Shadow Traffic
ELI5 — The Vibe Check
Shadow traffic sends a copy of real production requests to a new service version without affecting users. The shadow service processes the requests and you compare its responses to the real ones. It's like a dress rehearsal with real traffic — if the understudy messes up, the audience never knows.
Real Talk
Shadow traffic (traffic mirroring/shadowing) duplicates production requests to a test service for validation without impacting users. The shadow service's responses are discarded but compared against the production service for correctness and performance. This enables testing with real traffic patterns, data, and load without risk. Implemented via service meshes (Istio), proxies (Envoy), or application-level mirroring.
When You'll Hear This
"Mirror production traffic to the new service and compare response bodies for differences." / "Shadow traffic revealed our rewrite handles edge cases differently than the original."
Related Terms
Canary Backend
A canary backend routes a small percentage of traffic to a new version of your service while most users stay on the old one.
Chaos Testing
Chaos testing is intentionally breaking things in production to see if your system can handle it. Kill a server. Slow the network.
Load Test
A load test is when you throw a huge crowd of fake users at your app to see how it handles them.
Service Mesh
In a microservices system, every service needs to handle retries, timeouts, mutual TLS, and send traces. You could code all that in every service.