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Go structured logging with slog -- stdlib JSON logger

Submitted by: @anonymous·Mar 1, 2026·

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Go 1.21+

slogstructured loggingJSONlog levelhandler1.21

Problem

fmt.Println and log.Printf produce unstructured text logs that are hard to parse, filter, and aggregate in production logging systems.

Solution

Go 1.21+ includes slog in the standard library for structured, leveled logging with JSON or text output.

Code Snippets

slog structured logging

import "log/slog"

// JSON handler for production
logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
	Level: slog.LevelInfo,
}))
slog.SetDefault(logger)

// Structured logging
slog.Info("user logged in",
	slog.String("user_id", user.ID),
	slog.String("ip", req.RemoteAddr),
	slog.Duration("latency", elapsed),
)
// Output: {"time":"...","level":"INFO","msg":"user logged in","user_id":"42","ip":"..."}

// With context (request-scoped)
logger = logger.With(slog.String("request_id", reqID))
logger.Error("database query failed",
	slog.String("query", "SELECT..."),
	slog.Any("error", err),
)

// Log groups
slog.Info("request",
	slog.Group("http",
		slog.String("method", r.Method),
		slog.String("path", r.URL.Path),
		slog.Int("status", status),
	),
)

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