Picture ordering breakfast by phoning the kitchen, then the bakery, then the coffee shop separately — each call ringing for ages before someone picks up. You'd spend more time on hold than eating. Far better to tell one person your whole order and let them coordinate the rest behind the counter.
Gateway Aggregation does this for your app. Instead of a client firing off a separate request to every service a screen needs, it sends one request to the gateway, which gathers everything from the backends and hands back a single combined reply.
The problem
A single screen often needs data from several services at once — a product page might want details from the catalog service, the price from pricing, the stock count from inventory, and reviews from a fourth. If the client calls each one directly, that's four separate round trips over the network before the page can render.
On a fast wired connection that's annoying; on a phone with 150 ms of latency per round trip, it's painful. Each call also re-pays the cost of connection setup, authentication, and TLS. The client becomes a chatty coordinator, juggling partial results and failure handling for every backend — work it shouldn't have to do.
Firing the four calls in parallel from the phone helps less than you'd hope: each is still a separate request over a slow, lossy link, the page still waits for the slowest of them, and in practice many apps load a screen piece by piece, one call after another.
Step through that page load below and predict when it can render. At the end you'll predict once more, for a version with a gateway, and flip the switch to check.
How it works
The gateway absorbs the coordination. The client makes one request — "give me everything for this product page." The gateway then fans out to each backend service, ideally firing the calls in parallel rather than one after another. Because these calls happen inside the data center, the network hops between gateway and services are fast and cheap compared to the client's distant connection.
As the responses come back, the gateway merges them into a single payload shaped the way the client wants, and returns it in one reply. The client made one slow round trip instead of four; the four fast round trips all happened on the gateway's side. That's the switch you flipped above: the same four services, but the slow link is crossed once instead of four times.
A screen needs five services. Each call over the phone's link takes 200 ms there and back, and each service answers in about 10 ms. Roughly how long does the screen take with gateway aggregation, when the gateway calls all five at once?
When one backend is slow
Aggregation has a catch. The merged reply can't leave until every part is in, so the page is now only as fast as the slowest service behind it. When one backend has a bad day, everything waits for it, and while it waits the gateway holds a connection and memory for that request. Under load, one slow dependency can tie up the gateway itself.
The fix is to give each backend call its own timeout and decide in advance what to send when it misses: an empty list, a cached value, a placeholder. Below, the Reviews service takes three seconds to answer. Predict what the user sees with a gateway that waits for everything; at the end, predict again for a 100 ms budget with a default, then flip the switch.
Pair timeouts with a circuit breaker. A timeout caps how long one request waits for a sick backend, but every request still waits that long. A circuit breaker notices the pattern and stops calling the sick service for a while, so the gateway serves the default instantly and the struggling service gets room to recover.
Only default what's safe to default. An empty reviews list or a hidden recommendations row is a fine fallback. A missing price is not: defaulting it to $0, or to last week's number, can sell something at the wrong price. Decide for each part of the response whether it can be left out, served stale, or must fail the whole request, and mark partial responses so the client can show a placeholder instead of pretending everything is there.
Your aggregated product page sometimes takes 8 s, and traces show the Recommendations call is always the slow part. What's the best fix in the gateway?
When to use it
Aggregation pays off whenever clients are making many round trips to assemble one view, especially over high-latency mobile links. It's a core trick of an API gateway and a natural fit for a backends-for-frontends setup, where each client type gets responses pre-shaped for its needs.
It's not always the answer. If the calls genuinely depend on one another and must run in sequence, you lose the parallelism that makes aggregation fast. And don't let the gateway swell into a place where business logic lives — its job is to combine, not to compute. Keep that boundary clean and pair it with gateway routing for sending requests onward and gateway offloading for shared concerns.