Availability Guide for Application Design

Availability in the Pathway Transaction-Processing
Environment
Availability Guide for Application Design—525637-004
6-20
Availability of Pathsend Applications
2. The Pathsend requester formulates a request and sends it to a server class using
a server-class send operation. The transaction identifier is carried with the I/O
request to involve the actions of the server in the transaction.
3. The link manager function checks whether a link already exists with a server
process in the specified server class. If not, working with the PATHMON process, it
creates one. When a link exists, the I/O request proceeds.
4. The server processes the request and replies to the requester.
5. The requester might issue further I/O requests to the same server class or other
server classes, repeating Steps 2, 3, and 4 for each such request.
The following paragraphs explain how the Pathsend facility affects the availability of
applications. For a more detailed overview of the Pathsend facility, refer to the
Introduction to NonStop Transaction Processing.
Availability of Pathsend Applications
The Pathsend application environment depends 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. Pathsend requesters can run as
processes with immediate persistence, process pairs with initialized persistence, or as
process pairs with continual checkpointing to ensure the availability of the requester
process.
What Recovery Action Is Needed?
Either the system or application must take recovery action if any of the following failure
conditions arises; otherwise, the availability of the application is threatened.
•
A processor critical to the application fails.
•
The server process stops.
•
The Pathsend process stops.
The remainder of this subsection discusses how the Pathsend facility, the application
code, and appropriate operational procedures combine to protect the application from
these potential failures.
How Availability Works
The Pathsend facility has no features that automatically ensure availability of a
Pathsend requester. However, high availability of Pathsend applications can be
achieved through transaction protection, and process-pair requesters (or persistent
requesters) along with server replication.
Server replication is provided by NonStop TS/MP. Refer to NonStop TS/MP and Highly
Available Server Processes on page 6-4.