NHibernate Session Flushing Timing Versus Transaction Commit

Direct Answer

Yes. With the default flush mode, NHibernate runs ISession.Flush() synchronously before ITransaction.Commit() finalizes the underlying ADO.NET transaction. Pending inserts, updates, and deletes tracked by the session are written to the database during that pre-commit flush, so they become part of the committed transaction.

Likely Explanation Versus Confirmed Facts

Likely explanation

NHibernate's transaction integration calls flush as part of commit processing when FlushMode is Auto. This keeps the persistence context synchronized with the database before the ADO.NET transaction is committed.

Confirmed facts, version-assumed

• Default FlushMode is Auto.
• FlushMode.Commit defers automatic flushing until the transaction commit boundary.
• FlushMode.Never suppresses automatic flushing; explicit ISession.Flush() is required.
• A pre-commit flush failure typically causes transaction rollback rather than a commit with unsynchronized state.

[Review needed] The research brief states this behavior is stable across NHibernate 3.x through 5.x, but the exact wording for your installed release has not been verified against current documentation.

Steps for This Case

1. Confirm the installed NHibernate assembly version.
2. Verify configuration uses FlushMode.Auto unless you explicitly require deferred flushing.
3. Make tracked entity changes, then call ITransaction.Commit().
4. Inspect the database before disposing the session to confirm the rows are present.

Verification

session.FlushMode = FlushMode.Auto;
using (var tx = session.BeginTransaction())
{
    var entity = session.Load<MyEntity>(id);
    entity.Name = "updated";
    tx.Commit();
}

• Enable NHibernate SQL logging and confirm INSERT/UPDATE/DELETE statements appear before the ADO.NET Commit call.
• Repeat with FlushMode.Never and no explicit Flush(); changes should be absent after commit unless flushed manually.

Missing Diagnostic Detail

Which NHibernate version and FlushMode are configured? This determines whether automatic pre-commit flushing applies or whether you must call ISession.Flush() explicitly.

Assumptions and Uncertainty

• Behavior assumes NHibernate 3.x–5.x with standard ADO.NET transaction handling.
• Nested or ambient transactions may alter when the physical commit occurs relative to flush logic.
• Flush failure rollback behavior should be verified against your database provider.