a timestamp. In some embodiments, the client driver may receive an indicator from a memory caching node  620  that a configuration has changed during the use phase  614 , such as a secondary return value. 
       FIG. 7  shows an illustrative example of a process  700  that may be used to update configuration in accordance with at least one embodiment. This process may be accomplished, in one embodiment, by computing resources such as those seen in  FIG. 2  including application  202 , client driver  204 , configuration endpoint  206  and memory caching nodes  210 . A cache cluster may provide  702  a configuration endpoint to provide an alias to a memory caching node for configuration distribution. Upon modifying  704  one or more distributed memory caching nodes, such as provisioning memory caching nodes, de-provisioning memory caching nodes, movement of keys, changes to key placement or other changes affecting a client configuration, a new configuration may be determined  706  based on the changes performed. Configurations stored in memory caching nodes may be updated  710  as well as the configuration endpoint alias updated  708 , if needed. The update to a configuration may be a replacement, concatenation, overwrite or other modification to the configuration data stored in a memory caching node. 
     Some or all of the process  700  (or any other processes described herein, or variations and/or combinations thereof) may be performed under the control of one or more computer systems configured with executable instructions and may be implemented as code (e.g., executable instructions, one or more computer programs or one or more applications) executing collectively on one or more processors, by hardware or combinations thereof. The code may be stored on a computer-readable storage medium, for example, in the form of a computer program comprising a plurality of instructions executable by one or more processors. The computer-readable storage medium may be non-transitory. 
       FIG. 8  shows an illustrative example of a process  800  that may be used to configure a client in accordance with at least one embodiment. This process may be accomplished, in one embodiment, by computing resources such as those seen in  FIG. 2  including application  202 , client driver  204 , configuration endpoint  206  and cache cluster  213 . A client driver may receive  802  initialization information regarding a configuration endpoint. Using the configuration endpoint information, the client driver may request  804  use an alias provided by the configuration endpoint to receive  804  memory caching node information. The client driver may then request  805  a current configuration from the memory caching node. The request may include explicit or implicit identification of the application, client and/or customer requesting the configuration. Explicit identification may include credentials or account information. Implicit identification may include origin, destination, requesting IP address, destination IP address or other inherent characteristics of the requestor or the request. The identification is useful if the configuration endpoint serves multiple aliases for different distributed caching clusters. The client driver may load  806  the configuration and service  808  application requests until a configuration change is needed  810 . The need for a configuration update may be checked periodically, noted in a return value from a memory caching node, messaged to the driver or otherwise noticed by or notified to the driver, application or client. If needed, the configuration may be requested from a memory caching node. 
       FIG. 9  shows an illustrative example of a process  900  that may be used to manage caching in accordance with at least one embodiment. This process may be accomplished by computing resources such as those seen in  FIG. 2  including application  202 , client driver  204 , configuration endpoint  206  and memory caching nodes  210 . A management system may monitor memory caching nodes  902  for indicators of a problem. If a problem is discovered  904 , new memory caching nodes  906  may be provisioned  908  and/or existing memory caching nodes to be removed  910  may be de-provisioned  912 . For example, problems requiring new memory caching nodes to the cache cluster may include a growth in the need for caching bandwidth and/or caching storage. Problems requiring the removal of memory caching nodes from the cache cluster may be the decrease in need of caching bandwidth, failing hardware and/or caching storage. For example, caching may be increased during seasons of high use, such as a shopping website in December. Caching may also be decreased during seasons of low use, such as a ski retailer commerce application in summer. Some problems may also require the provisioning of new memory caching nodes and the removal of other memory caching nodes. This may include failure of a memory caching node and/or the migration from one size of memory caching node to another size of memory caching nodes. After provisioning and/or de-provisioning memory caching nodes, a new configuration may be determined  914  based at least in part on the changes in response to the problem. The new configuration may be pushed to update  920  the memory caching nodes for storage and/or update  918  the alias used by a configuration endpoint. 
     Turning now to  FIG. 10 , an illustrative example of a process  1000  that may be used to manage memory caching node behavior in accordance with at least one embodiment is shown. This process may be accomplished by computing resources such as those seen in  FIG. 2  including application  202 , client driver  204 , configuration endpoint  206  and memory caching nodes  210 . During the monitoring  1002  of memory caching nodes, one or more key/value pairs are identified  1003  to make redundant. This redundancy may be used to distribute a high load among memory caching nodes and/or increase durability of a cached key-value pair. A second memory caching node may be selected  1004  and caused to service  1006  the identified key-value pair. The second memory caching node may be identified by load, use patterns, durability or other attributes that make the memory caching node desirable. In some embodiments, important key-value pairs are stored on three or more memory caching nodes. Using the changes, a new configuration may be determined  1008 . Using the new configuration, memory caching nodes may be updated  1010  by receiving and storing the configuration. The configuration endpoint may also be updated  1012  by updating its alias. 
