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The final topics we cover in this chapter are the various primitive mechanisms used for threads, shared-memory concurrency, and signaling. In many ways, these are the assembly language of concurrency. In this chapter we ve concentrated mostly on techniques that work well with immutable data structures. That is not to say you should always use immutable data structures. It is, for example, perfectly valid to use mutable data structures as long as they are accessed from only one particular thread. Furthermore, private mutable data structures can often be safely passed through an asynchronous workflow, because at each point the mutable data structure will be accessed by only one thread, even if different parts of the asynchronous workflow are executed by different threads. This does not apply to workflows that use operators such as Async.Parallel or Async.SpawnChild that start additional threads of computation. This means that we ve largely avoided covering shared-memory primitives so far, because F# provides powerful declarative constructs such as asynchronous workflows and message passing that often subsume the need to resort to shared-memory concurrency. However, a working knowledge of thread primitives and shared-memory concurrency is still very useful, especially if you want to implement your own basic constructs or highly efficient concurrent algorithms on shared-memory hardware.

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Whereas we get only one TX lock per transaction, we can get as many TM locks as the objects we modify. Here, the interesting thing is that the ID1 column for the TM lock is the object ID of the DMLlocked object, so it is easy to find the object on which the lock is being held. An interesting aside to the TM lock: the total number of TM locks allowed in the system is configurable by you (for details, see the DML_LOCKS parameter definition in the Oracle Database Reference manual). It may, in fact, be set to zero. This does not mean that your database becomes a read-only database (no locks), but rather that DDL is not permitted. This is useful in very specialized applications, such as RAC implementations, to reduce the amount of intra-instance coordination that would otherwise take place. You can also remove the ability to gain TM locks on an object-by-object basis using the ALTER TABLE TABLENAME DISABLE TABLE LOCK command. This is a quick way to make it harder to accidentally drop a table, as you will have to re-enable the table lock before dropping the table. It can also be used to detect a full table lock as a result of the unindexed foreign key we discussed previously.

DDL locks are automatically placed against objects during a DDL operation to protect them from changes by other sessions. For example, if I perform the DDL operation ALTER TABLE T, the table T will in general have an exclusive DDL lock placed against it, preventing other sessions from getting DDL locks and TM locks on this table.

In this chapter we ve avoided showing how to work with threads directly, instead relying on abstractions such as BackgroundWorker and the .NET thread pool. If you do want to create threads directly, here is a short sample:

Note Oracle Database 11g has modified what used to be a rule. In the past, ALTER TABLE T would have an

exclusive DDL lock placed against it. In this example, table T prevents other sessions from performing DDL and acquiring TM locks (used to modify the contents of the table). Now, many ALTER commands can be performed online without preventing modifications.

open System.Threading let t = new Thread(ThreadStart(fun _ -> printfn "Thread %d: Hello" Thread.CurrentThread.ManagedThreadId)); t.Start(); printfn "Thread %d: Waiting!" Thread.CurrentThread.ManagedThreadId t.Join(); printfn "Done!" When run, this gives the following: val t : Thread Thread 1: Waiting! Thread 10: Hello Done!