The attributes of a data item determine if threads that access it can contend for it. Threads can never contend for data items defined:
Any other data items defined within the program are shared between threads, so contention is possible. Contention might occur for:
Data defined in the Local-Storage Section is traditionally used in recursive COBOL programs; for every recursion of the program a new instance of this data is allocated on the stack. Since each thread in a multi-threaded application has its own stack, this attribute is ideal for defining contention-free temporary and work items within reentrant programs. By design, a reentrant COBOL program also allows recursion within a single thread.
The disadvantage of using data defined in the Local-Storage Section is that a data item disappears when the reentrant or recursive program exits. If a program has exited, then on the next entry into that program the data items defined in the Local-Storage Section have undefined values. If a thread requires the program to preserve state across calls, some other mechanism must be used.
Data defined as thread-local, either in the Thread-Local-Storage Section, or by the THREAD-LOCAL attribute, resolves these problems. Thread-local data is unique to each thread, persists across calls and can be initialized with value clauses. Thread-local data can be viewed as a thread-specific Working-Storage data item. This kind of data is very useful for resolving contention problems in most reentrant programs. In many cases, a program that does no file handling can be made completely reentrant by simply changing the Working-Storage Section header to a Thread-Local-Storage Section header. You can fine-tune data allocation by defining all read-only constants in the Working-Storage Section, and all read-write data items in Thread-Local-Storage Section.
The use of thread-local data has the following disadvantages:
Sometimes threads require more than just private data. Most multi-threaded applications communicate and coordinate thread execution through shared data that is accessed by each thread under a strict protocol. In COBOL this is accomplished through the use of Working-Storage Section or File Section data in conjunction with the use of various synchronization primitives that resolve any possibility for destructive contention on the data. See the next section for details of the synchronization primitives.