How to Handle Errors in Go Middleware Chains?

Middleware Pattern with Error-Returning Handlers

Question: How can I combine the Go middleware pattern with request handlers that return errors?

Answer:

The middleware pattern in Go allows one to create reusable components that can be applied to HTTP handlers. However, traditional middleware functions don't natively handle errors.

To enable error handling in middleware, it's recommended to use a separate middleware function dedicated to this purpose. This function should be placed at the end of the middleware chain and handle errors returned by the handlers within the chain.

// Pattern for a middleware function that checks for errors from the next handler.
func errorHandler(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        err := next.ServeHTTP(w, r)
        if err != nil {
            // Handle the error, e.g., by logging or returning an appropriate HTTP response.
        }
    })
}

Example:

To combine error-returning handlers with the logging middleware from the original example:

type errorHandler func(http.ResponseWriter, *http.Request) error

// Create a special error-aware logging middleware.
func loggingWithErrorHandler(next errorHandler) errorHandler {
    return func(w http.ResponseWriter, r *http.Request) error {
        // Before executing the handler.
        start := time.Now()
        log.Printf("Started %s %s", r.Method, r.URL.Path)
        err := next(w, r)
        // After executing the handler.
        log.Printf("Completed %s in %v", r.URL.Path, time.Since(start))
        return err
    }
}

// Define an error-returning handler func.
func errorHandlerFunc(w http.ResponseWriter, r *http.Request) error {
    w.Write([]byte("Hello World from errorHandlerFunc!"))
    return nil
}

// Assemble the middleware chain and error-aware middleware.
http.Handle("/", loggingWithErrorHandler(errorHandlerFunc))

This combination allows for error handling while retaining the benefits of the middleware pattern. The wrapped error-aware middleware will handle any errors returned by the wrapped handler.

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