Go HTTP Middleware

Quick Reference

Topic	Reference
Context keys, request IDs, user metadata	references/context-propagation.md
slog setup, logging middleware, child loggers	references/structured-logging.md
AppHandler pattern, domain errors, recovery	references/error-handling-middleware.md

Middleware Signature

All middleware follows the standard func(http.Handler) http.Handler pattern. This is the composable building block for cross-cutting concerns in Go HTTP servers.

// Standard middleware signature
func RequestID(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        id := r.Header.Get("X-Request-ID")
        if id == "" {
            id = uuid.New().String()
        }
        ctx := context.WithValue(r.Context(), requestIDKey, id)
        w.Header().Set("X-Request-ID", id)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

// Type-safe context keys
type contextKey string
const requestIDKey contextKey = "request_id"

func RequestIDFromContext(ctx context.Context) string {
    id, _ := ctx.Value(requestIDKey).(string)
    return id
}

Key points:

Accept http.Handler, return http.Handler -- always
Call next.ServeHTTP(w, r) to pass control to the next handler
Work before the call (pre-processing) or after (post-processing) or both
Use r.WithContext(ctx) to propagate new context values downstream

Context Propagation

Use context.WithValue for request-scoped data that crosses API boundaries (request IDs, authenticated users, tenant IDs). Always use typed keys to avoid collisions.

type contextKey string

const (
    requestIDKey contextKey = "request_id"
    userKey      contextKey = "user"
)

Provide typed helper functions for extraction:

func RequestIDFromContext(ctx context.Context) string {
    id, _ := ctx.Value(requestIDKey).(string)
    return id
}

See references/context-propagation.md for user metadata patterns, downstream propagation, and timeouts.

Structured Logging

Use slog (standard library, Go 1.21+) for structured logging in middleware. Wrap http.ResponseWriter to capture the status code.

func Logger(logger *slog.Logger) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            start := time.Now()
            wrapped := &statusWriter{ResponseWriter: w, status: http.StatusOK}

            next.ServeHTTP(wrapped, r)

            logger.Info("request completed",
                "method", r.Method,
                "path", r.URL.Path,
                "status", wrapped.status,
                "duration_ms", time.Since(start).Milliseconds(),
                "request_id", RequestIDFromContext(r.Context()),
            )
        })
    }
}

See references/structured-logging.md for JSON/text handler setup, log levels, and child loggers.

Centralized Error Handling

Define a custom handler type that returns error so handlers don't need to write error responses themselves:

type AppHandler func(w http.ResponseWriter, r *http.Request) error

func (fn AppHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    if err := fn(w, r); err != nil {
        handleError(w, r, err)
    }
}

Map domain errors to HTTP status codes in a single handleError function. Never leak internal error details to clients.

See references/error-handling-middleware.md for the full pattern with AppError, errors.As, and JSON responses.

Recovery Middleware

Catch panics to prevent a single bad request from crashing the server:

func Recovery(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        defer func() {
            if rec := recover(); rec != nil {
                slog.Error("panic recovered",
                    "panic", rec,
                    "stack", string(debug.Stack()),
                    "request_id", RequestIDFromContext(r.Context()),
                )
                writeJSON(w, 500, map[string]string{"error": "internal server error"})
            }
        }()
        next.ServeHTTP(w, r)
    })
}

Recovery must be the outermost middleware so it catches panics from all inner middleware and handlers. See references/error-handling-middleware.md for details.

Middleware Chain Ordering

Apply middleware outermost-first. The first middleware in the chain wraps all others.

// Nested style (outermost first)
handler := Recovery(
    RequestID(
        Logger(
            Auth(
                router,
            ),
        ),
    ),
)

// Or with a chain helper
func Chain(h http.Handler, middleware ...func(http.Handler) http.Handler) http.Handler {
    for i := len(middleware) - 1; i >= 0; i-- {
        h = middleware[i](h)
    }
    return h
}

handler := Chain(router, Recovery, RequestID, Logger(slog.Default()), Auth)

Recommended Order

Recovery -- outermost; catches panics from all inner middleware
RequestID -- assign early so all subsequent middleware can reference it
Logger -- logs the completed request with ID and status
Auth -- after logging so failed auth attempts are recorded
Application-specific middleware -- rate limiting, CORS, etc.

Anti-patterns

Using string or int context keys

// BAD: collisions with other packages
ctx = context.WithValue(ctx, "user", user)

// GOOD: unexported typed key
type contextKey string
const userKey contextKey = "user"
ctx = context.WithValue(ctx, userKey, user)

Writing response before calling next

// BAD: writes response then continues chain
func Bad(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.WriteHeader(http.StatusOK) // too early!
        next.ServeHTTP(w, r)
    })
}

Forgetting to call next.ServeHTTP

// BAD: swallows the request
func Bad(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        log.Println("got request")
        // forgot next.ServeHTTP(w, r)
    })
}

Storing large objects in context

Context values should be small, request-scoped metadata (IDs, tokens, user structs). Never store database connections, file handles, or large payloads.

Using context.WithValue for function parameters

If a function needs a value to do its job, pass it as an explicit parameter. Context is for cross-cutting metadata that passes through APIs, not for avoiding function signatures.

Recovery middleware in the wrong position

If recovery is not the outermost middleware, panics in outer middleware will crash the server. Always apply recovery first.

go-middleware