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An async route can yield during nonblocking I/O, allowing the event loop to serve other work. A normal def route is typically run in a thread pool. Calling blocking I/O inside async def blocks the event loop; using async syntax alone does not improve CPU-bound work. Choose an execution model based on the library and bottleneck.

Concurrent coroutines can interleave operations on the same Session, mixing transaction state and identity-map contents. Create a session per request or task, scope its transaction clearly, and size the connection pool across all worker processes and replicas. A global Engine/pool is different from a shared mutable Session.

CompletableFuture represents a value that may complete later, with dependent stages triggered on completion. Async methods without an explicit executor commonly use a shared pool; blocking calls there can starve unrelated work. Prefer non-blocking composition, choose an executor suited to the task, set timeouts, and observe exceptional completion rather than calling join() throughout request paths.

Use async when the code is mostly waiting on IO such as database calls, HTTP requests, or message queues. Use threads or process pools for CPU-intensive work or code that cannot cooperate with the event loop. The key is matching the concurrency model to the bottleneck, not adding async everywhere because it seems modern.