Isolation level serializable + EntityFramework 6.0 Concurrent Add

Question

Hi!
I'm new to database programming and entity framework and currently running into
some database behavior I don't understand. Maybe someone can explain it to me?
I'm using a very simple table to isolate the behavior:
ID(Primary Key), SSI(Indexed, duplicates allowed)
        static void AddTalkgroups(object state)
            for (int i = 0; i < 1000; i++)
                Talkgroup talk = new Talkgroup();
                talk.ID = Guid.NewGuid();
                talk.SSI = i;
                using (var context = new LeitstelleContext())
                    using (var transaction = context.Database.BeginTransaction(System.Data.IsolationLevel.Serializable))
                       // context.Database.Log = ShowCache;
                        try
                            if (context.Talkgroups.SingleOrDefault(t => t.SSI == i) == null)
                                context.Talkgroups.Add(talk);
                                context.SaveChanges();
                            transaction.Commit();
                            Console.Write("+");
                        catch (Exception e)
                            transaction.Rollback();
                            Console.Write("-");
If I start the method in 2 concurrent threads, one of both will run into deadlocks
over and over again. As intended, no duplicates of SSI are added, but I don't understand
the database behavior.(The deadlocks)
As far as I understand. The database should place a RangeI-N lock on datasets
not found in the table during the "transaction" so no other transactions would be able
to add a new dataset matching the "select"-statement of the active transaction.
If I issue a "sp_lock" command during the transaction I'm seeing some RangeS-S locks.
thanks alot for looking into this!
best regards
Johnny

Answer 1

Hi David!
If I just switch to IsolationLevel.ReadCommited I get 968 duplicates in
my example. (And no PK violations)
Talkgroup.Id  = PrimaryKey
Talkgroup.Ssi is configured as "ununique index" in the SQL-Server 2012.
class Program
        static object lockobject = new object();
        static Barrier barrier = new Barrier(3);
        static void Main(string[] args)
            LeitstelleContext context = new LeitstelleContext();
            //Talkgroup talk = new Talkgroup();
            //talk.ID = Guid.NewGuid();
            //talk.SSI = 1234;
            //context.Talkgroups.Add(talk);
            //context.SaveChanges();
            context.Database.ExecuteSqlCommand("delete Talkgroups");
            DateTime start = DateTime.Now;
            System.Threading.ThreadPool.QueueUserWorkItem(AddTalkgroups);
            System.Threading.ThreadPool.QueueUserWorkItem(AddTalkgroups);
            barrier.SignalAndWait();
            Console.WriteLine(DateTime.Now.Subtract(start).TotalSeconds.ToString());
            var liste = context.Talkgroups.GroupBy(t => t.SSI).Where(mygroup => mygroup.Count() > 1);
            Console.WriteLine(liste.Count().ToString());
            //foreach (var item in liste)
            //    Console.WriteLine(item.Key);
            Console.ReadLine();
        static void AddTalkgroups(object state)
            for (int i = 0; i < 1000; i++)
                ////context.Database.Log = ShowCache;
                //Parallel.For(6500, 7500, i =>
                Talkgroup talk = new Talkgroup();
                talk.ID = Guid.NewGuid();
                talk.SSI = i;
                using (var context = new LeitstelleContext())
                    using (var transaction = context.Database.BeginTransaction(System.Data.IsolationLevel.ReadCommitted))
                       // context.Database.Log = ShowCache;
                        try
                            if (context.Talkgroups.SingleOrDefault(t => t.SSI == i) == null)
                                context.Talkgroups.Add(talk);
                                context.SaveChanges();
                            transaction.Commit();
                            Console.Write("+");
                        catch (Exception e)
                            transaction.Rollback();
                            Console.Write("-");
            barrier.SignalAndWait();
br
Johnny