12.3.2. Extended session and automatic versioning

A single Session instance and its persistent instances that are used for the whole conversation are known as session-per-conversation. Hibernate checks instance versions at flush time, throwing an exception if concurrent modification is detected. It is up to the developer to catch and handle this exception. Common options are the opportunity for the user to merge changes or to restart the business conversation with non-stale data.

The Session is disconnected from any underlying JDBC connection when waiting for user interaction. This approach is the most efficient in terms of database access. The application does not version check or reattach detached instances, nor does it have to reload instances in every database transaction.

// foo is an instance loaded earlier by the old session
Transaction t = session.beginTransaction(); // Obtain a new JDBC connection, start transaction

foo.setProperty("bar");

session.flush();    // Only for last transaction in conversation
t.commit();         // Also return JDBC connection
session.close();    // Only for last transaction in conversation

The foo object knows which Session it was loaded in. Beginning a new database transaction on an old session obtains a new connection and resumes the session. Committing a database transaction disconnects a session from the JDBC connection and returns the connection to the pool. After reconnection, to force a version check on data you are not updating, you can call Session.lock() with LockMode.READ on any objects that might have been updated by another transaction. You do not need to lock any data that you are updating. Usually you would set FlushMode.MANUAL on an extended Session, so that only the last database transaction cycle is allowed to actually persist all modifications made in this conversation. Only this last database transaction will include the flush() operation, and then close() the session to end the conversation.

This pattern is problematic if the Session is too big to be stored during user think time (for example, an HttpSession should be kept as small as possible). As the Session is also the first-level cache and contains all loaded objects, we can probably use this strategy only for a few request/response cycles. Use a Session only for a single conversation as it will soon have stale data.

Keep the disconnected Session close to the persistence layer. Use an EJB stateful session bean to hold the Session in a three-tier environment. Do not transfer it to the web layer, or even serialize it to a separate tier, to store it in the HttpSession.

The extended session pattern, or session-per-conversation, is more difficult to implement with automatic current session context management. You need to supply your own implementation of the CurrentSessionContext for this. See the Hibernate Wiki for examples.