Threading Models in Visual Basic Language

1. Threading Models in Visual Basic Language Student Name: Danyu Xu Student ID:98044

2. Process Execution of an individual program 3 Components - An executable program - Associated data needed by the Program -Execution by the Program All information the O/S needs to manage the process Thread A dispatchable unit of work An execution state(running,ready,etc.) Some per-thread static storage for local variables Access to the memory and resources of its process Process/Thread

3. Benefits of a Thread • Less time to create a new thread than a process • Less time to terminate than a process • Less time to switch between two threads within the same process • Since threads within the same process share same files and memory, they can communicate with each other without invoking the kernel

4. COM(Component Object Model) in Visual Basic • a software architecture that allows applications to be built from binary software components. • the underlying architecture that forms the foundation for higher-level software services , eg. OLE, ActiveX.

5. COM(Component Object Model) in Visual Basic (Continued) • A mechanism for keeping track of whether an object is in use and deleting it when no longer needed. • A standard error-reporting mechanism and set of error codes and values. • A mechanism for apps to exchange objects.

6. In-Process Server Component in VB-DLL • In-Process are always DLL (dynamic link libraries) files. DLL-A library containing API functions or procedures accessible to and called from an application. -ideal for implementing standard objects to reuse or share among apps. -ideal for defining interfaces implemented by other objects. -preferred to create high-performance objects without a user interface.

7. In-Process Server Component in VB-DLL (Continued) -Windows API(Application Programming Interface) has a collection of functions and procedures stored in Dynamic-Link Libraries.

8. Out-of-Process Server Component • Out-of-Process are always EXE (executable) files. -objects can execute in their own thread. -objects can be created and used both by client applications and by running the server as a standalone app.

9. Difference Between DLL and EXE • The way Windows interacts with them

10. Memory Organization of 16-bit OS Code for DLL A Data for DLL A Code for EXE B Data for EXE B Code for EXE A Data for EXE B -windows 3.x -if a program accidentally modifies another program’s memory, can bring down the system. -Every application has one and only one thread.

11. Memory Organization of 32-bit OS Code for EXE A Code for DLL A Data for EXE A Data for DLL A =============== Code for DLL A Data for DLL A Code for EXE A Data for EXE A --process space – the memory app runs in --walled off from other app --lower level physical memory is shared WinNT Each object could have its own thread, or the app ;might have a fixes number of threads and could manage its objects across the threads.

12. Communications Between Apps Under 32-bit OS • Mechanism of Marshalling • Proxy object – fake object exposes same interface as the actual object. -created by OLE. • Types of Marshalling: --Between Apps --Between networks(DCOM,overhead of marshalling function calls is negligible compared to transfer of large amount of data)

13. Features of In-Process and Out-of-Process server • EXE -It can run as a stand-alone application apart from the client -does not share same address space under the OS. • DLL -server component and client share some of the same memory. - share the same executable space.

14. Features of In-Process and Out-of-Process Server (Continued) • Performance --In-Process server shares the same process space with its client at run time --server and client share some of the same memory. --Public Creatable classes can be instantiated only as MultiUse objects. • Flexibility Out-of-Process server provide classes either SingleUse or MultiUse

15. Threading Models • Apartment Threading Model (single threaded apartments) • Free Threading Model (multi threaded apartments) • Thread Neutral Apartment Model

16. Apartment Threading Model A “logical container” that -creates an association between objects and, in some cases, threads. -a set of rules programmers must obey to receive the concurrency behavior they expect from COM environment. -system-supplied code that helps programmers manage thread concurrency with respect to COM objects. -object in each thread is unaware of objects in other threads and has its own separate copy of global data.

17. A SingleUse Class can only supply one instance of itself One Instance – one server component A MultiUse class can supply more than one instance of itself per copy of its component. One server component – many instance SingleUse/MultiUse

18. SingleUse/MultiUse (Continued) • Cons:high overhead • Pros:avoid blocking Blocking - >1 user try to call the same code at the same time.

19. Single-threaded apartment (STA) • a set of COM objects associated with a particular thread. • a place where an object or a proxy "lives."

20. Multi-Threaded Apartment (MTA) • a set of COM objects associated with a set of threads in the process such that any thread can call any object implementation directly without the interposition of system code.

21. Thread Neutral Apartment Model • MTS