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.…