Windows Azure Caching

Windows Azure Caching is an in-memory, distributed caching feature designed for Windows Azure applications. Caching is currently available as a part of the Windows Azure SDK.

Architecture

Windows Azure Caching allows a cloud service to host Caching on a Windows Azure role. The cache is distributed across all running instances of that role. Therefore, the amount of available memory in the cache is determined by the number of running instances of the role that hosts Caching and the amount of physical memory reserved for Caching on each instance.

There are two deployment topologies for Caching:

• Dedicated
• Co-located

Dedicated Topology

In the dedicated topology, you define a worker role that is dedicated to Caching. This means that all of the worker role's available memory is used for the Caching and operating overhead.

The following diagram shows Caching in a dedicated topology. The cloud service shown has three roles: Web1, Worker1, and Cache1. There are two running instances of each role. The cache is distributed across all instances of the dedicated Cache1 role.

A dedicated topology has the advantage of scaling the caching tier independently of any other role in the cloud service. It also is recommended for caches with higher capacity or throughput requirements.

Co-located Topology

In a co-located topology, you use a percentage of available memory on existing web or worker roles for Caching.

The following diagram shows Caching in a co-located topology. The cloud service has two roles: Web1 and Worker1. There are two running instances of each role. The cache is distributed across all instances of the Web1 role. Because this role also hosts the web front-end for the cloud service, the cache is configured to use only a percentage of the physical memory on each instance of the Web1 role.

A co-located cache is a cost-effective way to make use of existing memory on a role within a cloud service.

Examples

The following sections show Windows Azure Caching configuration and code examples.

Configuration Example

In Visual Studio, Caching on roles is configured in the Caching tab of the properties of the role that hosts Caching. This makes underlying changes to the ServiceConfiguration.cscfg file. These settings determine the topology used (dedicated or co-located) and the number of named caches and their settings.

Other roles must be configured to use Caching. One way to do this is with a NuGet package. This includes modifying the web.config to contain a properly configured dataCacheClients section. The following example dataCacheClients section specifies that the role that hosts Caching is named “CacheWorker1”.

<dataCacheClients>
  <dataCacheClient name="default">
    <autoDiscover isEnabled="true" identifier="CacheWorkerRole1" />
  </dataCacheClient>
</dataCacheClients>

Code Example

When hosting Caching on roles, the DataCache constructor can be used to specify both the named cache and the dataCacheClient section for the cache client settings. The following code shows these variations.

DataCache cache = new DataCache();
cache = new DataCache("default");
cache = new DataCache("NamedCache1", "default");
cache = new DataCache("NamedCache2", "customClient");

The following method shows how to retrieve data from the cache. In this example, a user identifier (userid) is the key for the associated user information object. The code first attempts to get this user information from the cache using the key. If that does not succeed, the code retrieves the information with a database query and then stores the returned user data in the cache. The next time the same code is run, the user information will be returned from the cache rather than the database. This assumes that the cached data has not been expired or evicted.

function get_foo(int userid) {
    /* first try the cache */
    data = (dataType)cache.Get(userid) ;
    if (!data) {
       /* not found : request database */
       data = db_select("SELECT * FROM users WHERE userid = ?", userid);
       /* then store in cache until next get */
       Cache.Add(userid, data);
    }
    return data;
}

The following method shows how to update data already in the cache.

function update_foo(int userid, string dbUpdateString) {
   /* first update database */
    result = db_execute(dbUpdateString);
    if (result) {
       /* database update successful : fetch data to be stored in cache */
       data = db_select("SELECT * FROM users WHERE userid = ?", userid);
       /* the previous line could also look like data = createDataFromDBString(dbUpdateString); */
       /* then store in cache until next get */
       Cache.Put(userid, data);
    }
}

The following call removes the item from the cache.

Cache.Remove(userid);

Shared Caching

Windows Azure Shared Caching provides caching as a managed service. Unlike co-located or dedicated topologies, the cache is not hosted on Windows Azure roles in a single cloud service. Instead, the cache is provided as a multitenant service with usage quotas. The service is divided into tiers that range from 128 MB to 4 GB. In addition to storage capacity, each tier provides increasing processor and network capabilities. Shared Caching provides a way for multiple cloud services to access the same cache.

History

Windows Azure Caching has its roots in an on-premises technology, AppFabric. Many of the assembly names, namespaces, and APIs are identical between the two products. The first release of Caching for Windows Azure in 2011 provided caching as a managed service in Windows Azure. This offering is now called Shared Caching.

In October 2012, support was added for hosting Caching on roles within a cloud service deployment. This is now called Window Azure Caching.

Related Caching Technologies

Windows Azure Caching is related to other Microsoft caching technologies. These technologies share similar features, such as the assembly name, namespace, and types. However, there are some differences. The table below describes these technologies.

Caching Technology	Target	Installed By	Description
AppFabric Caching	On-Premises	AppFabric	Distributed on-premises cache that uses servers that the user provisions and manages.
Windows Azure Caching	Cloud	Windows Azure SDK	Caching is distributed across the instances of a single role in a Windows Azure cloud service deployment.
Windows Azure Caching	Cloud	Windows Azure SDK	Caching is provided through several tiers as a service for use by Windows Azure cloud applications.