NHibernate - Best Practice for just select

Question

A have an action on my MVC application that have an id and returns a person's name.

What is the best practice for that? I'm following NHProf tips, but code sounds a little strange or something for me.

using (var session = Helper.SessionFactory.OpenStatelessSession())
{
    using (var tran = session.BeginTransaction(IsolationLevel.ReadCommitted))
    {
        return session.Query<Person>().Where(x => x.Id == id).Select(x => x.Name).SingleOrDefault();
        tran.Rollback();
    }
}

I would not use `OpenStatelessSession`the stateless session is for bulk scenarios and ignores the L1 cache. Instead of doing a linq query I would simply call `.Load(1)` or `.Get(1)` which expresses intend more than a linq query. — Andreas, May 10 '12 at 18:12

score 4 · Answer 1 · answered May 10 '12 at 13:12

4

The NHProf alert page explains it quite well I think -

http://nhprof.com/Learn/Alerts/DoNotUseImplicitTransactions

Basically it's saying if you don't manage transactions yourself, the database will create an "implicit transaction" and auto-commit for every statement, including queries. The misconception is that transactions are useful only for insert / update operations.

In your example above, it's not much of an issue as your transaction only executes a single statement anyway. If your method was running several statements however it would be good practice to wrap them in a transaction.

answered May 10 '12 at 13:12

richeym

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OK, but for simple methods should I follow NHProf and create a transaction or not? – Zote May 10 '12 at 13:21
1

The answer is only partial true, you should __always__ warp your database query in an transaction and indeed it would be an issue even if you execute a single statement. Implicit transactions are expensive! and NH won't use the L2 cache without a transaction. – Andreas May 10 '12 at 18:05

score 0 · Answer 2 · edited May 23 '17 at 11:52

0

The following is how I would approach this select:

    using (var session = Helper.SessionFactory.OpenStatelessSession())
    using (var tran = session.BeginTransaction(IsolationLevel.ReadCommitted))
    {
        try
        {
            string personName = session.Query<Person>()
            .Where(x => x.Id == id)
            .Single(x => x.Name);

            tran.Commit();
            return personName;
        }
        catch(Exception ex)
        {
            // handle exception
            tran.Rollback();
        }
    }

This SO answer gives good advice to dealing with transaction commits:

NHibernate - Is ITransaction.Commit really necessary?

With regards to your LINQ this is an interesting article on how not to approach queries using the extension method style syntax:

http://compiledexperience.com/blog/posts/how-not-to-use-linq

edited May 23 '17 at 11:52

Community

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answered May 10 '12 at 13:22

Peadar Doyle

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-1 for catching everything, doing redundant work (using and try catch). – Andreas May 10 '12 at 17:58
@Andreas as far as I'm aware using has no way of processing a thrown exception and the choice is to use a redundant try or manually disposing. Also how would you implement error handling in this case since there is a number of layers that could throw an error (NHibernate, ADO.NET, LINQ). – Peadar Doyle May 10 '12 at 22:24
2

http://stackoverflow.com/questions/6418992/is-it-a-better-practice-to-explicitly-call-transaction-rollback-or-let-an-except and for exceptions http://stackoverflow.com/questions/426346/is-this-a-bad-practice-to-catch-a-non-specific-exception-such-as-system-exceptio http://stackoverflow.com/questions/114658/catching-base-exception-class-in-net http://blogs.msdn.com/b/ericlippert/archive/2008/09/10/vexing-exceptions.aspx and don't tell me that you can properly handle stackoverflows, out of memory exc with your `tran.Rollback();`. – Andreas May 11 '12 at 07:55
2

Thanks for the links, I wasn't aware that transactions were implicitly rolled back. Also I should have put the "//handle exception" after Rollback() since I intended it as a place holder for error handling. I appreciate what you're saying with regards to catch all exception handling and it probably isn't the best thing to do here. – Peadar Doyle May 11 '12 at 08:26

score 0 · Answer 3 · answered May 10 '12 at 17:27

do not create multiple sessions over one HTTP request, ideally what you need to do is open a session and a corresponding transaction at the request scope and use this session throughout all your actions.

Here is a blogpost explaining how to achieve it: http://hackingon.net/post/NHibernate-Session-Per-Request-with-ASPNET-MVC.aspx

I would suggest the use of an IOC container and create a class that creates the session for you and scope this class to the request scope.

There are lots of resources on the web to approach this problem.. Google it..

While one session per request is sound advice, it's not a hard and fast rule, and having one transaction per request is bad advice, plain and simple. — Spivonious, Jul 22 '16 at 16:41

NHibernate - Best Practice for just select

3 Answers3