     For example, durability of key-value pairs that are costly to re-calculate may be factored in a decision to make the key-value pair redundant. An algorithm running on a monitoring system within the management system measures frequency of access to determine which key-value pairs will be made redundant. Using the result of the algorithm, the management system may cause a provisioning system to distribute the key-value pairs across two or more memory caching nodes. After distributing the key-value pairs, the management system may then update a configuration and cause the configuration to be stored by memory caching nodes that form part of a cache cluster. In some embodiments, this algorithm may be modified to weigh the costs of key-value pairs, such that costly and frequently accessed key-value pairs may be made redundant. In some embodiments that require further redundancy, multiple configuration endpoints may be used to increase durability. 
     In some embodiments, the key-value pair may be transferred rather than made redundant. For example, a memory caching node under load may select a range of keys to offload to a second memory caching node. In some cases, the memory caching node under load may have to continue servicing the range of keys until all or most of clients update their configuration. 
     In some embodiments, configurations may be propagated between memory caching nodes. For example, once a memory caching node receives a configuration, the memory caching node may attempt to distribute the configuration to other memory caching nodes in the configuration. In this way the memory caching nodes may work in using peer-to-peer communication to propagate configuration to each memory caching node. In one embodiment, memory caching nodes in a cache cluster may track nodes within the cache cluster such that changes to the cache cluster are monitored by the memory caching nodes themselves. A memory caching node that notices an addition or subtraction of a memory caching node, or is the subject of the addition or subtraction, may create a new configuration to distribute to the other memory caching nodes. 
     It should be recognized that the use of the term client driver does not necessarily refer to software that directly supports hardware. The client driver is code executed by a computing resource that at least manages communication between an application and a distributed cache cluster. In some embodiments, this is accomplished by a library. For example, a developer may call functions within a library to perform the phases seen and discussed in relation to  FIG. 6 . 
     It should be recognized that the use of the term memory caching node is used as a broad term that covers more than just the specific examples above. Other caching types are included in this term. Other examples of memory caching nodes include persistent caching systems and disk caching systems. In one embodiment, a persistent caching system is used such that a cache state is saved to avoid losing the cache. In another embodiment, a disk caching system may be used. 
       FIG. 11  is an illustrative example of a mapping  1100  that may be used to represent node locations in managed cache retrieval in accordance with at least one embodiment. This process may be accomplished by computing resources such as those seen in  FIG. 2 , including application  202 , client driver  204 , configuration endpoint  206  and memory caching nodes  210 . A range of possible hashes may be represented by a circle  1102 . Although differences in angles may graphically show similar spaces between nodes, any number of hashes may be included within a range of angles. For example, in one embodiment, there may be thousands of hashes contained within a small portion of the circle and in another embodiment, there may not be any hashes contained within a second portion of the circle. 
     Keys Z ( 1110 ), Y ( 1112 ), X ( 1114 ) and W ( 1116 ) correspond to caching angles measured from a reference angle  1101 , such as, for example, angles  1126 ,  1130 ,  1132  and  1134 , shown on circle  1102 . The keys may be input into a hashing function that returns a corresponding caching angle. A memory caching node may be assigned at least one caching angle along circle  1102 . Larger memory caching nodes may be assigned more caching angles, which may grant a larger coverage over the circle  1102 . It is understood that the number of hashes can differ per angle. For example, memory caching node assignments to memory caching node A include caching angle  1104  and caching angle  1106 . A key is assigned to a memory caching node first encountered travelling clockwise around the circle from a caching angle corresponding to the key. For example, caching angle  1130  determined from a hash of key Z  1110  is followed clockwise  1138  to the caching angle assignment 1 ( 1104 ) of memory caching node A. 
