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Spring / Spring gRPC Interview Questions

What are some best practices for evolving gRPC service contracts in a Spring microservices system over time?

The most important rule is protobuf-specific: never reuse or renumber an existing field tag. Add new fields with fresh tag numbers instead of repurposing old ones, and mark retired tags/names as reserved so nobody accidentally reintroduces the mistake later.

At the API level, prefer additive changes - new optional fields, new RPC methods - over breaking ones. When a genuinely breaking change is unavoidable, introduce it as a new method, service, or versioned package (e.g. orderv2) rather than mutating the existing contract out from under active consumers, and deprecate the old one on a clear timeline instead of deleting it immediately.

Operationally, keeping .proto files in a shared, versioned repository (or a schema registry) - rather than copy-pasted per service - keeps client and server teams working from the exact same contract and catches incompatible changes at build or review time rather than in production.

What is the single most important rule for evolving protobuf messages safely?
What is a recommended way to introduce an unavoidable breaking API change?

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