A Performance Metrics Domain Agent (PMDA) that exports event records
needs to track which clients are connected to it, in order that it
can track which events have been sent to which clients so far. Only
once an event has been sent to all monitoring tools that registered
an interest can the event be discarded and any memory reclaimed.
The PMDA(3) library provides callback routines for PMDA developers to
provide custom handling of client connections and disconnections. If
the PMDA is making use of the event queueing mechanism provided by
pmdaEventNewQueue(3) and friends, client connections and
disconnections must be tracked via calls to pmdaEventNewClient and
pmdaEventEndClient respectively. This allows the library to keep
track of when events can be discarded from a queue, for example, for
the context specified. This parameter is passed into the
e_endCallBack function directly, and for other callback functions is
available via the e_context field of the pmdaExt structure.
Additionally, it can be queried at any time using pmdaGetContext(3).
Sometimes it is useful for the PMDA to export a metric indicating the
current count of attached clients - this is available using the
pmdaEventClients routine, which will fill in the avp pmAtomValue
structure on behalf of a PMDA fetch callback routine.
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Performance Co-Pilot PCP PMDAEVENTCLIENT(3)