Routing Traffic Across Regions Without Guessing

The physically nearest region isn't always the fastest one to reach, because actual network paths depend on peering agreements and routing between providers, not straight-line distance. A user might get better real-world latency from a region that's farther away on a map but better connected to their actual internet service provider.

Route based on measured latency where you can, rather than assuming geographic distance is a reliable proxy for it. The difference is usually small, but for a small subset of users it can be significant enough to notice.

Routing based purely on geography sends users to their nearest region even when that region is degraded, which turns a partial regional problem into a bad experience for everyone geographically close to it, instead of routing them somewhere healthy at slightly higher latency. Combine geographic or latency-based routing with real health checks, so a struggling region actually sheds traffic instead of continuing to receive it at full volume.

This requires your health checks to be fast and accurate enough to trust for routing decisions, since a slow or flaky health check can cause routing to flap between regions in a way that's worse than not checking health at all.

Routing a user to whichever region answers fastest works cleanly for stateless requests, but breaks down the moment their data lives specifically in one region and a request lands somewhere else. Decide up front whether each type of request can be safely served from any region, or whether it needs to be pinned to wherever that user's actual data lives, and route accordingly rather than treating all traffic as interchangeable.

Getting this wrong is a common source of confusing, hard-to-reproduce bugs, where a request appears to succeed but returns stale or missing data because it landed in a region that didn't actually have the current state.

A handful of patterns account for most real multi-region routing problems:

• Health checks that are too shallow, confirming the service responds but not that it's actually functioning correctly
• DNS-based routing with a TTL long enough that a real regional failure takes far too long to actually redirect users away from it
• Session or state data that isn't accessible from every region a user might get routed to
• No monitoring of routing decisions themselves, so a misconfiguration silently sends a chunk of traffic to the wrong place for a long time before anyone notices

Each of these tends to look fine in testing and only shows up under a real regional problem.

Say a region degrades in a way that still returns a technically successful health check response, because the health check only confirms the service process is running, not that it can actually reach its own database. Traffic keeps routing there at full volume, and users in that region have a genuinely broken experience, while the monitoring dashboard shows a passing health check the entire time. A deeper health check, one that actually exercises the region's critical dependencies rather than just confirming the process is alive, is what would have caught this and rerouted traffic away.

Simulate a regional failure deliberately and confirm traffic actually reroutes the way you expect, rather than assuming the configuration is correct because it looks right on paper. This is easy to skip because it requires intentionally degrading something, even in a controlled way, which feels risky. It's worth doing anyway, on a schedule, since the alternative is discovering a routing gap during an actual regional incident, which is a far worse time to learn it.