Availability Guide for Application Design

What Is Application Availability?
Availability Guide for Application Design—525637-004
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Availability and Application Design
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The RSC/MP product each run partly on the workstation and partly on the server.
They enable client applications written in a standard language to access
transaction services directly.
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The Pathway Terminal Control Process (TCP) interprets traditional HP requester
programs running on NonStop systems. These programs are written in COBOL.
The Pathway/iTS product provides the TCP and the COBOL compiler.
Availability and Application Design
This subsection introduces aspects of application design that affect availability, and
points to later sections in this guide for details. The design issues outlined here
include:
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Designing highly available program modules
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Providing instrumentation
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Using a design methodology for reducing coding errors
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Changing the application online
Design Program Modules for Availability
Today, it is common for different parts of an application to run on equipment from more
than one vendor. Vendor equipment changes frequently and often requires portions of
an application to be moved from one platform to another. NonStop systems make it
easy to do this by providing open environments in which to develop and run
applications.
The Pathway transaction-processing environment also facilitates the development and
execution of traditional NonStop applications for execution on either:
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A network consisting of entirely HP NonStop systems
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A mixed network of HP NonStop systems and other systems
Fault tolerance in any application using an NonStop server can be provided by:
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Process monitors
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NonStop process pairs
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Transaction services
Process monitors are typically used to check on the continued operation of a process
or set of processes. If, for any reason, a monitored process should stop, then the
monitor simply restarts it. Process monitors are useful in making sure that server
processes are always available. Refer to Section 7, Availability Through Process-Pairs
and Monitors, for a discussion on process monitors.
NonStop process pairs provide a higher level of availability. These processes consist of
a primary process and a backup process that runs in a different processor. The backup
process takes over in the event of failure of the primary. To enable the backup to take
over, the primary process periodically checkpoints information to the backup. Refer to