Recovery and restart
CICS continually records information about the state of the region and about the state of each unit of work in the region. This information is preserved and used when a region is restarted, thus enabling CICS to restart with no loss of data integrity.
If CICS shuts down leaving any units of work in-flight, these units of work are backed out when CICS restarts and locks held by these units of work are released.
A CICS region can shut down in several ways:
- Normal (or warm) shutdown
- The CICS region shuts down in response to a CICS system management command. All transactions that have already started continue to run, and CICS waits indefinitely until they are complete. Long-running transaction can cause an unacceptable delay, or require intervention by the operator.
- Immediate shutdown
- The CICS region shuts down in response to a CICS system management command. All transactions are purged from the system; units of work are not backed out, and locks are not released, until the region restarts.
- Abnormal shutdown
- The CICS region shuts down in response to an unrecoverable error in a CICS system module, in response to a request from the operating system (arising, for example, from a program check or system abend), or following a machine check or power failure. In an abnormal shutdown, CICS might not be able to perform any shutdown processing. Units of work are not backed out and locks are not released until the region restarts.
Depending on whether the region has been started and shut down
in the past, and on the way it was shut down, a CICS region can start
in several ways:
- Initial start
- The CICS region starts with no reference to any previous activity. You perform an initial start when starting a new CICS region for the first time.
- Cold start
- The CICS region starts and restores recovery information that relates to systems and resource managers that were connected when the regions shut down. It does not recover the state of local resources.
- Warm restart
- A warm restart follows a normal shutdown and restores the region to its state it was in when shutdown completed.
- Emergency restart
- An emergency restart follows an immediate or abnormal shutdown of the CICS region. During an emergency restart, CICS backs out any transactions that were in-flight at the time of shutdown and thus frees any locks that were held at the protecting resources.
In normal operation, when a CICS region starts, it examines the information recorded before the region last shut down to determine in which way to start.