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A bounded queue limits waiting items; workers hold additional active items. task_done releases unfinished-item accounting, while business success and durable retries require separate records.

Child cancellation is cooperative, so cleanup must run and cancellation should normally be re-raised. A task group scopes lifetimes; it does not roll back remote effects or make partial success a business policy.

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.

Use bounded connection pools, semaphores or worker queues, request deadlines, and deliberate overload responses. Local limits must account for replica count because each replica has its own pool. Avoid unbounded in-memory tasks and retries that amplify load during an incident.

Cancellation is delivered at an await point. Use finally blocks or async context managers to close resources, then re-raise CancelledError unless there is a deliberate reason to suppress it. A blocking synchronous call may not respond promptly, so keep the event loop cooperative and bound background work.