PMJSONGET(3)

NAME
pmjsonGet, pmjsonPrint, pmjsonInit, pmjsonInitIndom - JSON string helpers and metrics extraction

C SYNOPSIS
#include <pcp/pmapi.h>
#include <pcp/pmjson.h>

typedef int (*json_get)(void *buffer, int length, void *data);
int pmjsonGet(json_metric_desc *json_metrics, int nmetrics, pmInDom indom, json_get callback, void *data);
int pmjsonPrint(FILE *fp, json_flag *output_flags, const char *json_pointer, json_get callback, void *data);

int pmjsonInit(int fd, json_metric_desc *json_metrics, int nmetrics);
int pmjsonInitIndom(int fd, json_metric_desc *json_metrics, int nmetrics, pmInDom indom);

cc ... -lpcp_web

DESCRIPTION
pmjsonGet performs metric value extraction from JSON strings for a Performance Metric Domain Agent (PMDA(3)). Metrics and instances are defined by the json_metrics parameter. The result of parsing is the extraction of values for the requested metrics (as defined in the json_pointer field) in the values field.

A simple diagnostic utility for reporting JSON strings is provided as a convenience as well. Its output_flags argument allows it to support a variety of alternate output formats, including minimal JSON (no optional whitespace), pretty-printed, and a simple YAML subset.

json_metrics is the array of json_metric_desc struct which contains the json pointer to the metric data, possible flags to check the data against, number of values (in the case of an array), pmAtomValue where the data is stored, as well as the instance domain in the event the PMDA has multiple instance domains. The number of elements in the json_metrics array is indicated by the nmetrics parameter.

Interfaces for common cases are provided - pmjsonInit specifies a call to the JSON parsing and metric value extraction functionality where there are no instance domains (or the PMDA wishes to perform instance domain operations itself) and where data can be read directly from an open file descriptor. pmjsonInitIndom is similar, but makes use of the pmdaCacheStore(3) function where possible.

The JSON string must be provided by the callback function. This function is passed a memory buffer of length bytes and an opaque user data pointer which is unchanged from the original call to pmjsonGet or pmjsonPrint. The returned result must be the number of bytes written into buffer, zero when no more data is available or less than zero on error conditions.

indom is the instance domain to be passed to pmdaCacheStore.

	typedef struct json_metrics_desc {
	    char          *json_pointer;  /* json pointer to metric */
	    int           flags;          /* flags to check if set */
	    int           num_values;     /* number of values */
	    pmAtomValue   values;         /* metric value */
	    char          *dom;           /* instance domain */
	} json_metric_desc;

The JSON Pointer syntax current follows RFC6901 in specifying a metrics location within the JSON document.

EXAMPLE
For the sample JSON document:

{
    "Name": "/clever_almeida",
    "State": {
        "Dead": false,
        "Error": "",
        "ExitCode": 0,
        "FinishedAt": "2016-07-18T21:21:20.332488706Z",
        "OOMKilled": false,
        "Paused": false,
        "Pid": 0,
        "Restarting": false,
        "Running": false,
        "StartedAt": "2016-07-18T14:10:58.52487316Z"
    }
}

A possible corresponding json_metric array would be as follows for the Pid, Name, and metrics to indicate whether the container is Running, Paused, or Restarting.

static json_metric_desc json_metrics[] = {
    { "State/Pid", 0, 1, {0}, ""},
    { "Name", 0, 1, {0}, ""},
    { "State/Running", CONTAINER_FLAG_RUNNING, 1, {0}, ""},
    { "State/Paused", CONTAINER_FLAG_PAUSED, 1, {0}, ""},
    { "State/Restarting", CONTAINER_FLAG_RESTARTING, 1, {0}, ""},
};

RETURN CODE
The functions return zero on success, otherwise an error code suitable for passing to pmErrStr_r(3) is returned.

SEE ALSO
PMDA(3), pmErrStr_r(3), pmdaCacheStore(3), https://www.json.org/, https://tools.ietf.org/html/rfc7159, and https://tools.ietf.org/html/rfc6901.