Availability Guide for Application Design

Availability in the Pathway Transaction-Processing
Environment
Availability Guide for Application Design—525637-004
6-6
Availability of NonStop TS/MP Server Processes
1. TMF backs out any uncommitted transactions that the server was involved in at the
time of the failure. In doing so, the database returns to a consistent state and each
requester process resumes at the start of the aborted transaction. Refer to
Section 4, Data Protection and Recovery, for a discussion of how TMF does this.
2. The requester or client restarts the transaction.
3. When the replayed transaction needs services from the server process, linkage
with a server process is transparently reestablished. The PATHMON process and
the TCP perform this function for Pathway/iTS requesters. The link manager
function works with the PATHMON process to perform this function for Pathsend
requesters. Note that servers must be designed so that the linkage can be
established with any available server process in the server class, not necessarily
the process that was restarted by the PATHMON process.
Restarting the Server
Restart of the failed server process depends on whether it was a static process or a
dynamic process. Static server processes are started when the server class is started.
Dynamic server processes are started only when the workload of the application needs
extra processing capability; dynamic processes stop again when they are no longer
required.
In the case of a failed processor, static server processes are immediately restarted in
the next configured processor. If only one processor is configured, then no servers will
be started until that processor is available.
The Role of the PATHMON Process
The PATHMON process is critical to keeping the Pathway environment running. Using
information stored in a configuration file, the PATHMON process automatically restarts
any stopped servers, TCPs, or terminals in a Pathway system. These objects are
restarted at their initialization level, allowing them to resume processing immediately.
Dynamic server processes are not restarted. However, the PATHMON process will
start new dynamic servers as required to handle the workload.
After the failed processor is reloaded, it will not be immediately repopulated with
servers. It will be used only when PATHMON needs to start new server processes
because of workload or failure of another processor.
If a static server process unexpectedly stops for some other reason—for example, it
terminates itself—then the process is restarted in the same processor.
The Role of the File System
Server and requester processes depend on the reliability of file-system software to
protect the application against lost messages. The file system transparently resends
any lost messages.