Checks

Checks validate boolean conditions in your test. Testers often use checks to validate that the system is responding with the expected content. For example, a check could validate that a POST request has a response.status_code == 201, or that the body is of a certain size.

Checks are similar to what many testing frameworks call an assert, but failed checks do not cause the test to abort or finish with a failed status. Instead, k6 keeps track of the rate of failed checks as the test continues to run.

Each check creates a checks rate metric. To make a check abort or fail a test, you can combine it with a threshold.

Check for HTTP response code

Checks are great for codifying assertions relating to HTTP requests and responses. For example, this snippet makes sure the HTTP response code is a 200:

load("check", "check")
load("requests", "get")

def default():
    resp = get("https://httpbin.test.k6.io/get")

    check(resp, {
        "is status 200": lambda r: r.status_code == 200,
    })

See percentage of checks that passed

When a script includes checks, the summary report shows how many of the tests’ checks passed:

$ k6 run script.js

  ...
    ✓ is status 200

  ...
  checks.........................: 100.00% ✓ 1        ✗ 0
  data_received..................: 11 kB   12 kB/s

In this example, note that the check “is status 200” succeeded 100% of the times it was called.

Fail a load test using checks

When a check fails, the script will continue executing successfully and will not return a ‘failed’ exit status. Checks are nice for codifying assertions, but unlike thresholds, checks do not affect the exit status of k6.

If you need the whole test to fail based on the results of a check, you have to combine checks with thresholds. This is particularly useful in specific contexts, such as integrating k6 into your CI pipelines or receiving alerts when scheduling your performance tests.

load("check", "check")
load("requests", "get")
load("time", "sleep")

options = {
    "vus": 5,
    "duration": "5s",
    "thresholds": {
        "checks": ["rate>0.9"],
    },
}

def default():
    resp = get("https://httpbin.test.k6.io/get")

    check(resp, {
        "is status 200": lambda r: r.status_code == 200,
    })

    sleep(0.5)

In this example, the threshold is configured on the checks metric, establishing that the rate of successful checks is higher than 90%.