k6 is a modern, developer-centric load testing tool maintained by Grafana Labs at github.com/grafana/k6. Written in Go for performance and using JavaScript (ES2015+) for test scripting, k6 bridges the gap between developer-friendly test authoring and high-performance load generation. With over 30,000 GitHub stars, it has become the go-to open-source alternative to commercial load testing platforms.

Test scripts are written in JavaScript and define virtual user scenarios. k6 supports HTTP/1.1, HTTP/2, WebSocket, and gRPC protocols natively, with extensions available for additional protocols. A typical test script defines a default function that represents a virtual user’s journey — making requests, checking responses, adding think times — and k6 handles the concurrent execution of thousands of these virtual users. Built-in checks and thresholds let you define pass/fail criteria directly in the test script.

The CLI provides multiple output modes: a real-time terminal summary with request rates, response times (p50, p90, p95, p99), error rates, and throughput metrics. Results can be exported to Grafana Cloud k6, InfluxDB, Prometheus Remote Write, Datadog, New Relic, and other backends for visualization and historical comparison. This makes k6 equally useful for one-off load tests and continuous performance testing in CI/CD pipelines.

k6’s extension system allows community and custom extensions written in Go to add new protocols, outputs, and JavaScript APIs. The Kubernetes operator (k6-operator) enables distributed load testing across cluster nodes. k6 runs on all major platforms (Linux, macOS, Windows) and can be installed via Homebrew, APT, YUM, Docker, or direct binary download. The AGPL-3.0 license keeps it fully open source while Grafana Labs offers the managed Grafana Cloud k6 service for teams wanting hosted test execution and result storage.