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This section covers the important aspect of metrics management in k6. How and what kind of metrics k6 collects automatically (built-in metrics), and what custom metrics you can make k6 collect.

Built-in metrics

The built-in metrics are the ones you can see output to stdout when you run the simplest possible k6 test, e.g. k6 run github.com/loadimpact/k6/samples/http_get.js which will output something like the below:

output-to-stdout

All the http_req_... lines and the ones after them are built-in metrics that get written to stdout at the end of a test.

The following built-in metrics will always be collected by k6:

Metric NameTypeDescription
vusGaugeCurrent number of active virtual users
vus_maxGaugeMax possible number of virtual users (VU resources are pre-allocated, to ensure performance will not be affected when scaling up the load level)
iterationsCounterThe aggregate number of times the VUs in the test have executed the JS script (the default function).
iteration_durationTrendThe time it took to complete one full iteration of the default/main function.
dropped_iterationsCounterIntroduced in k6 v0.27.0, the number of iterations that could not be started due to lack of VUs (for the arrival-rate executors) or lack of time (due to expired maxDuration in the iteration-based executors).
data_receivedCounterThe amount of received data. Read this example to track data for an individual URL.
data_sentCounterThe amount of data sent. Read this example to track data for an individual URL.
checksRateThe rate of successful checks.

HTTP-specific built-in metrics

built-in metrics will only be generated when/if HTTP requests are made:

Metric NameTypeDescription
http_reqsCounterHow many HTTP requests has k6 generated, in total.
http_req_blockedTrendTime spent blocked (waiting for a free TCP connection slot) before initiating the request. float
http_req_connectingTrendTime spent establishing TCP connection to the remote host. float
http_req_tls_handshakingTrendTime spent handshaking TLS session with remote host
http_req_sendingTrendTime spent sending data to the remote host. float
http_req_waitingTrendTime spent waiting for response from remote host (a.k.a. \"time to first byte\", or \"TTFB\"). float
http_req_receivingTrendTime spent receiving response data from the remote host. float
http_req_durationTrendTotal time for the request. It's equal to http_req_sending + http_req_waiting + http_req_receiving (i.e. how long did the remote server take to process the request and respond, without the initial DNS lookup/connection times). float

Accessing HTTP timings from a script

If you want to access the timing information from an individual HTTP request, the built-in HTTP timing metrics are also available in the HTTP Response object:

import http from 'k6/http';
export default function () {
  var res = http.get('http://httpbin.org');
  console.log('Response time was ' + String(res.timings.duration) + ' ms');
}

In the above snippet, res is an HTTP Response object containing:

PropertyDescription
res.bodystring containing the HTTP response body
res.headersobject containing header-name/header-value pairs
res.statusinteger containing HTTP response code received from server
res.timingsobject containing HTTP timing information for the request in ms
res.timings.blocked= http_req_blocked
res.timings.connecting= http_req_connecting
res.timings.tls_handshaking= http_req_tls_handshaking
res.timings.sending= http_req_sending
res.timings.waiting= http_req_waiting
res.timings.receiving= http_req_receiving
res.timings.duration= http_req_duration

Custom metrics

You can also create your own metrics, that are reported at the end of a load test, just like HTTP timings:

import http from 'k6/http';
import { Trend } from 'k6/metrics';

let myTrend = new Trend('waiting_time');

export default function () {
  let r = http.get('https://httpbin.org');
  myTrend.add(r.timings.waiting);
}

The above code will create a Trend metric named “waiting_time” and referred to in the code using the variable name myTrend. Custom metrics will be reported at the end of a test. Here is how the output might look:

custom metrics

Metric types

All metrics (both the built-in ones and the custom ones) have a type. The four different metric types in k6 are:

Metric typeDescription
CounterA metric that cumulatively sums added values.
GaugeA metric that stores the min, max and last values added to it.
RateA metric that tracks the percentage of added values that are non-zero.
TrendA metric that allows for calculating statistics on the added values (min, max, average and percentiles).

All values added to a custom metric can optionally be tagged which can be useful when analysing the test results.

Counter (cumulative metric)

import { Counter } from 'k6/metrics';

let myCounter = new Counter('my_counter');

export default function () {
  myCounter.add(1);
  myCounter.add(2);
}

The above code will generate the following output:

counter output

The value of my_counter will be 3 (if you run it one single iteration - i.e. without specifying --iterations or --duration).

Note that there is currently no way of accessing the value of any custom metric from within JavaScript. Note also that counters that have value zero (0) at the end of a test are a special case - they will NOT be printed to the stdout summary.

Gauge (keep the latest value only)

import { Gauge } from 'k6/metrics';

let myGauge = new Gauge('my_gauge');

export default function () {
  myGauge.add(3);
  myGauge.add(1);
  myGauge.add(2);
}

The above code will result in an output like this: gauge output

The value of my_gauge will be 2 at the end of the test. As with the Counter metric above, a Gauge with value zero (0) will NOT be printed to the stdout summary at the end of the test.

Trend (collect trend statistics (min/max/avg/percentiles) for a series of values)

import { Trend } from 'k6/metrics';

let myTrend = new Trend('my_trend');

export default function () {
  myTrend.add(1);
  myTrend.add(2);
}

The above code will make k6 print output like this:

trend output

A trend metric is a container that holds a set of sample values, and which we can ask to output statistics (min, max, average, median or percentiles) about those samples. By default, k6 will print average, min, max, median, 90th percentile, and 95th percentile.

Rate (keeps track of the percentage of values in a series that are non-zero)

import { Rate } from 'k6/metrics';

let myRate = new Rate('my_rate');

export default function () {
  myRate.add(true);
  myRate.add(false);
  myRate.add(1);
  myRate.add(0);
}

The above code will make k6 print output like this: rate output

The value of my_rate at the end of the test will be 50%, indicating that half of the values added to the metric were non-zero.

Notes

  • custom metrics are only collected from VU threads at the end of a VU iteration, which means that for long-running scripts, you may not see any custom metrics until a while into the test.

Metric graphs in k6 Cloud Results

If you use k6 Cloud Results, you have access to all test metrics within the Analysis Tab. You can use this tab to further analyze and compare test result data, to look for meaningful correlations in your data.

k6 Cloud Analysis Tab