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An unsafe procedure can be made thread safe and able to beserialized by surrounding the procedure with statements to lock and unlock a mutex.Example 7-1 shows three simplified implementations of fputs() , initially thread unsafe.

Next is a serializable version of this routine with a single mutexprotecting the procedure from concurrent execution problems. Actually, the single mutex is stronger synchronizationthan is usually necessary. When two threads are sending output to different filesby using fputs(), one thread need not wait for the other thread. Thethreads need synchronization only when sharing an output file.

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The last version is MT-safe. This version uses one lock for eachfile, allowing two threads to print to different files at the same time.So, a routine is MT-safe when the routine is thread safe, and theroutine's execution does not negatively affect performance.

When no sharing is intended, give each thread a private copy of the data. When sharing is important, provide explicit synchronization to make certain that the program behaves in a deterministic manner.

An unsafe procedure can be made thread safe and serializable by surrounding it with statements to lock and unlock a mutex. Example 6-1 shows three simplified implementations of fputs(), initially thread unsafe.

Next is a serializable version of this routine with a single mutex protecting the procedure from concurrent execution problems. Actually, this is stronger synchronization than is usually necessary. When two threads are sending output to different files using fputs(), one need not wait for the other--the threads need synchronization only when they are sharing an output file.

The last version is MT-safe. It uses one lock for each file, allowing two threads to print to different files at the same time. So, a routine is MT-safe when it is thread safe and its execution does not negatively affect performance.

password and session management using additional options available with OCISessionBegin().OCI support for thread safety and multithreaded application development. TransactionsRelease 8i of the Oracle Call Interface provides a set of API calls to support operations on both local and global transactions. These calls include object support, so that if an OCI application is running in object mode, the commit and rollback calls will synchronize the object cache with the state of the transaction.

When multiple threads make OCI calls, there are no side effects between threads.Users who do not write multithreaded programs do not pay a performance penalty for using thread-safe OCI calls.Use of multiple threads can improve program performance. Gains may be seen on multiprocessor systems where threads run concurrently on separate processors, and on single processor systems where overlap can occur between slower operations and faster operations. Thread Safety and Three-Tier ArchitecturesIn addition to client-server applications, where the client can be a multithreaded program, a typical use of multithreaded applications is in three-tier (also called client-agent-server) architectures. In this architecture the client is concerned only with presentation services. The agent (or application server) processes the application logic for the client application. Typically, this relationship is a many-to-one relationship, with multiple clients sharing the same application server.

If an application (running in OCI_THREADED mode) maintains multiple environment handles, each of which has one connection which can spawn multiple threads, the programmer has the following options:Pass a value of OCI_NO_MUTEX for the mode of OCIEnvInit(). In this case the application must mutex OCI calls made on the same environment handle by itself. This has the advantage that the mutexing scheme can be optimized based on the application design. The programmer must also insure that only one OCI call is in process on the environment handle connection at any given time.Pass a value of OCI_DEFAULT to OCIEnvInit(). In this case, the OCI library automatically gets a mutex on every OCI call on the environment handle.If an application running in OCI_THREADED mode maintains one or more environment handles, each of which has multiple connections, it also has the following options:Pass a value of OCI_NO_MUTEX for the mode of OCIEnvInit(). In this case the application must mutex OCI calls by made on the same environment handle itself. This has the advantage that the mutexing scheme can be optimized based on the application design. The programmer must also insure that only one OCI call is in process on the environment handle connection at any given time.Pass a value of OCI_DEFAULT to OCIEnvInit(). In this case, the OCI library automatically gets a mutex on every OCI call on the same environment handle.In this case, however, the programmer should be aware that if the application has two calls on the same environment handle, and one call operating on the server is mutexed, application performance can degrade if the mutexed call is long-running, thus tying up the server connection.

By default, ThreadSafe is set to No to specify that your connectiondoes not use the threadsafe client libraries. If you set ThreadSafeto Yes, your connection takes advantage of the threadsafe clientlibraries.

When to use Oracle and Informix provide support for thread safety in their clientlibraries. When you are using the Oracle or Informix database interface tobuild multi-threaded applications in PowerBuilder, set the ThreadSafe parameterto Yes to use threadsafe client libraries. This prevents possibleside effects among multiple threads of execution making calls tothe database server. Your application might incur a performancepenalty when you use the threadsafe client libraries.

By default, the client software (and, thus, PowerBuilder) assumesthat you are building an application in a single-threaded environmentthat does not need the threadsafe client libraries. This defaultensures that single-threaded applications do not incur the performancepenalty associated with using threadsafe libraries. Therefore, ifyou are building single-threaded applications, you can leave theThreadSafe parameter set to No (the default).

Threads are lightweight processes that exist within a larger process. Threads share the same code and data segments but have their own program counters, machine registers, and stacks. Global and static variables are common to all threads, and a mutual exclusivity mechanism is required to manage access to these variables from multiple threads within an application.

Once spawned, threads run asynchronously with respect to one another. They can access common data elements and make OCI calls in any order. Because of this shared access to data elements, a synchronized mechanism is required to maintain the integrity of data being accessed.

The mechanism to manage data access takes the form of mutexes (mutual exclusivity locks), that is implemented to ensure that no conflicts arise between multiple threads accessing shared internal data that are opaque to users. In OCI, mutexes are granted for each environment handle.

The thread safety feature of the Oracle Database server and OCI libraries allows developers to use the OCI in a multithreaded environment. Thread safety ensures code can be reentrant, with multiple threads making OCI calls without side effects.

Use of multiple threads can improve program performance. Gains may be seen on multiprocessor systems where threads run concurrently on separate processors, and on single processor systems where overlap can occur between slower operations and faster operations.

In addition to client/server applications, where the client can be a multithreaded program, a typical use of multithreaded applications is in three-tier, or client-agent-server, architectures. In this architecture the client is concerned only with presentation services. The agent, or application server, processes the application logic for the client application. Typically, this relationship is a many-to-one relationship, with multiple clients sharing the same application server.

The server tier in this scenario is a database. The applications server, or agent, is very well suited to being a multithreaded application server, with each thread serving a single client application. In an Oracle environment this application server is an OCI or precompiler program.

In order to take advantage of thread safety, an application must be running on a thread-safe operating system. The application specifies that it is running in a multithreaded environment by making an OCIEnvNlsCreate() call with OCI_THREADED as the value of the mode parameter.

If an application is single-threaded, whether or not the operating system is thread-safe, the application must pass a value of OCI_DEFAULT to OCIInitialize() or OCIEnvNlsCreate(). Single-threaded applications that run in OCI_THREADED mode may incur lower performance.

If a multithreaded application is running on a thread-safe operating system, the OCI library will manage mutexes for the application for each environment handle. An application can override this feature and maintain its own mutex scheme by specifying a value of OCI_NO_MUTEX in the mode parameter of the OCIEnvCreate() call.

If an application is mixing later release and 7.x OCI calls, and the application has been initialized as thread-safe (with the appropriate calls of the later release), it is not necessary to call opinit() to achieve thread safety. The application will get 7.x behavior on any subsequent 7.x function calls.

The OCIThread package provides a number of commonly used threading primitives. It offers a portable interface to threading capabilities native to various operating systems, but does not implement threading on operating systems that do not have native threading capability.

OCIThread does not provide a portable implementation, but it serves as a set of portable covers for native multithreaded facilities. Therefore, operating systems that do not have native support for multithreading will only be able to support a limited implementation of the OCIThread package. As a result, products that rely on all of OCIThread's functionality will not port to all operating systems. Products that must port to all operating systems must use only a subset of OCIThread's functionality. 2351a5e196