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Custom-Cache-stores-(deployable).md

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Deployable Cache Stores high level design

This document describes Deployable Cache Stores implementation details.

Client perspective

The client will be able to deploy a custom Cache Store jar into Hotrod server (put it into $HOTROD_SERVER/standalone/deployments). The jar will need to contain one of the following service entries:

  • /META-INF/services/org.infinispan.persistence.spi.AdvancedCacheWriter
  • /META-INF/services/org.infinispan.persistence.spi.AdvancedCacheLoader
  • /META-INF/services/org.infinispan.persistence.spi.CacheLoader
  • /META-INF/services/org.infinispan.persistence.spi.CacheWriter
  • /META-INF/services/org.infinispan.persistence.spi.ExternalStore
  • /META-INF/services/org.infinispan.persistence.spi.AdvancedLoadWriteStore

Those services might used later used in the configuration.

Implementation details

The implementation is based on Deployable Filters and Converters.

Currently all writers and loaders are instantiated in PersistenceManagerImpl#createLoadersAndWriters. This implementation will be modified to use CacheStoreFactoryRegistry, which will contain a list of CacheStoreFactories. One of the factories will be added by default - the local one (which will the same mechanism as we do now - Util.getInstance(classAnnotation). Other CacheStoreFactories will be added after deployment scanning.

PersistenceManagerImpl is attached to each Cache separately and has a reference to Configuration object. During the startup process PersistenceManagerImpl scans for configured stores (PersistenceManagerImpl#createLoadersAndWriters) and creates separate references for CacheLoaders and CacheWriters. After modifications, not only local instances will be available (more precisely from local Classloader), but also deployed.

FAQ

Q: What happens when you have both a cache loader and a cache writer deployed? A: PersistenceManagerImpl uses separate instances for CacheLoaders and CacheWriters. It is perfectly legal that a custom Cache Store class implements both of those interfaces. In that case the same instance will be used for loading and writing persistence data. Here is a sample block of code from, which illustrates this idea:

PersistenceManagerImpl#createLoadersAndWriters:
Object instance = Util.getInstance(classAnnotation);
CacheWriter writer = instance instanceof CacheWriter ? (CacheWriter) instance : null;
CacheLoader loader = instance instanceof CacheLoader ? (CacheLoader) instance : null;
...
loaders.add(loader);
writers.add(writer);

Q: What happens when you have a deployed Cache Store and it is also accessible locally? A: The deployed one will be used. There are 3 options we could use here:

  • throw some DuplicatedCacheStore exception to indicate such situation (since this is not a total disaster I wouldn't do that)
  • Use the local one
  • Use the deployed one (my vote) I think using the deployed one might be useful for upgrades (for example you have a MyCustomStore v. 1.0 accessible locally and MyCustomStore v. 1.1 deployed - you would probably prefer using the newer one).

Q: How to tie a deployed cache loader/writer with the configuration itself? A: It is already done via configuration. PersistenceManagerImpl#createLoadersAndWriters scans configuration and extracts Cache Store class name from:

  • ConfigurationFor#value
  • CustomStoreConfiguration#customStoreClass Later on this name is used to instantiate loader/writer.

Q: Are deployed Cache Stores "active" by default? A: Yes and no. They are accessible and can be instantiated, but without any configuration - they won't be attached to any Cache.