Metrics

Wiki ▸ API Reference ▸ Evaluator ▸ Metrics

Metrics

Metrics are a way of identifying events of interest, grouping them into bins, and reducing them to a single value. For example, given a set of "request" events, you might want to compute the number of requests in each five-minute interval over the last week. This metric, sum(request) tells you the rate of requests over time. For more details on how to define metrics, see the documentation on metric expressions.

Metrics are computed by the evaluator, via the metric/get endpoint. This endpoint returns an array of JSON-encoded metric values, which have two required properties:

• time - the metric time, in ISO 8601 format, UTC.
• value - the metric value; a number, or undefined or NaN.

If you use the HTTP GET endpoint, the metrics are returned in chronological order. For example:

[
  {"time": "2012-04-16T16:00:00.000Z", "value": 20},
  {"time": "2012-04-16T16:01:00.000Z", "value": 21},
  {"time": "2012-04-16T16:02:00.000Z", "value": 20},
  {"time": "2012-04-16T16:03:00.000Z", "value": 20},
  {"time": "2012-04-16T16:04:00.000Z", "value": 21},
  {"time": "2012-04-16T16:05:00.000Z", "value": 20},
  {"time": "2012-04-16T16:06:00.000Z", "value": 21},
  {"time": "2012-04-16T16:07:00.000Z", "value": 20},
  {"time": "2012-04-16T16:08:00.000Z", "value": 20},
  {"time": "2012-04-16T16:09:00.000Z", "value": 21},
  {"time": "2012-04-16T16:10:00.000Z", "value": 21}
]

However, if you use the WebSockets endpoint, the metrics are returned in arbitrary order, so as to give you results as quickly as possible; if some of the metrics are cached, they will be returned immediately even while Cube computes the non-cached metrics.

If you use the WebSockets endpoint, you can multiplex requests on the same socket by using a request id. For example, to get both cube_request and cube_compute metrics simultaneously:

var socket = new WebSocket("ws://localhost:1081/1.0/metric/get");

socket.onopen = function() {
  socket.send(JSON.stringify({
    "expression": "sum(cube_request)",
    "start": "2012-04-16T16:00:00Z",
    "stop": "2012-04-16T17:00:00Z",
    "step": 6e4,
    "id": "request"
  }));
  socket.send(JSON.stringify({
    "expression": "sum(cube_compute)",
    "start": "2012-04-16T16:00:00Z",
    "stop": "2012-04-16T17:00:00Z",
    "step": 6e4,
    "id": "compute"
  }));
};

socket.onmessage = function(message) {
  console.log(JSON.parse(message.data));
};

Of course, you may find it simpler to just open multiple WebSockets, rather than multiplex.