     In  FIG. 11  a caching angle is shown to be measured clockwise from the reference angle  1101 . For example, caching angle  1130  may have a smaller angle than caching angle  1126  as measured from the reference angle. To determine which memory caching node is responsible for a key, the key is first processed through a hash function to determine a caching angle. The caching angle may then be followed clockwise until the first memory caching node assignment occurs. For example, key X  1114  resolves to the caching angle  1126  shown. The caching angle is then swept clockwise along line  1124  until the first memory caching node assignment occurs at caching angle  1104  which is assignment number 1 for memory caching node A. Therefore key X is assigned to memory caching node A. Similarly, the caching angle of key Z ( 1110 ) sweeps  1138  to caching angle  1104  which is assignment 1 of memory caching node A. For the same reasons, Key Y  1112  is assigned to memory caching node A, assignment 2 ( 1106 ) because of the sweep  1136  to the assignment of caching angle  1106 . Key W  1116  is assigned to memory caching node B because sweeping  1140  clockwise arrives at the assignment of a caching angle  1108  assigned to assignment 1, memory caching node B. 
     Memory caching node assignments may be accomplished by several different methods. In one embodiment, the client driver comprises code configured to assign memory caching nodes within the mapping. In such an embodiment, the client may be aware of the angles on the mapping that are “hot” and as such require an additional node to offload one or more requests. In another embodiment, a management system may aid a client driver in assigning caching angles. For example, a management system monitors the access of keys and determines an optimum placement of assignments to reduce server loads on memory caching nodes. The management system may be aware of one or more aspects of the cache cluster in general as well as added nodes that it may provide as “hints” to the client driver. 
     In another embodiment, one or more clients monitor usage of the memory caching nodes. If needed, a client may request provisioning of a new memory caching node to add to the cache cluster. For example, a client may determine that a latency of a response from a memory caching node has increased beyond an acceptable threshold. As another example, the client may query the memory caching node using a protocol extension or reviewing an access log, the client determines that one or more keys are accessed with a frequency above a threshold. The client may then request that a provisioning system provision a new memory caching node. The client may then assign the memory caching node one or more caching angles. 
     In one example, shown in  FIG. 12 , a memory caching node C is added to the cache cluster illustrated in  FIG. 11 . Memory caching node C is able to support three caching angles: caching angle assignment 1 ( 1118 ), caching angle assignment 2 ( 1120 ) and caching angle assignment 3 ( 1122 ). This ability to support three caching angles may be due to the size, processing ability and/or placement of memory caching node C. Further, as illustrated, nodes in the cluster may not necessarily be equidistant from each other in terms of the angular distances between them. Some nodes may be closer to each other than others (in terms of angular distance in the representation in  FIGS. 11-12 ) due to various factors in connection with the key spaces served by the nodes. In the example shown, Key X  1114  and Key Z  1110  may be “hot” keys that are frequently accessed (i.e., accessed with a frequency in a range designated as “hot”), therefore, causing the utilization of caching angle  1104  to be responsible for the high utilization of memory caching node A. A management system may cause a new memory caching node C to receive a caching angle assignment 2 ( 1120 ). Due to the new assignment of caching angle  1120 , Key X  1114  may now be serviced by memory caching node C, as a sweep of caching angles along line  1128  leads to caching angle  1120  that is assigned to memory caching node C, assignment 2. Key Z  1110  may remain with memory caching node A due to an assignment of caching angle  1104 . 
     In another example, an access history of Key W  1116  may be such that Key W  1116  should be serviced by more than one memory caching node. This replication of the key space may be due to load, difficulty of calculation of the underlying cached value or other replication need. As shown in  FIG. 12 , memory caching node C, assignment 1 ( 1118 ) has been assigned the same caching angle assignment  1118  as the caching angle assignment  1108  of memory caching node B, assignment 1 ( 1108 ). Thus, memory caching node B and memory caching node C share responsibility for the same key space. In some embodiments, only a portion of the key space is noted as replicated. 
     In yet another example, a management system may also determine that the key space covered by caching angle  1105  assigned to memory caching node B, assignment 2 should be smaller. A caching angle  1122  assignment 3 to memory caching node C is added between assigned caching angles  1105  and  1106 . As may be noted in  FIG. 12 , the range responsibility between caching angles  1105  and  1122  need not be symmetric. In some cases, memory caching node C&#39;s range may be smaller than memory caching node A&#39;s range, but may be more frequently accessed. Considerations, such as range and frequency of access, may be used to determine the assignment of caching angle assignments. It should be recognized that in each of the above embodiments, the client driver may be in control of the determination of the location of the nodes and as such, the management system may provide information which may be used by the client driver in making the determinations. 
     It should be recognized that while the memory caching angle assignments have been discussed in terms of three or less assignments, actual use may be higher including hundreds, thousands, millions or more of caching assignments. The few assignments shown are for simplifying discussion. 
