Typically, only one production control region is started to support the entire online community. There are three ways to start the IMS online control region:
A Cold start starts IMS from the beginning. It is done when the nucleus of IMS changed, as a new version of IMS or when fixes have been applied to a current release. All IMS files are formatted in the startup, and nothing is recovered. Cold starts are rarely needed.
Warm starts are used most often to start IMS. It restores the environment and system files to the point at which IMS was properly brought down.
Emergency restarts are used after an uncontrolled shutdown such as a power failure. It restores IMS to the point of failure. Any data entered after the last successful commit point is lost.
A list of IMS databases available to the online system is submitted to the control region. A table of databases, called the Dynamic Allocation Table, is provided upon IMS online control region start-up. The IPC and data base administrator (DBA) have the ability to update this table once the control region becomes available. The databases are allocated exclusively to the online IMS control region and in order to access and process the databases, a program must be of the type that executes under the IMS control region. The most common types are MPP and BMP.
A list of transactions and programs available to the online system is also submitted to the control region and must be defined to IMS prior to processing. This is done at system generation time. Once the control region becomes available, the IPC and data base administrator can alter the state of transactions and programs as to their availability to IMS.
The IMS control region starts online message regions that schedule transactions for execution. The number of message regions started depends on the client's requirements. Depending on the releases of MVS and IMS/VS, as many as 255 dependent regions can be started at once. Basically, two types of message regions are started: Message Processing Regions and Batch Message Processing Regions.
The IMS message-processing region (or message region) processes online transactions that have been queued from clients or the master terminal. The message region polls the message queues searching for a transaction to process. The criteria for the search includes the transaction class, the transaction position in the queue, and the transaction classes defined for the region. Once a transaction is selected for processing, the region is no longer available to process any other transaction until the current transaction terminates.
IMS batch message processing regions process batch programs defined to have access to the control region and their allocated databases. Typically, only one or two of these regions are started, depending on the amount of batch processing required.
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