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After committing a transaction, the NHibernate session replaces it with a new transaction. This means you should keep a reference to the transaction you re committing if you think you ll need it afterward. This is necessary if you need to call transaction.WasCommitted.session. Transaction.WasCommitted always returns false.
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Here is another version showing in detail where exceptions can be thrown and how to deal with them (this version is more complex than the one presented in chapter 2):
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ISession session = sessions.OpenSession(); ITransaction tx = null;
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Transactions, concurrency, and caching
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try { tx = session.BeginTransaction(); ConcludeAuction(); tx.Commit(); } catch (Exception e) { if (tx != null) { try { tx.Rollback(); } catch (HibernateException he) { //log here } } throw; } finally { try { session.Close(); } catch (HibernateException he) { throw; } }
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As you can see, even rolling back an ITransaction and closing the ISession can throw an exception. You shouldn t use this example as a template in your own application, because you should hide the exception handling with generic infrastructure code. You can, for example, wrap the thrown exception in your own InfrastructureException. We discuss this question of application design in more detail in section 8.1.
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You must be aware of one important aspect: the ISession has to be immediately closed and discarded (not reused) when an exception occurs. NHibernate can t retry failed transactions. This is no problem in practice, because database exceptions are usually fatal (constraint violations, for example), and there is no well-defined state to continue after a failed transaction. An application in production shouldn t throw any database exceptions, either.
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We ve noted that the call to Commit() synchronizes the ISession state with the database. This is called flushing, a process you automatically trigger when you use the NHibernate ITransaction API.
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Flushing the session
The NHibernate ISession implements transparent write-behind. This means changes to the domain model made in the scope of an ISession aren t immediately propagated to the database. Instead, NHibernate can coalesce many changes into a minimal number of database requests, helping minimize the impact of network latency. For example, if a single property of an object is changed twice in the same ITransaction, NHibernate needs to execute only one SQL UPDATE. NHibernate flushes occur only at the following times:
When an ITransaction is committed Sometimes before a query is executed
Understanding database transactions
When the application calls ISession.Flush() explicitly
Flushing the ISession state to the database at the end of a database transaction is required in order to make the changes durable and is the common case. NHibernate doesn t flush before every query. But if changes are held in memory that would affect the results of the query, NHibernate will, by default, synchronize first. You can control this behavior by explicitly setting the NHibernate FlushMode to the property session.FlushMode. The flush modes are as follow:
FlushMode.Auto The default. Enables the behavior just described. FlushMode.Commit Specifies that the session won t be flushed before query
execution (it will be flushed only at the end of the database transaction). Be aware that this setting may expose you to stale data: modifications you made to objects only in memory may conflict with the results of the query. FlushMode.Never Lets you specify that only explicit calls to Flush() result in synchronization of session state with the database.
We don t recommend that you change this setting from the default. It s provided to allow performance optimization in rare cases. Likewise, most applications rarely need to call Flush() explicitly. This functionality is useful when you re working with triggers, mixing NHibernate with direct ADO.NET, or working with buggy ADO.NET drivers. You should be aware of the option but not necessarily look out for use cases. We ve discussed how NHibernate handles both transactions and the flushing of changes to the database. Another important responsibility of NHibernate is managing actual connections to the database. We discuss this next.
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