Architecture & System Design › Events & Integration

Schema Registry

A central store of event schemas and their compatibility rules.

Backend Engineer · Senior Data Engineer · Mid-level

Also known as: schema registry, event schema registry, avro schema registry

A schema registry centralises event/message schemas: producers register versions, consumers fetch them, and compatibility rules (backward/transitive) reject breaking publishes in CI — before they reach production. It’s version control plus contract enforcement for the event-driven estate.

producer CI → register v3 → compatibility check (vs v2, v1) → pass/fail
consumer → fetch schema → deserialize safely (upcasters handle gaps)

Registries pair with binary formats (Avro, Protobuf) where schemas aren’t self-describing, but govern JSON too. References by id (not embedded schemas) keep payloads lean while retaining evolvability.

The classic mistakes:

Registry as documentation only. Schemas registered but compatibility unchecked decay into suggestion boxes. Enforce in CI; block breaking publishes.
Incompatible defaults. Permissive modes (“none”) during early development fossilise into production anarchy. Start strict (backward-transitive); relax deliberately per subject.
Unregistered producers. Shadow topics bypassing the registry fragment governance — unknown schemas breaking unknown consumers. All production schemas registered, no exceptions.
Deleting schemas. Removing old versions breaks replay, new consumers and audits. Schemas are immutable history; deprecate, never delete.
Subject strategy chaos. Topic-per-subject vs global subjects mixing breaks compatibility scoping. One strategy, documented, enforced.
Ignoring consumer lag. Compatibility windows must cover the slowest consumer’s upgrade cadence; aggressive sunsets strand laggards. Version support follows consumer reality.
No disaster story. Registry outage blocking all deploys (or worse, deserialization) makes governance the SPOF. Cache, replicate, and degrade publishes safely.

How to run it: every schema registered, compatibility enforced in CI, versions immutable, subjects strategised. The registry is the event estate’s constitution — write it strictly, amend it carefully.