Availability Guide for Application Design

Availability in the Pathway Transaction-Processing
Environment
Availability Guide for Application Design—525637-004
6-3
Availability Concepts Used in Pathway Applications
•
Process pairs with initialized persistence
A process runs with a backup process that takes over if the primary process stops.
The backup process is preinitialized by a single checkpoint operation that occurs
when the primary process finishes its initialization phase.
•
Process pairs with continual checkpointing
A process runs with a backup process that takes over if the primary process stops.
In this case, state information is continually checkpointed to the backup process so
that the backup process is able to take over processing from where the primary
process left off.
•
File system retries
If a failure occurs during an I/O operation, the file system has the capability to send
the I/O request again when function is restored. This technique is used, for
example, when sending nontransactional requests to the disk process and the
primary disk process fails.
•
ransaction Management Facility (TMF) transactions
The transaction concept, as explained in Section 4, Data Protection and Recovery,
provides a known point of consistency for restart following a failure. Availability is
enhanced because the original transaction is backed out, leaving the database
exactly as the transaction found it. The new transaction can then execute as it
would have the first time.
•
Client/requester context
The client process or requester process can save its execution context
immediately before starting a TMF transaction. The client or requester process can
use the saved context to restart a failed transaction.
•
Process replication
More than one instance of the same program runs in separate processors to make
sure that there is always one copy of the process available in the event of a
processor failure. Server processes often execute this way to provide server
availability in addition to providing the ability to service a greater number of
requesters and balance the load.
Each of the subsections that follow provides information on how these techniques are
used to provide availability to server programs, RSC/MP applications, Pathsend
applications, and TCP requesters. Summary and Comparison of Application
Components on page 6-28 provides tables that summarize the availability features that
apply to the major components of Pathway applications.