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Special attention must be given to programs that can run indefinitely. An example of such a program might be a server that continually listens for incoming mail connections and enqueues any received mail to PMDF. The following discussion is concerned with such programs. Programs which run and merely submit a few messages, loop over a queue of messages and then exit, or user interfaces should not take the steps described in this section.a
When PMDFinitialize is called, site-specific configuration
information is loaded. The life span of this information usually far
exceeds the running time of a program that uses it. However, this isn't
the case for a program that can run for weeks or months. When PMDF
configuration information changes, these programs need to be made aware
of the change so that they can reload this information. Subsequent
calls to PMDFdone and PMDFinitialize will not
accomplish this task: a program must exit and restart in order to
ensure that all configuration information is reloaded.
Also, a program which enqueues or dequeues messages will open the queue
cache database and possibly the PMDF log file,
mail.log_current. Care must be taken to ensure that these
files are not left open during periods of inactivity. Leaving these
files open might block activities that require exclusive access to
those files. Programs which run indefinitely enqueuing or dequeuing
messages should always call PMDFcloseQueueCache,
PMDFcloseLogFile, and, if doing message dequeue activity,
PMDFdequeueEnd before going idle. The queue cache and log
file will be automatically reopened when needed. The queue cache should
not be closed while in the middle of dequeue processing; i.e.,
PMDFcloseQueueCache should not be called while looping
over messages in a message queue with PMDFgetMessage.
PMDFcloseQueueCache should be called after
PMDFgetMessage has returned a PMDF__EOF
status and before again calling PMDFdequeueInitialize.
1.7.1 OpenVMS Considerations
The PMDF RESTART command is used after a change to the PMDF
configuration to restart components of PMDF which run indefinitely. In
addition, the PMDF CACHE/CLOSE command is used to force components of
PMDF to close the queue cache database should they have it open. One
such component of PMDF is BN_SLAVE. This component is a slave channel
program which runs as a detached process. It starts running at system
startup and continues to run, processing incoming BITNET mail, until
Jnet or the system is shut down. When configuration changes are made or
the queue cache needs to be rebuilt, a PMDF RESTART command is issued
to inform BN_SLAVE of this fact. BN_SLAVE then either exits and
restarts or closes the queue cache database at its earliest convenience.
The routine PMDFsetCallBack provides a communication path
whereby a running program can be notified when a PMDF RESTART, PMDF
SHUTDOWN, or PMDF CACHE/CLOSE command has been issued. When a RESTART
or SHUTDOWN command is issued, a running program should note this fact
and, as soon as is convenient, exit and restart or simply exit. In
response to a CACHE/CLOSE command, the queue cache should be closed as
soon as it is convenient to do so. This is accomplished with the
PMDFcloseQueueCache routine. The cache will be reopened
automatically when it is again needed. This is generally done by
PMDFenqueueMessage and PMDFgetMessage. The
queue cache should not be closed while in the middle of dequeue
processing; i.e., it should not be called while looping over
messages in a message queue. PMDFcloseQueueCache should be
called after PMDFgetMessage has returned a
PMDF__EOF status and before again calling
PMDFdequeueInitialize.
On OpenVMS systems, the communication path established by
PMDFsetCallBack is implemented using cluster-wide resource
locks. Thus, PMDF RESTART and PMDF SHUTDOWN commands issued anywhere on
an OpenVMS cluster will be seen by all users of
PMDFsetCallBack throughout the cluster.
1.7.2 UNIX & Windows Considerations
On UNIX and Windows systems, the PMDFsetCallBack facility
is non-functional. Calls to it will merely return PMDF__OK
without doing anything. Likewise for the
PMDFcancelCallBack routine.
Also on UNIX and Windows systems, the pmdf restart and
shutdown commands cannot be used to restart or shutdown
site-supplied API clients. Such clients must supply their own mechanism
for being signalled to either restart or shutdown.
a User interfaces should specify
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