Schema Registry Compatibility Gate

Design a compatibility gate for event and table contracts. A merged PR is not compatible until old consumers, new producers, and the warehouse sink are considered together. Formats: [Avro / Protobuf / JSON Schema / Iceberg] Registry or catalog: [Confluent, Glue, in-repo, other] Compatibility mode today: [backward / forward / full / none] Producers and consumers with deploy lag: [hours/days] Warehouse or lake sink: [details] Current schemas and CI: [Paste schemas and pipeline YAML] Recent breakages: [field rename, type change, default missing, or none] Produce: 1. Compatibility matrix per topic/table: which mode is actually required 2. Change classes: additive, optional-to-required, type change, rename, enum, default 3. CI checks: registry dry-register, consumer contract tests, sink DDL diff 4. Dual-publish / expand-contract sequence when a breaking change is unavoidable 5. Default, nullability, and union pitfalls by format 6. Runtime detection for poison messages that passed CI 7. Ownership, freeze windows, and exception process 8. Rollback when a compatible-looking change still breaks a slow consumer 9. Example failing and passing PRs for the team's last three change types 10. Metrics: incompatible-register attempts, consumer lag after schema deploys If compatibility mode is FULL but consumers parse JSON loosely, say the registry is lying and add payload tests. Do not recommend "just bump the version" as a strategy.