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Designing Contracts with Protocol Buffers: .proto Files, Field Numbers and Backward Compatibility

In gRPC the contract travels by field numbers, not field names, so adding a field is safe while changing a number or a type silently corrupts data. We write Bookstore Inventory's inventory.proto and show the enum zero rule, presence with optional, a DecimalValue for money, reserved, and C# generation with Grpc.Tools, all with real output.