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All Things WebSphere

Concerns and issues relating to all versions of WebSphere Application Server

Tuesday, May 10, 2011


Dynacache Questions and Answers on Data Replication Service (DRS) replication best practices

Questions on Dynacache best practices vis a vis replication ... 

Q We plan to use one or more replication domains that span a large number of servers with a number of object cache instances, and to use sharing policy PUSH.
Unfortunately PUSH does not scale. Go with NOT_SHARED.

Q We have observed that it takes a little more than one second for the replication to begin after a cache is being looked up the first time. We want to lookup all cache instances early at the server start up process, in order to fill the caches with existing data in the cluster, making it available to the application as soon as the data is requested.
In WAS v7 Dynacache has a JVM generic customer property called "com.ibm.ws.cache.CacheConfig.createCacheAtServerStartup" which when set to "true" will create cache instances automatically at server startup instead of the default on-demand behavior. This definitely should be set if deploying to WAS7. If on an earlier version of WAS look at  technote  http://www-01.ibm.com/support/docview.wss?rs=180&uid=swg21313480 .
See http://wasdynacache.blogspot.com/2010/04/dynacache-and-drs-replication-faq.html .

Q We are concerned about the push mechanism and whether the initial push of say 100-150 Mb existing data distributed over perhaps 30-40 cache instances into a new-started JVM could have (temporary) performance implications on one or multiple JVM:s that are up and running.
Your concern is warranted and that is why I said PUSH does not scale. When a new JVM starts up *ALL* the other JVMs send it cached data.  So the same cached data is sent by ALL the running JVMs to the new one.  If this is a lot of cache data, I have often seen the older JVMs that are pushing the data die due to OOMs. If you so desire you can completely disable bootstrap by calling DistributedMap.setDRSBootstrap(false). This will prevent any JVM from sending data to the upcoming JVM.

Q When data is pushed into a newborn cache, will all cached element come from the same JVM, or could some elements be pushed from JVM 1 and others from JVM 2
Unfortunately there is no workload distribution when sending data by the existing JVMs to the new one. We don't even optimize and have one JVM send the data, instead ALL the JVMs send all the cache data in that object cache instance to the new JVM.
Some customers write cache population scripts to populate the cache for each JVM after it comes up and configure the cache in the NOT_SHARED mode which only replicates invalidations to keep the cache consistent. Other option is to use PUSH with drs bootstrap disabled.


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