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What is STA/MTA vs apartments/free threads vs UI threads/worker threads? Why the name changes?

I am reading Inside COM by Dale Rogerson, and it uses the terms apartment threads and free threads to describe the different types of COM threads.

He also clarifies that these correspond directly to UI threads and worker threads:

COM uses the same two types of threads, although COM has different names for them. Instead of calling one a user-interface thread, COM uses the term apartment thread. The term free thread is used instead of worker thread. [...]

However, lots of other documentation refers to STAs and MTAs. "Single-Threaded Apartments" and "Multi-Threaded Apartments."

  • Do "apartments/free threads" and "STA/MTA" mean different things? Does Rogerson's book (1997) no longer reflect COM's threading model?
  • Why has the naming changed?

Disclaimer: I work for Microsoft.

Solution

  • It seems that the terms are interchangeable:

    COM synchronizes calls COM does not synchronize calls
    STA (preferred name) MTA (preferred name)
    "Apartment thread" Free thread
    (often) UI thread (often) Worker thread

    Descriptions and workings of OLE threading models indicates STA == apartment thread and MTA == free thread (even though both new terms use the word "apartment"):

    1. Single-threaded Apartment model (STA): One or more threads in a process use COM and calls to COM objects are synchronized by COM. Interfaces are marshaled between threads. A degenerate case of the single-threaded apartment model, where only one thread in a given process uses COM, is called the single-threading model. Previous have sometimes referred to the STA model simply as the "apartment model."

    2. Multi-threaded Apartment model (MTA): One or more threads use COM and calls to COM objects associated with the MTA are made directly by all threads associated with the MTA without any interposition of system code between caller and object. Because multiple simultaneous clients may be calling objects more or less simultaneously (simultaneously on multi-processor systems), objects must synchronize their internal state by themselves. Interfaces are not marshaled between threads. Previous have sometimes referred to this model as the "free-threaded model."

    And as quoted above apartment thread == UI thread and free thread == worker thread according to Rogerson:

    COM uses the same two types of threads, although COM has different names for them. Instead of calling one a user-interface thread, COM uses the term apartment thread. The term free thread is used instead of worker thread. [...]

    I'd love to know why the terminology changed, though.