.Net Session Nhibernate Flush

Understanding the `ISession.Flush` in NHibernate: When and Why to Use It

Working with NHibernate can sometimes feel overwhelming, especially with functions like ISession.Flush. Many developers struggle to grasp its utility and effective implementation. In this guide, we’ll break down what session.Flush is, when to use it, and why it matters in your database transactions.

The Core Challenge: Navigating Session Management

The confusion often arises around the relationship between three critical operations in NHibernate: Flush, Commit, and Close. Below is a brief overview:

Flush: Updates the database with changes made in-memory.
Commit: Completes a transaction, ensuring all changes are either confirmed or rolled back in case of an error.
Close: Ends the session and releases any connections.

Understanding when to use Flush alongside Commit is essential for effective data handling.

When to Use `ISession.Flush`

Here’s a straightforward guideline to help you decide when to use Flush effectively:

Always Use Transactions: Begin by enclosing your database operations in a transaction. This provides an easy rollback mechanism for error handling.
Avoid Using Close() Directly: Instead, opt for using statements that manage the ISession lifecycle automatically. This helps to prevent resource leaks and improves code readability.

NHibernate Default Flushing Behavior

According to NHibernate documentation, flushing of the ISession happens automatically in several scenarios:

During certain invocations of Find() or Enumerable() methods.
Upon calling NHibernate.ITransaction.Commit(); this flushes and commits the transaction in one go.
Explicitly by calling ISession.Flush().

The Flush Execution Order

When a flush occurs, NHibernate executes SQL statements in the following order:

Inserts for all entities (in the order they were saved).
Updates for all entities.
Deletions for collections.
Updates or insertions for collection elements.
Finally, deletions of entities (in the order corresponding to their deletion calls).

Important Considerations

Implicit vs. Explicit Flushing: If you do not explicitly call Flush(), there are no guarantees about when the session execution occurs, just the order.
Flush Modes: You can configure the FlushMode for your sessions with three modes: flush only on commit, flush automatically, or never flush unless specific calls are made. This is particularly useful for long-running transactions.

Flushing, Committing, and Closing Phases

Ending a session involves these distinct phases, as highlighted in the documentation:

Flushing the Session: Call Flush() to ensure changes synchronize with the database when not using the ITransaction API.
Committing the Transaction: If using the ITransaction API, this is automatically handled, otherwise, it must be done manually.
Closing the Session: Always close your sessions to release connections properly.

Code Snippet Illustrations

Here’s how you might typically manage these phases with NHibernate:

using (var session = sessionFactory.OpenSession())
{
    using (var transaction = session.BeginTransaction())
    {
        // Your database operations here
        session.Flush(); // Ensure changes are applied
        transaction.Commit(); // Commit transaction
    }
}

If you manage ADO.NET transactions directly:

session.Flush(); // Explicitly flush
currentTransaction.Commit(); // Commit the ADO.NET transaction

Conclusion: Mastering NHibernate Flushing

While ISession.Flush can seem daunting, understanding its relationship with transaction management is crucial. Following best practices—like always using transactions, properly handling session closure, and knowing when to flush—will make your experience with NHibernate easier and more productive.

By following the guidance in this article, you’ll be better equipped to manage your database operations effectively, ensuring your data integrity and session efficiency is always top-notch.

Understanding the ISession.Flush in NHibernate: When and Why to Use It