slog utilities

Utilities and handlers for the Go log/slog package.

go get github.com/networkteam/slogutils

Features

CLI handler

• Unobstrusive minimal logging output for humans (no timestamps, no log levels)
• Supports ReplaceAttr as in slog.HandlerOptions
• Grouping and quoting of attributes
• Prefixes, colors and paddings can be fully customized
• Supports an additional slogutils.LevelTrace level that is below slog.LevelDebug and can be used for tracing

Example

package main

import (
	"context"
	"errors"
	"log/slog"
	"os"
	"time"

	"github.com/networkteam/slogutils"
)

func main() {
	slog.SetDefault(slog.New(
		slogutils.NewCLIHandler(os.Stderr, &slogutils.CLIHandlerOptions{
			Level: slogutils.LevelTrace,
		}),
	))

	slog.Info("Starting server", "addr", ":8080", "env", "production")
	slog.Debug("Connected to DB", "db", "myapp", "host", "localhost:5432")
	slog.Warn("Slow request", "method", "GET", "path", "/users", "duration", 497*time.Millisecond)
	slog.Error("DB connection lost", slogutils.Err(errors.New("connection reset")), "db", "myapp")
	// This is a non-standard log level that is below debug
	slog.Log(context.Background(), slogutils.LevelTrace, "Trace message", "foo", "bar")
}

Context helper

Setting a logger instance with groups / attributes on a context is very useful e.g. in request processing or distributed tracing.

• Use slogutils.FromContext to get a logger instance from a context (or slog.Default() as a fallback)
• Use slogutils.WithLogger to set a logger instance on a context

Example

package main

import (
	"log/slog"
	"net/http"
	"time"
)

func main() {
	// An HTTP middleware with a simple request ID
	mux := http.NewServeMux()
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		logger := slogutils.FromContext(r.Context())
		logger.Info("Handling main route")
	})

	http.ListenAndServe(":8080", withRequestID(mux))
}

func withRequestID(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		requestID := time.Now().UnixNano() // Note: use a better request ID based on UUIDs in production

		// Get the logger from the request context and add the request ID as an attribute,
		// then set the logger back to a new context.
		logger := slogutils.FromContext(r.Context()).With(slog.Group("request", "id", requestID))
		ctx := slogutils.WithLogger(r.Context(), logger)

		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

PGX tracelog adapter for slog

See adapter/pgx/v5/tracelog.

Acknowledgements

• The output of the CLI handler is based on the CLI handler of the github.com/apex/log package.
• Some code and tests of the CLI handler implementation were adapted from github.com/lmittmann/tint.