Master backend interviews
with real code.

Rate Limiter for API Gateway

Design a sliding window rate limiter for an API gateway handling 1M+ requests/min. Must be thread-safe with O(1) allow() and support 10K+ concurrent clients.

func (rl *RateLimiter) Allow(clientID string) bool {
    rl.mu.Lock()
    defer rl.mu.Unlock()
    sw := rl.getWindow(clientID)
    now := time.Now()
    sw.removeExpired(now, rl.window)
    if len(sw.timestamps) >= rl.rate {
        return false
    }
    sw.timestamps = append(sw.timestamps, now)
    return true
}

Build a Backend-for-Frontend (BFF)

Build a BFF that aggregates data from 3 downstream microservices in parallel with timeouts, error isolation, and request tracing. Failed sections degrade gracefully instead of crashing the page.

func handleHomePage(w http.ResponseWriter, r *http.Request) {
    ctx, cancel := context.WithTimeout(r.Context(), 500*time.Millisecond)
    defer cancel()
    sections := fetchAll(ctx, services, requestID)
    json.NewEncoder(w).Encode(PageResponse{Sections: sections})
}