vaquarkhan Data Contract Testing With Schema Registry
Overview
Use this skill when contracts must be tested through registry-backed compatibility rules, not only by convention. It helps agents make event contract validation a blocking part of build and release behavior with automated testing, consumer impact analysis, and evidence-driven release gates.
When to Use
- testing event contract changes before production release
- validating schema compatibility as part of CI/CD pipelines
- coordinating producer and consumer schema evolution across teams
- enforcing registry-backed contract quality gates before deployment
- building release evidence that proves schema safety
Do not use this when schemas are not managed through a registry or when the system has no shared event contracts.
Workflow
Define the contract and compatibility target per subject. Include:
- which schema registry subjects are in scope
- the compatibility level required:
BACKWARD,FORWARD,FULL, orTRANSITIVEvariants - who owns the contract (producing team, platform team, or shared)
- which consumers depend on each subject and their minimum supported version
Implement contract tests in CI.
- register the proposed schema against the registry's compatibility endpoint
- use the registry's
/compatibilityAPI to check before merging - fail the build if compatibility is violated
- produce test output showing which rules passed or failed
- run tests against both the latest registered version and specific consumer versions
Test representative producer and consumer scenarios.
- serialize sample records with the new schema and deserialize with the old
- verify that default values produce correct behavior for removed or added fields
- test edge cases: null handling, enum additions, nested record changes
- for Protobuf: test field number stability and reserved field behavior
- for Avro: test union type evolution and logical type compatibility
Validate downstream consumer impact before release.
- identify all registered consumers of the subject
- test that each consumer's current schema can read the proposed producer schema
- document which consumers need upgrades for breaking changes
- define the deployment order: consumer upgrade before or after producer change
- communicate schema changes through established team channels
Tie contract test results to release gates.
- schema compatibility check must pass before deployment proceeds
- produce structured evidence (JSON report) of compatibility test results
- store evidence alongside the release artifact for audit
- block production deploys on compatibility failures — no manual overrides without review
- include contract test results in pull request checks
Plan for contract migrations and breaking changes.
- when breaking changes are necessary, define a migration plan
- create a new subject or version namespace for the breaking schema
- run both old and new contracts in parallel during migration
- define a deprecation timeline for the old contract
- document the consumer migration path with explicit deadlines
Common Rationalizations
| Rationalization | Reality |
|---|---|
| "We check compatibility manually before releasing." | Manual checks are inconsistent and skip edge cases. Automated registry-backed testing catches issues that humans miss under time pressure. |
| "Adding a field is always safe." | Adding a required field without a default breaks backward compatibility. Adding an optional field is safe only if consumers handle absence correctly. |
| "We only have one consumer so contract testing is overhead." | Consumers multiply over time. Establishing contract testing early prevents breaking changes from accumulating technical debt. |
| "The registry will reject incompatible schemas anyway." | Registry rejection at deploy time is too late. Testing in CI catches issues before code is merged and before release pressure builds. |
Red Flags
- no contract tests in CI — compatibility is checked only at deploy time
- proposed schemas are registered directly without prior compatibility validation
- consumer dependency map does not exist — nobody knows who reads the events
- breaking changes are deployed with "we'll fix consumers later" attitude
- test output is not stored — no evidence trail for audits
- compatibility policy is set to
NONEin production subjects - contract test failures are routinely overridden without review
- migration plans for breaking changes have no timeline or consumer notification
Verification
- Compatibility targets are defined per subject and match consumer expectations
- Contract tests run in CI and block merges on compatibility failures
- Producer and consumer serialization/deserialization are tested with sample records
- Consumer dependency map is documented and up to date
- Contract test evidence is produced, stored, and linked to releases
- Breaking changes have explicit migration plans with timelines
- Deployment order (producer-first vs consumer-first) is defined for each change type