DocumentationGrafana k6ExtensionsCreate an extensionOutput Extensions

Output Extensions

k6 provides many metrics and output formats, but it cannot directly support all possibilities. To store or alter metrics captured during an active k6 test, you can create a custom output extension.

Output extension binaries can use the --out flag to send metrics to a custom place. Some potential reasons for a custom extension could include:

• To support a time-series database not already supported
• To add derived metrics data for storage
• To filter metrics to only the data you care about

Like JavaScript extensions, output extensions rely on the extension author to implement specific APIs.

Before you start:

To run this tutorial, you’ll need the following applications installed:

• Go
• Git

You also need to install xk6:

bash Copy

$ go install go.k6.io/xk6/cmd/xk6@latest

Write a simple extension

1. Set up a directory to work in.

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$ mkdir xk6-output-logger; cd xk6-output-logger; go mod init xk6-output-logger

1. The core of an Output extension is a struct that implements the output.Output interface.

Create a simple example that outputs each set of metrics to the console as received by the AddMetricSamples(samples []metrics.SampleContainer) method of the output interface.

Go Copy

package log

import (
    "fmt"
    "strings"
    "time"

    "go.k6.io/k6/metrics"
    "go.k6.io/k6/output"
)

// AddMetricSamples receives metric samples from the k6 Engine as they're emitted.
func (l *Logger) AddMetricSamples(samples []metrics.SampleContainer) {
    for _, sample := range samples {
        all := sample.GetSamples()
        fmt.Fprintf(l.out, "%s [%s]\n", all[0].GetTime().Format(time.RFC3339Nano), metricKeyValues(all))
    }
}

// metricKeyValues returns a string of key-value pairs for all metrics in the sample.
func metricKeyValues(samples []metrics.Sample) string {
    names := make([]string, 0, len(samples))
    for _, sample := range samples {
        names = append(names, fmt.Sprintf("%s=%v", sample.Metric.Name, sample.Value))
    }
    return strings.Join(names, ", ")
}

1. Register the module to use these from k6 test scripts.

Go Copy

import "go.k6.io/k6/output"

// init is called by the Go runtime at application startup.
func init() {
    output.RegisterExtension("logger", New)
}

The final extension code looks like this:

Go

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Go Copy

package log

import (
    "fmt"
    "io"
    "strings"
    "time"

    "go.k6.io/k6/metrics"
    "go.k6.io/k6/output"
)

// init is called by the Go runtime at application startup.
func init() {
    output.RegisterExtension("logger", New)
}

// Logger writes k6 metric samples to stdout.
type Logger struct {
    out io.Writer
}

// New returns a new instance of Logger.
func New(params output.Params) (output.Output, error) {
    return &Logger{params.StdOut}, nil
}

// Description returns a short human-readable description of the output.
func (*Logger) Description() string {
    return "logger"
}

// Start initializes any state needed for the output, establishes network
// connections, etc.
func (o *Logger) Start() error {
    return nil
}

// AddMetricSamples receives metric samples from the k6 Engine as they're emitted.
func (l *Logger) AddMetricSamples(samples []metrics.SampleContainer) {
    for _, sample := range samples {
        all := sample.GetSamples()
        fmt.Fprintf(l.out, "%s [%s]\n", all[0].GetTime().Format(time.RFC3339Nano), metricKeyValues(all))
    }
}

// metricKeyValues returns a string of key-value pairs for all metrics in the sample.
func metricKeyValues(samples []metrics.Sample) string {
    names := make([]string, 0, len(samples))
    for _, sample := range samples {
        names = append(names, fmt.Sprintf("%s=%v", sample.Metric.Name, sample.Value))
    }
    return strings.Join(names, ", ")
}

// Stop finalizes any tasks in progress, closes network connections, etc.
func (*Logger) Stop() error {
    return nil
}

Notice a couple of things:

• The module initializer New() receives an instance of output.Params. With this object, the extension can access the output-specific configuration, interfaces to the filesystem, synchronized stdout and stderr, and more.

• AddMetricSamples in this example writes to stdout. This output might have to be buffered and flushed periodically in a real-world scenario to avoid memory leaks. Below we’ll discuss some helpers you can use for that.

Compile your extended k6

To build a k6 binary with this extension, run:

bash Copy

$ xk6 build --with xk6-output-logger=.

Use your extension

Now we can use the extension with a test script.

1. In new JavaScript file, make some simple test logic.

JavaScript

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JavaScript Copy

import http from 'k6/http';
import { sleep } from 'k6';

export default function () {
  http.get('https://test-api.k6.io');
  sleep(0.5);
}

1. Now, run the test.

bash Copy

$ ./k6 run test.js --out logger --quiet --no-summary --iterations 2

Your output should look something like this:

shell Copy

2022-07-01T08:55:09.59272-05:00 [http_reqs=1, http_req_duration=117.003, http_req_blocked=558.983, http_req_connecting=54.135, http_req_tls_handshaking=477.198, http_req_sending=0.102, http_req_waiting=116.544, http_req_receiving=0.357, http_req_failed=0]
2022-07-01T08:55:09.917036-05:00 [vus=1, vus_max=1]
2022-07-01T08:55:10.094196-05:00 [data_sent=446, data_received=21364, iteration_duration=1177.505083, iterations=1]
2022-07-01T08:55:10.213926-05:00 [http_reqs=1, http_req_duration=119.122, http_req_blocked=0.015, http_req_connecting=0, http_req_tls_handshaking=0, http_req_sending=0.103, http_req_waiting=118.726, http_req_receiving=0.293, http_req_failed=0]
2022-07-01T08:55:10.715323-05:00 [data_sent=102, data_received=15904, iteration_duration=620.862459, iterations=1]

Things to keep in mind

• Output structs can optionally implement additional interfaces that allow them to receive thresholds or run-status updates and even interrupt a test.
• Consider using output.SampleBuffer and output.PeriodicFlusher to improve performance given the large amounts of data produced by k6. Refer to statsd output for an example.
• Use the project template as a starting point for your output extension.

Questions? Feel free to join the discussion on extensions in the k6 Community Forum.