clog/gcp: structured logging for Google Cloud using slog

Contrary to the documented "standard" approach for logging, this doesn't use any third-party logging package for logging.

Instead, it relies on Google Cloud's support for ingesting structured logs by simply printing JSON to stderr.

This method of emitting structured logs is supported by:

• Cloud Run
• Kubernetes Engine
• Cloud Functions
• App Engine (standard and flexible)
• and in other products, using the Cloud Logging agent and Ops agent.

Basic Usage

To use this, underscore-import gcp/init, which will configure slog to use the GCP-optimized JSON handler for all log messages:

Then when you use slog, all log messages will be output in JSON format to standard error, which is automatically ingested by Cloud Logging.

import _ "github.com/chainguard-dev/clog/gcp/init"

...

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
  slog.InfoContext(r.Context(), "my message",
    "mycount", 42,
    "mystring", "myvalue",
  )
})

This logs the message, with the additional structured logging fields in Cloud Logging:

Correlating Logs with Requests

You can also use this to correlate log lines with the request that generated them, by associating the log message with the request's trace context header.

import "github.com/chainguard-dev/clog/gcp"

...

http.Handle("/", gcp.WithCloudTraceContext(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
  slog.InfoContext(r.Context(), "my message",
    "mycount", 42,
    "mystring", "myvalue",
  )
})))

This logs the message, associated with the request's trace, in Cloud Logging:

Other logs with the same trace attribute are generated by the same incoming request.

See https://cloud.google.com/trace/docs/trace-log-integration for more information.

Critical Logging

Cloud Logging supports a CRITICAL logging level, which doesn't map cleanly to slog's built-in levels.

To log at this level:

slog.Log(ctx, gcp.LevelCritical, "I have a bad feeling about this...")

See ./cmd/example for a deployable example.

This repo is forked from https://github.com/remko/cloudrun-slog, which originated this idea and implementation.