NAME

       pmdaEventNewQueue, pmdaEventNewActiveQueue, pmdaEventQueueHandle, pmdaEventQueueAppend,
       pmdaEventQueueShutdown, pmdaEventQueueRecords, pmdaEventQueueClients, pmdaEventQueueCounter,
       pmdaEventQueueBytes, pmdaEventQueueMemory - utilities for PMDAs managing event queues

C SYNOPSIS

       #include <pcp/pmapi.h>
       #include <pcp/pmda.h>

       int pmdaEventNewQueue(const char *name, size_t maxmem);
       int pmdaEventNewActiveQueue(const char *name, size_t maxmem,  int nclients);
       int pmdaEventQueueHandle(const char *name);
       int pmdaEventQueueAppend(int handle, void *buffer, size_t bytes, struct timeval *tv);
       int pmdaEventQueueShutdown(int handle);

       typedef int (*pmdaEventDecodeCallBack)(int, void *, int, struct timeval *, void *);
       int  pmdaEventQueueRecords(int  handle,  pmAtomValue  *avp, int context, pmdaEventDecodeCallBack decoder,
               void *data);
       int pmdaEventQueueClients(int handle, pmAtomValue *avp);
       int pmdaEventQueueCounter(int handle, pmAtomValue *avp);
       int pmdaEventQueueBytes(int handle, pmAtomValue *avp);
       int pmdaEventQueueMemory(int handle, pmAtomValue *avp);

       cc ... -lpcp_pmda -lpcp

DESCRIPTION

       A Performance Metrics Domain Agent (PMDA) that exports  event  records  must  effectively  act  an  event
       multiplexer.   Events  consumed by the PMDA may have to be forwarded on to any number of monitoring tools
       (or "client contexts").  These tools may be requesting events at different sampling  intervals,  and  are
       very  unlikely  to request an event at the exact moment it arrives at the PMDA, making some form of event
       buffering and queueing scheme a necessity.  Events must be held by the PMDA until either  all  registered
       clients  have been sent them, or until a memory limit has been reached by the PMDA at which point it must
       discard older events as new ones arrive.

       The routines described here are designed to assist the PMDA developer in managing  both  client  contexts
       and  queues  of  events at a high level.  These fit logically above lower level primitives, such as those
       described in pmdaEventNewArray(3), and shield the average PMDA from  the  details  of  directly  building
       event record arrays for individual client contexts.

       The  PMDA  registers  a  new  queue  of events using either pmdaEventNewQueue or pmdaEventNewActiveQueue.
       These  are  passed  an  identifying  name  (for  diagnostic  purposes,  and  for  subsequent  lookup   by
       pmdaEventQueueLookup)  and maxmem, an upper bound on the memory (in bytes) that can be consumed by events
       in this queue, before beginning to discard them (resulting in "missed" events for any client that has not
       kept  up).   If  a queue is dynamically allocated (such that the PMDA may already have clients connected)
       the pmdaEventNewActiveQueue interface should be used, with the additional numclients parameter indicating
       the  count  of  active  client connections.  The return is a negative error code on failure, suitable for
       decoding by the pmErrStr(3) routine.  Any non-negative value indicates success,  and  provides  a  handle
       suitable for passing into the other API routines.

       For  each new event received by the PMDA, the pmdaEventQueueAppend routine should be called, placing that
       event into the queue identified by handle.  The event itself must be contained in the passed  in  buffer,
       having  bytes  length.   The  timestamp  associated  with the event (time at which the event occurred) is
       passed in via the final tv parameter.

       In the PMDAs specific implementation of its fetch callback, when values for an  event  metric  have  been
       requested,  the  pmdaEventQueueRecords routine should be used.  It is passed the queue handle and the avp
       pmAtomValue structure to fill with event records, for the client making that fetch request (identified by
       the  context  parameter).   Finally,  the  PMDA  must  also  pass  in an event decoding routine, which is
       responsible for decoding the fields of a single event into the individual event parameters of that event.
       The  data parameter is an opaque cookie that can be used to pass situation-specific information into each
       decoder invocation.

       Under some situations it is useful for the PMDA to export state about the queues under its control.   The
       accessor    routines    -    pmdaEventQueueClients,    pmdaEventQueueCounter,   pmdaEventQueueBytes   and
       pmdaEventQueueMemory provide a mechanism for querying a queue by its handle and filling in a  pmAtomValue
       structure that the pmdaFetchCallBack method should return.

SEE ALSO

       PMAPI(3), PMDA(3), pmdaEventNewClient(3) and pmdaEventNewArray(3).