First Level and Second Level Cache in Hibernate

Hibernate is actually a very powerful, consistent, and reliable database mapping tool. Mapping between objects in Java to relational databases has many facets that you must be aware of. Hibernate does a particularly good job of making the process simple to start, and providing the facilities to allow it to scale well and meet exceedingly complex mapping demands. 

Caching is all about application performance optimization and it sits between your application and the database to avoid the number of database hits as many as possible to give a better performance for performance critical applications.
Caching is important to Hibernate as well which utilizes a multilevel caching schemes as explained below:

 

 

 

One of the primary concerns of mappings between a database and our Java application is performance.  This is the common concern of the all guys who working with hibernate and spent the more time in ORM tools for performance-enhancing changes to particular queries and retrievals. Today I want to discuss about some facets of the Hibernate infrastructure that are implemented to handle certain performance concerns - 

1. The first-level cache - Session (Earlier hibernate already provide this level of cache)
   
2. The second-level cache -Session-factory-level cache
3. and the query cache.

The first-level cache: The first level cache type is the session cache. The session cache caches object within the current session but this is not enough for long level i.e. session factory scope.

The second-level cache: The second-level cache is called 'second-level' because there is already a cache operating for you in Hibernate for the duration you have a session open. A Hibernate Session is a transaction-level cache of persistent data. It is possible to configure a SessionFactory-level cache on a class-by-class and collection-by-collection basis.

                 second-level cache
                     -- Across sessions in an Application
                     -- Across applications (different applications on same servers with same database)
                     -- Across clusters (different applications on different servers with same database)

NOTE: Be careful Caches are never aware of changes made to the persistent store by another application i.e. suppose one application deploy one server with using hibernate and get the data from database and put to the cache for further using purpose but another application deployed another server which does not using any ORM tool so it does mot know about Cache and direct interacting with database and may be update data of database. Now data in Cache is invalid.

Hibernate uses first-level cache by default and you have nothing to do to use first-level cache. Let's go straight to the optional second-level cache. Not all classes benefit from caching, so it's important to be able to disable the second-level cache.

The 'second-level' cache exists as long as the session factory is alive. The second-level cache holds on to the 'data' for all properties and associations (and collections if requested) for individual entities that are marked to be cached.