Availability Guide for Application Design

Availability Through Process-Pairs and Monitors
Availability Guide for Application Design—525637-004
7-9
Receiving Information in the Passive Backup
Receiving Checkpoint Information
The CHECKMONITOR procedure receives all checkpoint information from the primary,
including:
•
File-open information
•
Critical data
•
Control-flow information
On receipt of a file-open message from the primary process, the CHECKMONITOR
procedure performs a backup open on the same file and gets an access control block
for it. A backup open is a special kind of open that allows the process or device to
identify the backup open with the open from the primary process.
Critical data is received when the primary process sends such information as part of a
restart or nonrestart checkpoint using the CHECKPOINT, CHECKPOINTMANY,
CHECKPOINTX, or CHECKPOINTMANYX procedure. CHECKMONITOR makes
copies of the checkpointed data blocks at the corresponding addresses in the backup
process and copies any file synchronization information into its open access control
block for the file.
The backup process receives control-flow information when a restart checkpoint is
issued by the primary process. The control-state information is received by way of the
stack marker for the CHECKPOINT, CHECKPOINTMANY, CHECKPOINTX, or
CHECKPOINTMANYX procedure which CHECKMONITOR uses to establish the
restart program address for the backup process.
For applications that use only nonrestart checkpoints, the backup process receives no
control-state information. Consequently, the backup process simply returns from the
CHECKMONITOR procedure if it needs to take over.
Receiving Messages From the Operating System
The backup process receives messages from the operating system to indicate that:
•
The primary process has stopped; the backup process receives the Process
Deletion message.
•
The processing unit in which the primary process was running has failed or is
otherwise inaccessible from the processor in which the backup process is running;
the backup process receives the Processor Down message.
Receipt of the Process Deletion message in the backup is automatic. The operating
system ensures that the mom process of any process receives this message. The
mom process, for unnamed processes, is usually the creator of the process. But for
process pairs, the mom of the process is always the other member of the pair. In other
words, if the primary process stops, the backup process receives the Process Deletion
message; if the backup process stops, the primary process receives a Process
Deletion message.