     While memory caching nodes have been used for illustration of various aspects of the present disclosure, it should be recognized that the structures and processes described may also be more broadly applied to storage nodes and clusters of computing resources in general. For example, a storage node may include a memory caching node, databases and read-replicas. In one embodiment, membership information of a cluster of nodes is shared with clients of the nodes. For example, the processes and structures may be used in database scaling. Configuration of read-replicas may be stored in a configuration space on a database server. Clients of the database server may detect changes, such as additions or subtractions, to the read-replicas by requesting the configuration from a read-replica, using client configuration update techniques described above. In another example, the processes and structures may be used in database clustering. A cluster configuration may be stored in the database itself alongside the data that makes up the distributed data store of the cluster, which may be retrieved by clients of the database. This allows the client initialization to be decoupled from server resources. 
       FIG. 13  illustrates aspects of an example environment  1300  for implementing aspects in accordance with various embodiments. As will be appreciated, although a Web-based environment is used for purposes of explanation, different environments may be used, as appropriate, to implement various embodiments. The environment includes an electronic client device  1302 , which can include any appropriate device operable to send and receive requests, messages or information over an appropriate network  1304  and convey information back to a user of the device. Examples of such client devices include personal computers, cell phones, handheld messaging devices, laptop computers, set-top boxes, personal data assistants, electronic book readers and the like. The network can include any appropriate network, including an intranet, the Internet, a cellular network, a local area network or any other such network or combination thereof. Components used for such a system can depend at least in part upon the type of network and/or environment selected. Protocols and components for communicating via such a network are well known and will not be discussed herein in detail. Communication over the network can be enabled by wired or wireless connections and combinations thereof. In this example, the network includes the Internet, as the environment includes a Web server  1306  for receiving requests and serving content in response thereto, although for other networks an alternative device serving a similar purpose could be used as would be apparent to one of ordinary skill in the art. 
     The illustrative environment includes at least one application server  1308  and a data store  1310 . It should be understood that there can be several application servers, layers, or other elements, processes or components, which may be chained or otherwise configured, which can interact to perform tasks such as obtaining data from an appropriate data store. As used herein the term “data store” refers to any device or combination of devices capable of storing, accessing and retrieving data, which may include any combination and number of data servers, databases, data storage devices and data storage media, in any standard, distributed or clustered environment. The application server can include any appropriate hardware and software for integrating with the data store as needed to execute aspects of one or more applications for the client device, handling a majority of the data access and business logic for an application. The application server provides access control services in cooperation with the data store, and is able to generate content such as text, graphics, audio and/or video to be transferred to the user, which may be served to the user by the Web server in the form of HTML, XML or another appropriate structured language in this example. The handling of all requests and responses, as well as the delivery of content between the client device  1302  and the application server  1308 , can be handled by the Web server. It should be understood that the Web and application servers are not required and are merely example components, as structured code discussed herein can be executed on any appropriate device or host machine as discussed elsewhere herein. 
     The data store  1310  can include several separate data tables, databases or other data storage mechanisms and media for storing data relating to a particular aspect. For example, the data store illustrated includes mechanisms for storing production data  1312  and user information  1316 , which can be used to serve content for the production side. The data store also is shown to include a mechanism for storing log data  1314 , which can be used for reporting, analysis or other such purposes. It should be understood that there can be many other aspects that may need to be stored in the data store, such as for page image information and to access right information, which can be stored in any of the above listed mechanisms as appropriate or in additional mechanisms in the data store  1310 . The data store  1310  is operable, through logic associated therewith, to receive instructions from the application server  1308  and obtain, update or otherwise process data in response thereto. In one example, a user might submit a search request for a certain type of item. In this case, the data store might access the user information to verify the identity of the user, and can access the catalog detail information to obtain information about items of that type. The information then can be returned to the user, such as in a results listing on a Web page that the user is able to view via a browser on the user device  1302 . Information for a particular item of interest can be viewed in a dedicated page or window of the browser. 
     Each server typically will include an operating system that provides executable program instructions for the general administration and operation of that server, and typically will include a computer-readable storage medium (e.g., a hard disk, random access memory, read only memory, etc.) storing instructions that, when executed by a processor of the server, allow the server to perform its intended functions. Suitable implementations for the operating system and general functionality of the servers are known or commercially available, and are readily implemented by persons having ordinary skill in the art, particularly in light of the disclosure herein. 
