Architecture & System Design › Cloud Design Patterns

Gateway Aggregation

One gateway call that fans out to many services and combines the results.

Backend Engineer · Senior Frontend Engineer · Senior

Also known as: gateway aggregation, api composition, backend aggregation

Gateway aggregation collapses multiple downstream calls into one client-facing request: the gateway fans out to services in parallel, assembles their responses, and returns the composed result. Mobile and web clients get one round trip instead of a chatty handful — latency falls, client logic simplifies.

client → gateway → (orders ∥ users ∥ inventory, parallel) → one composed response

It’s the BFF’s workhorse mechanism, distinct from mere routing: aggregation owns composition logic (merging, defaults, partial-failure handling) that clients shouldn’t duplicate. Timeouts per downstream, hedged slowness, and partial responses (data with gaps marked) keep one slow service from spoiling the composition.

The classic mistakes:

Sequential fan-out. Awaiting downstream calls one-by-one sums their latencies; parallelise with per-call timeouts. Aggregation must be concurrent to matter.
All-or-nothing composition. One failed downstream failing the whole response converts partial degradation into total failure. Degrade per section (cached/defaults/gaps marked).
Business logic accretion. Composition growing into pricing, eligibility and workflow rules turns the gateway into an untested monolith. Aggregate and shape; decide in services.
No per-route budgets. Slow downstream without a deadline holds the composition hostage. Budget per call; fail fast past it.
Cache blindness. Composed responses cached whole (invalidation pain) or never (repeated fan-out cost). Cache fragments with precise keys; compose from cache where possible.
Client-specific gateways multiplying. A gateway per screen fragments composition logic across near-identical aggregators. Aggregate per client class, version the compositions.
Hiding failures. Swallowing downstream errors into 200-with-gaps confuses debugging and clients. Mark gaps explicitly; expose health honestly.

How to use it: parallel fan-out, per-call budgets, partial responses by design, composition without business rules. One round trip for clients, honest degradation inside.