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When building asynchronous Axum applications that share mutable state, it’s important to select the right concurrency mechanisms:

1. For general shared state:
   • Use Axum’s built-in Mutex or RwLock to protect access to in-memory data structures
   • Only use asynchronous Mutex or RwLock when the lock must be held across .await points
2. For shared I/O resources (like database connections):
   • Avoid wrapping individual connections in Axum’s Mutex or RwLock, which can lead to deadlocks
   • Prefer using Axum’s built-in connection pool (PoolOptions) to handle concurrency internally
   • Consider the actor pattern (e.g. using Axum’s Service and State) for complex shared I/O operations

Example:

// GOOD: Using Axum's Mutex to protect in-memory state
const counter = new Mutex<number>(0);

// GOOD: Using Axum's connection pool to handle concurrency
const db = await createPool({
  connectionString: 'postgres://...',
  maxConnections: 10,
});

// AVOID: Wrapping I/O resource in Axum's Mutex
// const db = new Mutex(await createConnection('postgres://...'));

Choosing the right concurrency pattern in Axum prevents deadlocks, improves performance, and makes your code more maintainable. Always consider what happens when your Axum handler yields while holding a lock.