How do you decide whether two backend capabilities communicate through a synchronous API call or asynchronous messaging?

Synchronous vs asynchronous service communication

Selects synchronous calls, asynchronous messaging, or a deliberate combination based on user feedback, coupling, latency, durability, and failure behavior.

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I would use a synchronous call when the caller needs an immediate result to continue, such as validating credentials or reading current product data. I would use asynchronous messaging when work is long-running, bursty, fan-out oriented, or can finish after the user receives an acknowledgement. An asynchronous API can durably record the request, return an accepted response with an operation id, and let a worker update status. A queue decouples time and absorbs bursts, but it also introduces duplicate delivery, delayed completion, retries, and operational state that the design must handle.

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Communication semantics, latency, temporal coupling, queues, durable intent, retries, idempotency, ordering, backpressure, and user-visible status.

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Why this question matters
Communication style changes both the user experience and the failure model. The useful choice comes from when the result is needed and what the system must do when the other component is slow or unavailable.
I would use a synchronous call when the caller needs an immediate result to continue, such as validating credentials or reading current product data. I would use asynchronous messaging when work is long-running, bursty, fan-out oriented, or can finish after the user receives an acknowledgement. An asynchronous API can durably record the request, return an accepted response with an operation id, and let a worker update status. A queue decouples time and absorbs bursts, but it also introduces duplicate delivery, delayed completion, retries, and operational state that the design must handle.
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Many flows benefit from both styles. The request path can synchronously validate authorization and business invariants, commit the intended state, and publish work through an outbox for asynchronous processing. Consumers need idempotency, bounded retries, dead-letter handling, correlation, and an ordering rule where order matters. The contract also needs to distinguish commands, which request an action, from events, which report a fact. Users need a truthful pending, failed, or completed state instead of an optimistic success message before durable work exists.

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Details

Using a queue only to make an endpoint look faster while hiding failure from the user.
Building long synchronous call chains with no timeout budget.
Publishing a message separately from the database commit.
Ignoring duplicate delivery, ordering, and poison messages.

How do you decide whether two backend capabilities communicate through a synchronous API call or asynchronous messaging?

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