How to test thread safety of adding to a hashtable?

Question

We're using a hashtable to store sessions in a web service. I want to test thread safety of adding to this session. I wrote a Console App which generates lots of threads and adds to this session. But I am neither getting an exception nor losing any data. Am I doing anything wrong or isn't this much of operation a threat to hashtable?!

• The key is generated randomly.

• The operations done on this hashtable are fairly simple: Add, Remove, ContainsKey, [] (getting an entry) and sometimes updating the value of an entry - no foreach or loop.

  private const int Threads = 10000;
  private const int SleepLimit = 10000;
  
  
  public static void Main(string[] args)
  {
      try
      {
          Console.WriteLine("Application Started.");
  
  
      Thread[] threads = new Thread[Threads];
      for (int i = 0; i < threads.Length; i++)
          threads[i] = CreateThread();
  
  
      Console.WriteLine("All Threads Started.");
  
  
      foreach (Thread t in threads)
          t.Join();
  
  
      Console.WriteLine("All Threads Joined.");
  }
  catch (Exception ex)
  {
      Console.WriteLine("Exception occurred: " + ex.Message);
      Console.ReadLine();
  }
  
  
  }
  
  
  private static Thread CreateThread()
  {
      Thread thread = new Thread(UserFunction);
      thread.Start();
      return thread;
  }
  
  
  private static void UserFunction()
  {
      Thread.Sleep(_random.Next(SleepLimit));
      SessionManagement.AddSession("Test");
  }
  
  
  public static void AddSession(string session)
  {
      ulong hash = NextHashValue();
      while (_sessionPool.ContainsKey(hash))
      {
          Console.WriteLine(string.Format("Duplicate Session Key: {0}-PoolSize: {1}",
              hash, _sessionPool.Count);
  
  
      hash = NextHashValue();
  }
  
  
  _sessionPool.Add(hash, session);
  
  
  if (!_sessionPool.ContainsKey(hash))
      Console.WriteLine("Race problem occurred - Session Key: " + hash);
  
  
  }
  

Answer 1

I had the same problem.

The only way I know how to test this is by slowly reviewing all the possible thread/lock interaction in the source code or on a piece of paper.

A long time ago I had an app with 8 threads interacting between them, and it crashed about once time a week of constant use. After a few of these crashes I started to suspect a threading issue. So I managed to simplify a lot the locking code, and then I proved on a piece of paper that all the possible thread/lock interactions were safe. The issue disappeared.

Answer 2

I'm not sure why you re going to such trouble to verify that concurrent writes to Hashtable are going to crash your app.

Even if your test app runs safely for a long time, you should not deploy code that does not protect your Hashtable from writes that are concurrent with other read or write operations.

10000 threads is a very large number, by the way - I hope your production app does not plan on using this many.