Availability Guide for Application Design

What Is Application Availability?
Availability Guide for Application Design—525637-004
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Design Program Modules for Availability
RSC/MP
The NonStop RSC/MP product provides for highly available applications on hardware
from multiple vendors. While not strictly open, RSC/MP allows the client part of the
application to be written in a standard language such as COBOL, C, or C++ while
gaining access to HP availability through its transaction semantics. The Transaction
Delivery Process (TDP) is the message control or gateway process that persistently
runs on the HP system and acts on behalf of the client process running on the
workstation.
Pathsend
The Pathsend interface to NonStop TS/MP allows requester programs to be written in
any supported language. While such processes can fulfill the client function of the
application itself, Pathsend requesters more often provide message control to client
processes running on equipment not made by HP. For example, the TDP part of
RSC/MP is a Pathsend requester and acts as a gateway to client processes that run
on workstations, PCs, and Macintosh computers.
A Pathsend process can run with persistence or as a process pair, providing a backup
process that can restart transactions if a failure in the primary Pathsend process
occurs.
Pathway/iTS
Pathway/iTS provides a terminal control process (TCP) that typically runs as a process
pair although it can run as a single process with immediate persistence. TCP is a
multithreaded requester process that interprets individual requester programs written in
COBOL as separate threads.
TCP helps provide availability through transaction semantics, which provide a stable
restart point for requesters if needed, and through running as a process pair. If a failure
of some kind does occur, the TCP can switch to its backup process, which restarts the
requesters at the beginning of any incomplete transactions.
The PATHMON process helps to ensure availability by always making sure that a
server process is available and by working with the TCP to provide the linkage
between the requester program and the server. Retries by the file system ensure that
messages between the TCP and PATHMON are not lost.
Refer to Section 6, Availability in the Pathway Transaction-Processing Environment, for
a discussion of RSC/MP, Pathsend, and Pathway/iTS and Section 4, Data Protection
and Recovery, for more information about transaction protection.
Applications That Run in a Mixed Environment
Many applications on HP NonStop systems are made of modules that execute in
different transaction-processing environments. For example, an application written in
the NonStop Tuxedo environment or NonStop DCE might need the services of a