Synchronous calls are easy to reason about: the caller waits for a response. They are appropriate when the user needs an immediate result, but latency and availability depend on the downstream path. If A calls B, which calls C, a slow C can consume resources in A and B.

Asynchronous communication lets a producer hand work to a broker or durable queue and continue. It improves burst absorption and decoupling, but requires retries, deduplication, dead-letter handling, event schemas, and a way to observe work that has not completed.

Set deadlines across a request chain. A downstream call should not be allowed to outlive the caller's remaining budget. Use asynchronous processing for tasks such as emails, analytics, and media transcoding when immediate completion is unnecessary.