Overview
Summarizing contextual logging using log/slog in Go.
What is log/slog?
A package for structured logging added in Go1.21.
Structured logging is the output of logs as structured data.
Previously, to perform structured logging in Go, one had to either use third-party packages or implement it from scratch. Now, the standard package is also becoming a viable option.
By default, it can output in text or JSON format.
slog can hold context, allowing you to include request-based information.
Contextual logging with slog
Let's write code to create a Logger that includes request-based information in the logs.
Below is the code that assumes logging with a trace ID in the context.
By implementing the slog.Handler interface, you can create your own Handler.[]
package main
import (
"context"
"fmt"
"log/slog"
"net/http"
"os"
"github.com/google/uuid"
)
// TraceIDHandler represents the singular of trace id handler.
type TraceIDHandler struct {
slog.Handler
}
type ctxTraceID struct{}
var ctxTraceIDKey = ctxTraceID{}
// Handle implements slog.Handler interface.
func (t TraceIDHandler) Handle(ctx context.Context, r slog.Record) error {
tid, ok := ctx.Value(ctxTraceIDKey).(string)
if ok {
r.AddAttrs(slog.String("trace_id", tid))
}
return t.Handler.Handle(ctx, r)
}
// WithTraceID returns a context with a trace id.
func WithTraceID(ctx context.Context) context.Context {
uuid, _ := uuid.NewRandom()
return context.WithValue(ctx, ctxTraceIDKey, uuid.String())
}
func main() {
mux := http.NewServeMux()
handler := TraceIDHandler{slog.NewJSONHandler(os.Stdout, nil)}
logger := slog.New(handler)
mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
ctx := WithTraceID(r.Context())
logger.InfoContext(ctx, "Log with TraceID")
w.WriteHeader(http.StatusOK)
fmt.Fprint(w, "Contextual Logging!")
})
http.ListenAndServe(":8085", mux)
}
When you run the above code, the following log is output:
{"time":"2023-10-08T17:06:44.423859+09:00","level":"INFO","msg":"Log with TraceID","trace_id":"4f9a0bb6-cf8d-4eef-82ea-2385b76d3a74"}
Impressions
I was using my own package for structured logging, but with the release of Go1.21, I switched to this.
