PMDF System Manager's Guide


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34.1 Basics

It is important that you understand some of the basics of how PMDF works. This section attempts to present a very basic overview.

When PMDF receives a message, PMDF writes the message as one or more disk files in the PMDF queue directories. These files represent copies of the message: at least one copy for each channel to which the message must be enqueued. It is crucial that the received message be written to a non-volatile medium such as a magnetic disk file: were the system to crash before the message could be sent on to its final destination, then the message might be lost. For this reason, PMDF always writes received messages to disk before giving a positive acknowledgement of receipt to the transmitter of the message. After a message is received, an entry is made in the queue cache database.

Once PMDF has received a message, it attempts to deliver it. This is done by submitting a processing job for each channel to which the message is enqueued. These jobs attempt to send the message to wherever it is next bound (as determined by PMDF's domain rewriting rules). If a job is successful, then the message copy it was handling is deleted and the corresponding entry removed from the queue cache database. If not successful, then the message copy is left on disk for a subsequent delivery attempt.

So, the normal mode of operation is: A message is received, it is written to a file, a record is added to the queue cache database, a processing job is started, the job reads the message file, the job then deletes the file, the record is removed from the queue cache database.

Given this basic scenario, several things should be clear:

The suggestions in the first bullet item above, as well as several others, are explored in the remainder of this chapter.
On OpenVMS, note that the queue cache database is an RMS keyed, indexed file. As such, it can be tuned using any of the standard RMS tuning tools. Should you want to tune it differently from the FDL parameters in the file PMDF_COM:queue_cache.fdl, you might first consult with Process Software.

Note

1 With certain provisos, however, storing the queue cache database on a virtual RAM disk can be safe enough --- see the discussion later in this chapter. And a reliable, battery-backed, solid-state RAM disk can be safe enough for the message store.


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