Validate Spring Boot Upgrades with Traffic Replay

Spring Boot upgrades can introduce subtle runtime regressions even when unit and integration tests pass, because changes in Jackson serialization, autoconfiguration behavior, transitive dependencies, removed APIs, and configuration properties may alter responses or request handling without causing obvious failures. Examples include Optional serialization errors after a Jackson update, changes to JSON field naming or numeric formatting, and behavioral shifts caused by dependency or module reorganizations in major releases such as Spring Boot 2 to 3 and 3 to 4. The proposed validation approach uses Speedscale traffic replay to record representative production traffic from the existing deployment, replay it against an upgraded build while mocking downstream dependencies, and compare responses, error rates, and latency at a detailed level. The workflow includes instrumenting a Kubernetes deployment, creating a traffic snapshot, deploying the upgraded application, replaying the snapshot, investigating response differences, iterating on fixes, and adding replay checks to CI/CD pipelines. For local testing, proxymock can record and replay inbound traffic, although outbound dependency capture may require additional configuration.

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