Data Contracts & Schema Governance

Beyond compatibility checks

Schema Registry stops a producer from breaking the wire format. It does not stop a team from changing what a field means, deprecating a topic nobody knew you depended on, or leaking PII into a shared stream. That gap is what data contracts close. A data contract is the schema plus the organisational metadata around it:

• Ownership — a named team, an on-call, an SLA for the topic.
• Semantics — field-level docs, units, allowed ranges, enumerations.
• Classification — which fields are PII / financial / public; retention rules.
• Quality guarantees — freshness, completeness, the compatibility policy.
• Change process — deprecation windows, who must approve a breaking change.

Modern registries support schema metadata, tags and rules (CEL/JSONata validation, field-level encryption directives) so the contract is enforced at serialize time, not just documented in a wiki nobody reads.

Promote a bare schema into a contract-bearing one: add doc strings, units, an ownership block, and tag the PII field so downstream tooling can enforce masking and retention.