Temporary storage

Temporary storage is the primary CICS® facility for storing data that must be available to multiple transactions. Data items that are in temporary storage are kept in queues whose names are assigned dynamically by the program that stores the data. You do not need to define temporary storage queues in the resource definitions. A temporary storage queue that contains multiple items can be thought of as a miniature file whose records can be addressed either sequentially, or directly by item number. If a queue contains only a single item, it can be thought of as a named scratchpad area.

Temporary storage is implemented in two different ways. Which one is used for a particular queue is determined by what is specified on the command that creates the first item. MAIN means that the queue is kept in memory, and AUXILIARY means that the queue is written to disk storage. For either method, CICS maintains an index of items in main storage.

Both these methods have characteristics that you should keep in mind:

• Main temporary storage requires much more virtual storage than auxiliary temporary storage does. In general, you should use it only for small queues that have short lifetimes, or are accessed frequently. Auxiliary temporary storage is specifically designed for relatively large amounts of data that have a relatively long lifetime, or that are accessed infrequently.
• You can make queues that are in auxiliary storage recoverable, but you cannot make queues that are in main storage recoverable.

  Only one transaction at a time can update a recoverable temporary storage queue. If you choose to make queues recoverable, keep in mind the possibility of enqueues.

• If a task tries to write to temporary storage and no space is available, CICS suspends it. The task is not resumed until some other task frees the necessary space in the file. This can produce unexplained response delays, especially if the waiting task owns exclusive-use resources, in which case all other tasks that need those resources must also wait. You can use the NOSUSPEND option to avoid this; see The NOSUSPEND option.
• It can be more efficient to use main temporary storage exclusively in low-volume systems that have no need for recovery.

The following points apply to temporary storage in general:

• You must use a CICS command every time data is written to, or read from, a temporary storage queue, and CICS must use its internal index to find, or insert, the data. This means that the overhead for using main temporary storage is greater than for the CWA or terminal user area. With auxiliary storage, often the most frequently used, file I/O usually also occurs, which increases the overhead even more.
• You do not need to allocate temporary storage until it is required; you need to keep it only as long as it is required, and the item size is not fixed until you issue the command that creates it. This makes it a good choice for relatively high-volume data, and for data that varies in length or duration.
• The fact that temporary storage queues are named when they are created provides a very powerful form of random access to saved data. You can access scratchpad areas for terminals, file records, and so on, simply by including the terminal name or record key in the queue name.
• Resource-level protection is available for temporary storage.