github.com/grafana/k6

k6 is fundamentally a load testing tool, but when embedding it as a Go library, you get direct access to its execution engine and metrics collection. The JavaScript execution environment is sandboxed using goja, which provides good isolation, though you need to be careful about what modules and extensions you expose to test scripts. The library doesn't handle authentication itself but provides solid primitives for implementing various auth flows in your tests.

From a security perspective, k6 handles TLS sensibly with reasonable defaults (TLS 1.2+) and gives you fine-grained control over certificate validation when needed. Input validation is your responsibility when passing data into test scripts - the library won't sanitize JavaScript code for you. Error messages are generally safe and don't leak sensitive information, though custom extensions need careful review. One gotcha: metrics endpoints can expose test parameters and URLs, so be mindful in production environments.

Dependency management is reasonable with a moderately-sized tree. The team is responsive to security issues, though updates sometimes lag behind upstream goja vulnerabilities. Documentation around secure extension development could be more comprehensive.

k6 is primarily a standalone load testing tool written in Go, but using it as a Go library is an advanced use case. The typical workflow involves writing test scripts in JavaScript, not importing k6 as a Go package. That said, if you're extending k6 or building custom tooling around it, the Go API is well-structured and the codebase is clean.

The documentation is outstanding for the main use case (JavaScript-based load testing) with tons of examples covering HTTP requests, WebSockets, gRPC, and browser testing. The error messages are helpful and specific, making debugging straightforward. The community is responsive on GitHub and the k6 community forum, though Stack Overflow coverage is lighter since most discussion happens in their dedicated channels.

For day-to-day load testing work, k6 is a joy to use. The JavaScript API is intuitive, thresholds and checks make validation simple, and the local execution with cloud integration option is flexible. However, if you're specifically looking to import k6 as a Go library rather than use it as a CLI tool, be prepared for less documentation and more source code reading.

K6 excels at scenario-based load testing with excellent JavaScript API ergonomics and Go extension capabilities. The execution model is well-designed for realistic traffic patterns - virtual users, ramp-up schedules, and distributed execution work reliably. Resource management is generally solid with configurable connection pooling, though you need to be careful with per-VU state and memory growth under sustained load with large datasets.

The built-in metrics and custom metric hooks provide good observability, streaming directly to various backends (Prometheus, InfluxDB, etc). However, debugging failed tests can be frustrating - error context from the JS runtime isn't always detailed, and timeout behaviors aren't consistently documented across all protocol modules. The gRPC and WebSocket implementations sometimes have surprising defaults that bite you in production.

Configuration management is flexible but can become unwieldy with many options split between code, CLI flags, and environment variables. Breaking changes between minor versions have occurred more frequently than expected, particularly around extension APIs and metric output formats. Overall, it's a robust choice for serious load testing despite occasional rough edges.