From beagle-go
Provides idiomatic Go HTTP middleware for context propagation, structured slog logging, error handling, and panic recovery. Use when writing middleware, adding request tracing, or cross-cutting concerns.
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| Topic | Reference |
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| 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 |
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:
http.Handler, return http.Handler -- alwaysnext.ServeHTTP(w, r) to pass control to the next handlerr.WithContext(ctx) to propagate new context values downstreamUse 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.
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.
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.
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.
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)
// 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)
// 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)
})
}
// 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)
})
}
Context values should be small, request-scoped metadata (IDs, tokens, user structs). Never store database connections, file handles, or large payloads.
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.
If recovery is not the outermost middleware, panics in outer middleware will crash the server. Always apply recovery first.