Availability Guide for Application Design

Availability in the Pathway Transaction-Processing
Environment
Availability Guide for Application Design—525637-004
6-26
Availability of TCP Applications
5. The COBOL program might perform Steps 2, 3, and 4 again with additional
requests to other servers.
6. The requester ends the transaction by starting the two-phase commit protocol.
Availability of TCP Applications
TCP requesters depend on a combination of availability techniques to keep the
application online. TMF transactions provide a known point of consistency for
application restart following a failure. Immediate persistence, initialized persistence,
and process pairs are three techniques that can be used to keep the TCP process
continuously available. Automatic saving of requester context allows a backup TCP
process to be able to restart failed transactions.
What Recovery Action Is Needed?
Either the system or the application must take recovery action if any of the following
failure conditions arises:
•
A processor that is critical to the application fails.
•
The server process stops.
•
The TCP stops.
The remainder of this section discusses how the TCP, the application code, and
appropriate operation combine to protect the application from these potential failures.
How Availability Works
Availability of the TCP can be ensured by configuring the TCP to run as a process pair
or with immediate persistence. Availability of server processes is ensured by NonStop
TS/MP. Refer to NonStop TS/MP and Highly Available Server Processes on page 6-4.
TMF provides support for starting, committing, and aborting transactions, thereby
ensuring data integrity even if a failure occurs and ensuring a known point of
consistency from which to restart the application.
You can configure each TCP to run with one of three levels of availability:
•
As a process pair with continual checkpointing (the default)
•
With immediate persistence
•
Without persistence
To configure the availability of the TCP, you set the following parameters using the
PATHCOM SET TCP command:
•
The NONSTOP parameter allows you to choose whether you want the TCP to run
as a process pair. Running as a process pair offers the highest level of availability.
•
The AUTORESTART parameter allows you to select immediate persistence by
choosing how many times you want the PATHMON process to try to restart a failed
TCP. While ensuring that the TCP is always available, this option provides the user
with a noninitialized TCP following failure.