Concurrency Violation in NHibernate( c#) example

Question

For quite some time , I was reading about the optimistic concurrency in NHibernate. If what i understood was correct then the below sample should hold good.

Consider two transactions T1 and T2.

1. When T1 and T2 are done simultaneously , the state(DB entries) gets updated with the values of the most latest update.(T1 or T2).

Though it seems to be conceptually sound , how do i simulate this for the purpose of understanding and integration testing.?

Can someone help me with a sample c# code.?

Thanks ,

vijay

Answer 1

Conceptually :

1. use 2 threads for performing T1 and 2; and a shared lock to synchronize access
2. thread1 acquires lock and performs T1
3. thread2 acquires the same lock and performs T2
4. one of the transactions will fail with Optimistic Concurrency Exception

Answer 2

Now after a lot of googling , i have found out a fairly simple way to do that .The following are the steps to reproduce this.

1. Have a Thread.Sleep() between the Get() and Update() methods, for only one user(process 1 ).

2. When process 1 is running , start process 2 which doesnt encounter the Thread.Sleep() and completes the update before the process 1 does .

3. Now process 2 has modified the data in the data base, now when process 1 tries to update the data, NHibernate throws a stale object exception.

Please refer the following code snippet.

public void Update(int empid)
    {
        Employee person = this.dalService.GetByEntityId(empid);
        person.Name = "Process 1";

        // At this point , Update the Person table  manually using raw sql query
        // such as this 'Update Person Set Name = 'Process 2'where empid = @empid;


        // When the update is performed , it is expected to throw the exception , as the in memory data
        // is different from the one in database.
        if (username.equals("Bob"))
        {
            Thread.Sleep(50000);
            // If this is a website, have the above condition, so that it is simulated only for one user.
        }

        this.dalService.Update(person);
    }