Structured Logging in Go with slog

Feb 5, 2025 / 4 minutes to read / Tags: go, observability

Go 1.21 shipped log/slog — structured, leveled logging in the standard library. No more reaching for zerolog or zap for basic use cases.

Why Structured Logging

Plain text logs are for humans. Structured logs (JSON or key-value) are for machines — log aggregators, alerting pipelines, and dashboards can query them without regex.

Basic Usage

import "log/slog"

slog.Info("request handled",
    "method", "GET",
    "path", "/api/users",
    "status", 200,
    "latency_ms", 12,
)

Output (JSON handler):

{"time":"2025-02-05T10:00:00Z","level":"INFO","msg":"request handled","method":"GET","path":"/api/users","status":200,"latency_ms":12}

JSON Handler

logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
    Level: slog.LevelInfo,
}))
slog.SetDefault(logger)

Adding Context

Use With to attach fields that apply to all subsequent logs — useful for request IDs:

reqLogger := logger.With("request_id", requestID)
reqLogger.Info("starting request")
reqLogger.Info("request done", "status", 200)

Both lines will carry request_id automatically.