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COM Threading Models

Author: Gopalan Suresh Raj
Date Submitted: Feb 07 2000
Level of Difficulty: Intermediate
Subjects Covered: COM
Pre-required Reading: None

Article ammended after feedback by Julian Templeman, thanks to Julian.

Introduction

COM uses the same threading mechanism that Win32 uses. It uses the Win32 API for creating and synchronizing threads. However, threading models are referred to differently in Win32 and COM.

The Win32 Threading Model

Win32 defines two types of threads. User-Interface threads and Worker threads. Each process in Win32 can have one or more User Interface and/or multiple Worker threads. User Interface threads have message-loops that receive events targetted to that window and are hence associated with one or more windows. Worker threads are used for background processing and are not associated with any window.

As a developer, there is one other major difference that you have to be aware of when developing Win32 threads. User Interface threads always own one or more windows. Whenever there is an event for a particular window, the UI thread takes care of calling the appropriate method. Since the message loop is executed on the same UI thread regardless of the thread that sent the message, synchronization is guaranteed by Windows. You as the programmer need not write any special code for thread synchronization. However, if you are developing worker threads, it is your responsibility to handle thread synchronization and prevent deadlocks or racing conditions.

The COM Threading Model

COM terminology to threads is slightly different. There are three types of threading models in COM. They are apartment threading, free threading and rental threading ( introduced for MTS ) The closest analogy to Win32 is that UI threads are analogically similar to the Apartment threading model and worker threads are analogically similar to the Free threading model.

Apartment Threading Model (single threaded apartments)

This model was introduced in the first version of COM with Windows NT3.51 and later Windows 95. The apartment model consists of a multithreaded process that contains only one COM object per thread. Single Threaded Apartments (STA)- This also means that each thread can be called an apartment and each apartment is single threaded. All calls are passed through the Win32 message processing system. COM ensures that these calls are synchronized. Each thread has its own apartment or execution context and at any point in time, only one thread can access this apartment. Each thread in an apartment can receive direct calls only from a thread that belongs to that apartment. The call parametes have to be marshalled between apartments. COM handles marshalling between apartments through the Windows messaging system.

Free Threading Model (multi threaded apartments)

This model was introduced with Windows NT 4.0 and Windows95 with DCOM. The free threaded model allows multiple threads to access a single COM object. Free threaded COM objects have to ensure thread synchronization and they must implement message handlers which are thread aware and thread safe. Calls may not be passed through the Win32 messaging system nor does COM synchronize the calls, since the same method may be called from different processes simultaneously. Free threaded objects should be able to handle calls to their methods from other threads at any time and to handle calls from multiple threads simultaneously. Parameters are passed directly to any thread since all free threads reside in the same apartment. These are also called Multi-Threaded Apartments ( MTA )

Both Apartment and Free Threaded Model

It is possible for a process to have both the apartment and free threaded model. The only restriction is that you can have only one free threaded apartment but you can have multiple single threaded apartments. Interface pointers and data have to be marshalled between apartments. Calls to objects within the STAs will be synchronized by Win32 whereas calls to the MTAs will not be synchronized at all.

Thread Neutral Apartment

Components that use the Thread Neutral Apartment model (TNA), mark themselves as Free Threaded or Both. Each instance of a COM class can run on a different thread each time a method is called. When a thread is executing a method in a COM object, and that method creates a new object, MTS will suspend the current thread and create a new thread to handle the new object. Like the MTA, TNA's allow more than one thread to enter an apartment. However, once a thread has entered an apartment, it obtains an apartment-wide lock and no other thread can enter that apartment until it exits. This model was introduced into MTS to ensure that context switches are faster. The MTS COM object is however oblivious to the fact about multithreading and assumes that it is single threaded.

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