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

Process/Thread 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

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

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.

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.

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.

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.

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.

Difference Between DLL and EXE  The way Windows interacts with them

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.

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.

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)

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.

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

Threading Models  Apartment Threading Model (single threaded apartments)  Free Threading Model (multi threaded apartments)  Thread Neutral Apartment Model

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.

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

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

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.

Thread Neutral Apartment Model  MTS