NAME

       pmdaInstance - return instance descriptions for a PMDA

C SYNOPSIS

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

       int pmdaInstance(pmInDom indom, int inst, char *name, pmInResult **result, pmdaExt *pmda);

       cc ... -lpcp_pmda -lpcp

DESCRIPTION

       pmdaInstance  uses  the  standard PMDA(3) data structures to return information concerning
       the instance domain indom.

       The result structure is constructed by pmdaInstance and will contain one or more  instance
       names and/or identifiers as specified by the inst and name arguments.

       If  inst  has  the value PM_IN_NULL and name is a null string, result will contain all the
       instances names and identifiers in the instance domain.

       If inst is PM_IN_NULL but name is the name of an instance in the  instance  domain  indom,
       then  result  will  contain  the instance identifier for instance name.  Note that if name
       contains no spaces, partial matching up to  the  first  space  in  the  instance  name  is
       performed,  i.e.   ``1''  will  match  instance  name  ``1  minute''.  If name contains an
       embedded space, then no partial matching is performed and name should  match  one  of  the
       instance names exactly.

       If  name is a null string but inst is an instance identifier in the instance domain indom,
       then result will contain the name for instance inst.  The result  structure  is  allocated
       with malloc(3) and should be released by the caller with free(3).

MULTI-DIMENSIONAL INSTANCE NAMING

       Further  to  the  above description of name, the set of rules describing external instance
       names is provided in the pmdaCacheStore(3) manual page.

       Instance domains adds another dimension (set of values) to metrics.  However, this may not
       suffice  to  describe complex multi-dimensional instance domain situations.  For this case
       the approach used by a number of PMDAs is to structure the external instance names using a
       delimiter  (``/''  or  ``::''  are  most  commonly  used) to allow separation of the other
       dimensions.  In this situation, instance domain labels should be used to define names  for
       each  instance  name  component.  This allows PMAPI(3) client tools to identify and refine
       value fetches to specific dimensions of interest.

       For example, some of the Linux kernel cgroup (control group) metric instance  domains  are
       multi-dimensional.   The  instance domain represents individual values across both control
       groups and CPUs, making this  a  two-dimensional  instance  domain.   The  instance  names
       associated  with this cgroup metrics indom have been structured using the ``::'' delimiter
       to separate the two dimensions.  The instance domain itself has been labeled  accordingly,
       as follows.

       $ pminfo --desc --fetch --labels cgroup.cpuacct.usage_percpu
       cgroup.cpuacct.usage_percpu
            Data Type: 64-bit unsigned int  InDom: 3.22 0xc00016
            Semantics: counter  Units: nanosec
            inst [0 or "/::cpu0"] value 713787
            inst [1 or "/::cpu1"] value 353969
            inst [2 or "/app::cpu0"] value 407816
            inst [3 or "/app::cpu1"] value 202747
            inst [0 or "/::cpu0"] labels {"device_type":"cpu","cgroup":"/","cpu":0}
            inst [1 or "/::cpu1"] labels {"device_type":"cpu","cgroup":"/","cpu":1}
            inst [2 or "/app::cpu0"] labels {"device_type":"cpu","cgroup":"/app","cpu":0}
            inst [3 or "/app::cpu1"] labels {"device_type":"cpu","cgroup":"/app","cpu":1}

       $ pminfo --labels 3.22
       InDom: 3.22 0xc00016
            labels {"device_type":"cpu"}

       As  shown  above the individual instances inherit the labels from the instance domain, and
       the PMDA also applies additional per-instance labels describing individual cgroup and  CPU
       names.   When  this  model  has  been used by the PMDA, PMAPI clients are able to restrict
       their queries to the cgroup metric instances - in the example,  restricting  to  processor
       "cpu0"  using  the  "cpu"  label,  perhaps, or to just the "/app" cgroup metrics using the
       "cgroup" label.

       Furthermore, using this labeling scheme client tools can also correlate related  instances
       across different instance domains.

       $ pminfo --desc --fetch --labels kernel.percpu.cpu.irq.soft
       kernel.percpu.cpu.irq.soft
            Data Type: 64-bit unsigned int  InDom: 60.0 0xf000000
            Semantics: counter  Units: millisec
            inst [0 or "cpu0"] value 6770
            inst [1 or "cpu1"] value 100
            inst [0 or "cpu0"] labels {"device_type":"cpu"}
            inst [1 or "cpu1"] labels {"device_type":"cpu"}

       $ pminfo --labels 60.0
       InDom: 60.0 0xf000000
            labels {"device_type":"cpu"}

       Although  these  two  metrics have different instance domains (60.0 and 3.22 respectively)
       and are sourced from different PMDAs, the "device_type" label identifies the common device
       to which these values relate.

DIAGNOSTICS

       If  any  errors  occur  during  the  execution  of  pmdaInstance,  the result structure is
       deallocated.  If the instance domain indom is not supported by the PMDA, pmdaInstance will
       return PM_ERR_INDOM.

       If the inst or name does not correspond to any instances in the indom domain, pmdaInstance
       will return PM_ERR_INST.

CAVEAT

       The PMDA must be using PMDA_INTERFACE_2 or later, as specified in the call  to  pmdaDSO(3)
       or  pmdaDaemon(3).  If labeling of multi-dimensional instance names is performed, the PMDA
       must use PMDA_INTERFACE_7 or later.

       Because of optional partial matching up to the first space in the instance name, the  PMDA
       developer  should  ensure that if instance names are allowed to have spaces, the names are
       unique up to the first space.

SEE ALSO

       malloc(3), PMAPI(3), PMDA(3), pmdaCacheStore(3), pmdaLabel(3) and pmGetInDom(3).