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OpenSessionInView Filter, Sessions, Transactions and Connections

By now, I have worked enough on OpenSessionInViewFilter, Hibernate and Spring in some projects.

Hibernate 3 uses Lazy Loading by default. So, We use for maintaining lazy loaded collections to display in view. But, lately, as I mentioned in earlier posts, we were having some issues with connections that were not returned to pool.

To understand where connections were held up we need to see how OSIVfilter, Session, HibernateTransactionManager and Spring work together - Where and when connections are picked and released. OSIVFilter guarentees that each request will have one and only one Hibernate Session. A Session is a lightweight abstraction of data in the database pulled into vm converted as objects. So for a session to fetch data from db, it needs connections. This connection is available from the datasource through a TransactionManager. In general, if you have one database, You would use Springs HibernateTransactionManager.

A Session generally contains one Transaction. However, In OSIV pattern, You keep the session for conversation. This means, You could have multiple transactions in a request. So What happens is, Session syncs with database on multiple transactions. However, Session *doesnot* need to hold the *same* connection throughout the request (it may happen that the pool returned the same connection int he second call, but it is not necessary)

But the TransactionManger will need the same connection all through the transaction. So IT is the one that borrows connection from the pool. When does it release the connection? Pre Hib 3, Session and Transaction was defacto considered the same - So the connections were released only at the end of the session. For Hib 3.1 and later, ConnectionReleaseMode was introduced, which defaults to on_transaction which releases after each transaction. So this way you could use OSIV pattern, scalably. You *can* (by all logical means should not) change it to on_session.

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