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 iss  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 pmdaCacheAdd(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 pmdaCacheAdd.

            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),  pmdaCacheAdd(3),  http://json.org/,  https://tools.ietf.org/html/rfc7159,   and
       https://tools.ietf.org/html/rfc6901.