PMDAINSTANCE(3)

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).