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   Programming a BEA Tuxedo Application Using COBOL

Before you start coding, make sure you have fulfilled or thought about the following:

• Prerequisites for a Multithreaded Application

• General Multithreaded Programming Considerations

• Concurrency Considerations

Prerequisites for a Multithreaded Application

Make sure your environment meets the following prerequisites before starting your development project.

• Your operating system must provide a suitable threads package supported by the BEA Tuxedo system.

  The BEA Tuxedo system does not supply tools for creating threads, but it supports various threads packages provided by different operating systems. To create and synchronize threads, you must use the functions native to your operating system. To find out which, if any, threads packages are supported by your operating system, see Platform Data Sheets, in Installing the BEA Tuxedo System.

• If you are using multithreaded servers, the resource managers used by those servers must support threads.

General Multithreaded Programming Considerations

Only experienced programmers should write multithreaded programs. In particular, programmers should already be familiar with basic design issues specific to this task, such as:

• The need for concurrency control among multiple threads

• The need to avoid the use of static variables in most instances

• Potential problems that may arise from the use of signals in multithreaded programs

These are just a few of the issues, too numerous to list here, with which we assume any programmer undertaking the writing of a multithreaded program is already familiar. These issues are discussed in many commercially available books on the subject of multithreaded programming.

Multithreading enables different threads of an application to perform concurrent operations on the same conversation. We do not recommend this approach, but the BEA Tuxedo system does not forbid it. If different threads perform concurrent operations on the same conversation, the system acts as if the concurrent calls were issued in some arbitrary order.

When programming with multiple threads, you must manage the concurrency among them by using mutexes or other concurrency-control functions. Here are three examples of the need for concurrency control.

• When multithreaded threads are operating on the same context, the programmer must ensure that functions are being executed in the required serial order. For example, all RPC calls and conversations must be compiled before tpcommit(3c) can be called. If tpcommit() is called from a thread other than the thread from which all these RPC or conversational calls are made, some concurrency control is probably required in the application.

• Similarly, it is permissible to call tpacall(3c) in one thread and tpgetrply(3c) in another, but the application must either:

  • Ensure that tpacall() is called before tpgetrply(), or

  • Manage the consequences if tpacall() is not called before tpgetrply()

• Multiple threads may operate on the same conversation but application programmers must realize that if different threads issue tpsend(3c) at approximately the same time, the system acts as though these tpsend() calls have been issued in an arbitrary order.

  For most applications, the best strategy is to code all the operations for one conversation in one thread. The second best strategy is to serialize these operations using concurrency control.

See Also

• Design Considerations for a Multithreaded and Multicontexted Application

• Writing Code to Enable Multicontexting in a Client

• Writing Code to Enable Multicontexting and Multithreading in a Server

• Writing a Multithreaded Client

• Writing a Multithreaded Server