dynamic cache in WebSphere

Dynamic Cache supports caching of Java servlets, JavaServer Pages (JSP), WebSphere command objects, Web services objects, and Java objects.

*  The concept of caching static information in a browser, proxy or a webserver provides an effective way to reduce network and processing requirements. A larger overhead of many web applications is related to the serving, not of static content, but of dynamic content based on user input and retrieval of data from backend resources such as databases.

How to enable Dynamic Cache
*  Dynamic cache is enabled by default on V6.1 WebSphere AppServers.
*  To enable Servlet and JSP caching from the admin console, navigate to the web container
Servers –> Application Servers -> <select server> -> webcontainer settings –> webcontainer
*  Check ‘Enable servlet caching’
*  Save and restart the Application Server to put the changes into effect.

Specify what to be catched
– Now servlet caching is enabled, if is necessary to define which dynamic content will be cached and the rules by which it will be cached or invalidated. servlet and JSP caching will be policy based using the cachespec.xml file. The preferred location for the cachespec.xml file is within the web applications WEB-INF folder.

<cache-instance name=”offloadtest”>
<property name=”consume-subfragments”>true
<component id=”name” type=”parameter”>

While the timeout directive dictates how long content can remain in cache before being refreshed, the Inactivity directive can cause a refresh prior to the timeout when a page is not used frequently.

The consume-subfragments property tells the cache not to stop saving content when it includes a child servlet. The parent entry will include all the content from all fragments in its cache entry, resulting in one big cache entry.

Use the <exclude> element to tell the cache to stop consuming for the excluded fragment and instead, create a placeholder for the include or forward.

A dependencyID identifies a cache entries dependency based on certain factors, such that when those factors occur they trigger an invalidation of all the cache entries that share this dependency.

How it works?
– The first request will result in a cache MISS. Full processing takes place for Time.jsp. The response is returned to the browser but also loaded into cache.
– Subsequent requests for Time.jsp result in a cache HIT until the cached copy of Time.jsp is invalidated.

Monitoring Dynamic cache
*  Once Dynamic caching is configured, and dynamic content is cached, the content of the cache can be examined and monitored using the cachemonitor.
*  An enterprise application called CacheMonitor is provided with WebSphere Application server. It is available in the folder <WebSphere InstallRoot>/installableApps and called CacheMonitor.ear.

Disk Offload
*  By default, when the number of cache entries reaches the configured limit for a given application server, cache entries are removed from the memory cache, allowing newer entries to be stored in the cache. Use disk offload to copy the cache  entries that are being removed from the memory cache to disk for potential future access. Disk offload is configured via the adminconsole either globally or for a specific cache instance.

Cache Instances
*  Since different webapplications or pages may have different caching requirements, unique cache instances can be created for each. This allows applications to have greater flexibility and better tuning of cache resources.
*  The characteristics of each servlet cache instance are a unique jndi name, disk offload policies, cache configuration and performance parameters.

Configure a new cache instance
goto Resources –> Cache Instances –> Servlet Cache Instance –> new
*  A jndi name must be given to the cache instance which will be used to refer back to the instance in cachespec.xml.
*  Check ‘Enable disk offload’ and specify a disk off load location, cache size and cache entry limits.

Cache Invalidation
*  It is essential that timely invalidations of cached content take place for the integrity of the website.
Mechanisms for invalidation are:
-timeout or inactivity directives within cachespec.xml
-group-based invalidation mechanism through dependency IDs.
-Programmatic invalidation via the cache API ‘com.ibm.websphere.cache.*’
-The CacheMonitor

Some Tips
*  WebSphere Application Server uses JVM memory to store cached objects. Therefore, it is important to know how much memory can be allocated for the cache and based on this information you can set the cache size to the proper value.
*  Increase the priority of cache entries that are expensive to regenerate.
*  Modify timeout of entries so that they stay in memory as long as they are valid.
*  If the estimated total size of all cached objects is bigger than the available memory, you can enable the disk offload option.
*  Increase the cache size if memory allows.


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