Three structures are declared in /usr/include/pcp/pmda.h which
provide a framework for declaring the metrics and instances
supported by the PMDA.
Every instance requires a unique integer identifier and a unique
name, as defined by the structure pmdaInstid:
/*
* Instance description: index and name
*/
typedef struct {
int i_inst; /* internal instance identifier */
char *i_name; /* external instance identifier */
} pmdaInstid;
An instance domain requires its own unique identification
(pmInDom), the number of instances the domain represents, and a
pointer to an array of instance descriptions. This is defined in
the structure pmdaIndom:
/*
* Instance domain description: unique instance id,
* number of instances in this domain, and the list of
* instances (not null terminated).
*/
typedef struct {
pmInDom it_indom; /* indom, filled in */
int it_numinst; /* number of instances */
pmdaInstid *it_set; /* instance identifiers */
} pmdaIndom;
The simple PMDA has one instance domain for simple.color with
three instances (red, green and blue), and a second instance
domain for simple.now with instances which can be specified at
run-time. These instance domains are defined as:
static pmdaInstid _color[] = {
{ 0, "red" }, { 1, "green" }, { 2, "blue" }
};
static pmdaInstid *_timenow = NULL;
static pmdaIndom indomtab[] = {
#define COLOR_INDOM 0
{ COLOR_INDOM, 3, _color },
#define NOW_INDOM 1
{ NOW_INDOM, 0, NULL },
};
The preprocessor macros COLOR_INDOM and NOW_INDOM are used in the
metric description table to identify the instance domains of
individual metrics. These correspond to the serial value in the
instance domain pmInDom structure (the domain field is set by
pmdaInit(3) at run-time). The serial value must be unique for
each instance domain within the PMDA.
The indom table shown above which is usually passed to
pmdaInit(3) does not need to be created if one wants to write
one's own Fetch and Instance functions. See pmdaInit(3) for more
details.