     The environment in one embodiment is a distributed computing environment utilizing several computer systems and components that are interconnected via communication links, using one or more computer networks or direct connections. However, it will be appreciated by those of ordinary skill in the art that such a system could operate equally well in a system having fewer or a greater number of components than are illustrated in  FIG. 13 . Thus, the depiction of the system  1300  in  FIG. 13  should be taken as being illustrative in nature, and not limiting to the scope of the disclosure. 
     The various embodiments further can be implemented in a wide variety of operating environments, which in some cases can include one or more user computers, computing devices or processing devices which can be used to operate any of a number of applications. User or client devices can include any of a number of general purpose personal computers, such as desktop or laptop computers running a standard operating system, as well as cellular, wireless and handheld devices running mobile software and capable of supporting a number of networking and messaging protocols. Such a system also can include a number of workstations running any of a variety of commercially-available operating systems and other known applications for purposes such as development and database management. These devices also can include other electronic devices, such as dummy terminals, thin-clients, gaming systems and other devices capable of communicating via a network. 
     Most embodiments utilize at least one network that would be familiar to those skilled in the art for supporting communications using any of a variety of commercially-available protocols, such as TCP/IP, OSI, FTP, UPnP, NFS, CIFS and AppleTalk. The network can be, for example, a local area network, a wide-area network, a virtual private network, the Internet, an intranet, an extranet, a public switched telephone network, an infrared network, a wireless network and any combination thereof. 
     In embodiments utilizing a Web server, the Web server can run any of a variety of server or mid-tier applications, including HTTP servers, FTP servers, CGI servers, data servers, Java servers and business application servers. The server(s) also may be capable of executing programs or scripts in response requests from user devices, such as by executing one or more Web applications that may be implemented as one or more scripts or programs written in any programming language, such as Java®, C, C# or C++, or any scripting language, such as Perl, Python or TCL, as well as combinations thereof. The server(s) may also include database servers, including without limitation those commercially available from Oracle®, Microsoft®, Sybase® and IBM®. 
     The environment can include a variety of data stores and other memory and storage media as discussed above. These can reside in a variety of locations, such as on a storage medium local to (and/or resident in) one or more of the computers or remote from any or all of the computers across the network. In a particular set of embodiments, the information may reside in a storage-area network (“SAN”) familiar to those skilled in the art. Similarly, any necessary files for performing the functions attributed to the computers, servers or other network devices may be stored locally and/or remotely, as appropriate. Where a system includes computerized devices, each such device can include hardware elements that may be electrically coupled via a bus, the elements including, for example, at least one central processing unit (CPU), at least one input device (e.g., a mouse, keyboard, controller, touch screen or keypad), and at least one output device (e.g., a display device, printer or speaker). Such a system may also include one or more storage devices, such as disk drives, optical storage devices, and solid-state storage devices such as random access memory (“RAM”) or read-only memory (“ROM”), as well as removable media devices, memory cards, flash cards, etc. 
     Such devices also can include a computer-readable storage media reader, a communications device (e.g., a modem, a network card (wireless or wired), an infrared communication device, etc.) and working memory as described above. The computer-readable storage media reader can be connected with, or configured to receive, a computer-readable storage medium, representing remote, local, fixed and/or removable storage devices as well as storage media for temporarily and/or more permanently containing, storing, transmitting and retrieving computer-readable information. The system and various devices also typically will include a number of software applications, modules, services or other elements located within at least one working memory device, including an operating system and application programs, such as a client application or Web browser. It should be appreciated that alternate embodiments may have numerous variations from that described above. For example, customized hardware might also be used and/or particular elements might be implemented in hardware, software (including portable software, such as applets) or both. Further, connection to other computing devices such as network input/output devices may be employed. 
     Storage media and computer readable media for containing code, or portions of code, can include any appropriate media known or used in the art, including storage media and communication media, such as but not limited to volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage and/or transmission of information such as computer readable instructions, data structures, program modules or other data, including RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disk (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices or any other medium which can be used to store the desired information and which can be accessed by the a system device. Based on the disclosure and teachings provided herein, a person of ordinary skill in the art will appreciate other ways and/or methods to implement the various embodiments. 
     The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims. 
     Other variations are within the spirit of the present disclosure. Thus, while the disclosed techniques are susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit the invention to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the invention, as defined in the appended claims. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the disclosed embodiments (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The term “connected” is to be construed as partly or wholly contained within, attached to, or joined together, even if there is something intervening. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate embodiments of the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this disclosure are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 
     All references, including publications, patent applications and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.