What is Edge Side Include (ESI) ?

ESI caching is an in-memory caching solution that is implemented through the web server plug-in. The ESI processor can cache pages or fragments of pages at the HTTP server layer.

The Web server plug-in contains a built-in ESI processor. The ESI processor can cache whole pages, as well as fragments, providing a higher cache hit ratio. The cache implemented by the ESI processor is an in-memory cache, not a disk cache, therefore, the cache entries are not saved when the Web server is restarted.

When a request is received by the Web server plug-in, it is sent to the ESI processor, unless the ESI processor is disabled. It is enabled by default. If a cache miss occurs, a Surrogate-Capabilities header is added to the request and the request is forwarded to the WebSphere Application Server. If servlet caching is enabled in the application server, and the response is edge cacheable, the application server returns a Surrogate-Control header in response to the WebSphere Application Server plug-in.

The value of the Surrogate-Control response header contains the list of rules that are used by the ESI processor to generate the cache ID. The response is then stored in the ESI cache, using the cache ID as the key. For each ESI include tag in the body of the response, a new request is processed so that each nested include results in either a cache hit or another request that forwards to the application server. When all nested includes have been processed, the page is assembled and returned to the client.

The ESI processor is configurable through the WebSphere Web server plug-in configuration file plugin-cfg.xml. The following is an example of the beginning of this file, which illustrates the ESI configuration options.

After the basic of ESI now the question is How do I use the application cache monitor to display static content on the plug-in Edge Side Include (ESI) cache and clear the plug-in ESI cache without a restart of the web server?

There is default application, the cache monitor application is located in the installableApps directory and is installed as "Dynamic Cache Monitor: The main functions of the "Dynamic Cache Monitor" (CacheMonitor) application as it relates to the static caching in the plug-in ESI cache are:

• You can manually change the state of the cache without having to restart the web server
• You can display what is being cached in the plug-in ESI Cache

These functions are only related to static caching in the plug-in ESI. There are additional function related to dynamic caching referenced in the information center topic Cache monitor.

To manage and monitor static caching in the plug-in ESI cache using the WebSphere Application Server application cache monitor go through the following steps.

1. Both the CacheMonitor.ear (Dynamic Cache Monitor ) and DyncCacheEsi.ear     (DyncCacheEsi) applications must be installed. 

2. Change the 3 critical values for management of the plug-in ESI cache and for monitoring using the cache monitor application in the plugin-cfg.xml file

Set esiEnable =true
esiEnable will enable or disable the ESI processor. ESI is enabled by default. If ESI is disabled, then the other ESI options are ignored.

Set esiMaxCacheSize= 1024
esiMaxCacheSize is the maximum size of the cache in 1K byte units. The default maximum size of the cache is 1 megabyte. Each web server process will contain its own cache, so consideration should be given to the cache size as it related to the number of plug-in processes. If the response content has a content-length response header, the web server plug-in checks for the response content size. If the size of the response content body is larger than the available ESI caching space, the response is passed through without being handled by ESI.

Set esiInvalidationMonitor =true
esiInvalidationMonitor, specifies if the ESI processor should receive invalidation from the application server.

3. Following are 3 methods by which entries are removed or made invalid from the ESI cache:

Expired timeout
The default Timeout values is set to 300 seconds. You can change the timeout value by adding the property com.ibm.servlet.file.esi.timeOut to the Java virtual machine (JVM) command line parameters. The following example shows how to set a one minute timeout on static data cached in the plug-in:

-Dcom.ibm.servlet.file.esi.timeOut=60

Purge entry to make room for newer entries:
When the plug-in ESI cache is filled, entries will be purged from the cache based on their pending expiration (those closest to expiration will be purged first).

Explicit invalidation:
The application server or cache monitor application sends an explicit invalidation for a group of entries. For this mechanism to be enabled, the esiInvalidationMonitor property must be set to true and the cache monitor DynaCacheEsi.ear applications must be installed on the application server. The DynaCacheEsi application is located in the installableApps directory and is named DynaCacheEsi.ear .

4. Go to the WebSphere Application Server > Server_Name > Web container and enable servlet Caching

5. To display the static plug-in ESI cache :

http://<hostName>:port/cachemonitor/
where hostname is the hostname of the application server or the web server (if the cache monitor application is mapped to a web server) port is the port of the application sever or the web server.

Notes: You must click Refresh from the Edge Statistics Panel to get updated statistics

“Effort only fully releases its reward after a person refuses to quit.”

Regards,

Akhilesh B. Humbe