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FastAPI Backend Development Bootcamp · Lesson

Schema Registry and Avro Contract Evolution

Enforce event contracts with a schema registry and evolve payloads using compatibility rules.

Why Event Contracts Need a Registry

In an event-driven FastAPI backend, your service publishes events to Kafka or Pulsar and many independent consumers read them. The event payload is a contract: producers and consumers must agree on field names, types, and structure.

  • If the producer renames user_id to userId, every consumer breaks silently.
  • Plain JSON has no enforced shape, so a typo ships straight to production.

A Schema Registry stores versioned schemas centrally and rejects messages that violate the agreed contract, decoupling teams while keeping data safe.

Avro: A Compact, Schema-First Format

Apache Avro is the most common serialization format used with schema registries. Each record is described by a JSON schema, and the binary payload itself carries no field names, only values, making it compact.

  • The schema defines name, type, fields, and optional default values.
  • Readers need the schema to decode the bytes, which is exactly why the registry exists.

Below is a minimal Avro schema for an OrderCreated event.

order_created_schema = {
    "type": "record",
    "name": "OrderCreated",
    "namespace": "com.shop.events",
    "fields": [
        {"name": "order_id", "type": "string"},
        {"name": "user_id", "type": "string"},
        {"name": "amount_cents", "type": "long"},
        {"name": "currency", "type": "string"},
    ],
}

print(order_created_schema["name"], "has", len(order_created_schema["fields"]), "fields")

All lessons in this course

  1. Producing and Consuming Kafka Events Asynchronously
  2. Schema Registry and Avro Contract Evolution
  3. The Transactional Outbox Pattern
  4. Idempotent Consumers and Exactly-Once Semantics
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