If you're building web APIs in Rust with Axum , you'll quickly hit a very relatable question: "How do I share a database connection, a config value, or an in-memory cache across my route handlers?" The answer involves four tools working together: State<T> , Arc<T> , Mutex<T> , and RwLock<T> . Each one does a specific job. By the end of this article, you'll understand what each one is for, when to use which, and how to combine them confidently in Axum 0.8. Let's go step by step. 🦀 The Problem: Handlers Don't Share Memory by Default In Axum, each request is handled by an async function. These handlers are independent — they don't automatically have access to shared resources like: A database connection pool An API key loaded from environment variables An in-memory counter or key-value store You need a way to inject shared data into your handlers safely — especially when multiple requests run concurrently on different threads.…