Design a schema registry service that stores, versions, and enforces the message schemas flowing through a streaming platform, so producers and consumers can evolve independently without breaking one another.

Operating context. Hundreds of streaming topics each carry messages under an evolving schema. A producer registers a schema and receives an immutable schema ID that it embeds in every message; a consumer fetches the schema by ID to deserialize. The registry enforces a compatibility policy (backward, forward, or full) before accepting a new version. The workload is overwhelmingly read-heavy — every producer and consumer caches schemas but must resolve an unknown ID quickly — and a registry outage must not halt already-cached serialization.

Out of scope. The serialization format specification, the message broker, the data records themselves, and access-control policy authoring.

What to produce. A high-level architecture covering: schema storage and per-subject versioning, the register / lookup API and how immutable schema IDs are assigned (and identical schemas deduplicated), compatibility-rule enforcement at registration time, client-side caching and why immutable IDs make it safe, high availability and read scaling including a degraded read path during a writer outage, and how a multi-region deployment keeps IDs globally consistent. Sketch the components and flow; we will probe specifics at checkpoints.