NHibernate 3.0 Cookbook

Get solutions to common NHibernate problems to develop high-quality performance-critical data access applications Master the full range of NHibernate features Reduce hours of application development time and get better application architecture and performance Create, maintain, and update your database structure automatically with the help of NHibernate Written and tested for NHibernate 3.0 with input from the development team distilled in to easily accessible concepts and examples Part of Packt’s Cookbook series: each recipe is a carefully organized sequence of instructions to complete the task as efficiently as possible

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(For more resources on NHibernate, see here.)

Using NHibernate Profiler

NHibernate Profiler from Hibernating Rhinos is the number one tool for analyzing and visualizing what is happening inside your NHibernate application, and for discovering issues you may have. In this recipe, I’ll show you how to get up and running with NHibernate Profiler.

Getting ready

Download NHibernate Profiler from http://nhprof.com, and unzip it. As it is a commercial product, you will also need a license file. You may request a 30-day trial license from the NHProf website.

Using our Eg.Core model, set up a new NHibernate console application with log4net. (Download code).

How to do it…

1. Add a reference to HibernatingRhinos.Profiler.Appender.dll from the NH Profiler download.
2. In the session-factory element of App.config, set the property generate_statistics to true.
3. Add the following code to your Main method:

   log4net.Config.XmlConfigurator.Configure();
   
   HibernatingRhinos.Profiler.Appender.
     NHibernate.NHibernateProfiler.Initialize();
   
   var nhConfig = new Configuration().Configure();
   
   var sessionFactory = nhConfig.BuildSessionFactory();
   
   using (var session = sessionFactory.OpenSession())
   {
     var books = from b in session.Query<Book>()
                where b.Author == "Jason Dentler"
                select b;
     
     foreach (var book in books)
       Console.WriteLine(book.Name);
   }

4. Run NHProf.exe from the NH Profiler download, and activate the license.
5. Build and run your console application.
6. Check the NH Profiler. It should look like the next screenshot. Notice the gray dots indicating alerts next to the Session #1 and Recent Statements.

   

7. Select Session #1 from the Sessions list at the top left pane.
8. Select the statement from the top right pane.
9. Notice the SQL statement in the following screenshot:

   

10. Click on See the 1 row(s) resulting from this statement.
11. Enter your database connection string in the field provided, and click on OK.
12. Close the query results window.
13. Switch to the Alerts tab, and notice the alert: Use of implicit transaction is discouraged.
14. Click on the Read more link for more information and suggested solutions to this particular issue.
15. Switch to the Stack Trace tab, as shown in the next screenshot:

    

16. Double-click on the NHProfTest.NHProfTest.Program.Main stack frame to jump to that location inside Visual Studio.
17. Using the following code, wrap the foreach loop in a transaction and commit the transaction:

    using (var tx = session.BeginTransaction())
    {
      foreach (var book in books)
        Console.WriteLine(book.Name);
      tx.Commit();
    }

18. In NH Profiler, right-click on Sessions on the top left pane, and select Clear All Sessions.
19. Build and run your application.
20. Check NH Profiler for alerts.

How it works…

NHibernate Profiler uses a custom log4net appender to capture data about NHibernate activities inside your application and transmit that data to the NH Profiler application.

Setting generate_statistics allows NHibernate to capture many key data points. These statistics are displayed in the lower, left-hand side of the pane of NHibernate Profiler.

We initialize NHibernate Profiler with a call to NHibernateProfiler.Initialize(). For best results, do this when your application begins, just after you have configured log4net.

There’s more…

NHibernate Profiler also supports offline and remote profiling, as well as command-line options for use with build scripts and continuous integration systems.

In addition to NHibernate warnings and errors, NH Profiler alerts us to 12 common misuses of NHibernate, which are as follows:

• Transaction disposed without explicit rollback or commit: If no action is taken, transactions will rollback when disposed. However, this often indicates a missing commit rather than a desire to rollback the transaction
• Using a single session on multiple threads is likely a bug: A Session should only be used by one thread at a time. Sharing a session across threads is usually a bug, not an explicit design choice with proper locking.
• Use of implicit transaction is discouraged: Nearly all session activity should happen inside an NHibernate transaction.
• Excessive number of rows: In nearly all cases, this indicates a poorly designed query or bug.
• Large number of individual writes: This indicates a failure to batch writes, either because adonet.batch_size is not set, or possibly because an Identity-type POID generator is used, which effectively disables batching.
• Select N+1: This alert indicates a particular type of anti-pattern where, typically, we load and enumerate a list of parent objects, lazy-loading their children as we move through the list. Instead, we should eagerly fetch those children before enumerating the list
• Superfluous updates, use inverse=”true”: NH Profiler detected an unnecessary update statement from a bi-directional one-to-many relationship. Use inverse=”true” on the many side (list, bag, set, and others) of the relationship to avoid this.
• Too many cache calls per session: This alert is targeted particularly at applications using a distributed (remote) second-level cache. By design, NHibernate does not batch calls to the cache, which can easily lead to hundreds of slow remote calls. It can also indicate an over reliance on the second-level cache, whether remote or local.
• Too many database calls per session: This usually indicates a misuse of the database, such as querying inside a loop, a select N+1 bug, or an excessive number of writes.
• Too many joins: A query contains a large number of joins. When executed in a batch, multiple simple queries with only a few joins often perform better than a complex query with many joins. This alert can also indicate unexpected Cartesian products.
• Unbounded result set: NH Profiler detected a query without a row limit. When the application is moved to production, these queries may return huge result sets, leading to catastrophic performance issues. As insurance against these issues, set a reasonable maximum on the rows returned by each query
• Different parameter sizes result in inefficient query plan cache usage: NH Profiler detected two identical queries with different parameter sizes. Each of these queries will create a query plan. This problem grows exponentially with the size and number of parameters used. Setting prepare_sql to true allows NHibernate to generate queries with consistent parameter sizes.

See also

• Configuring NHibernate with App.config
• Configuring log4net logging