How do you use schema evolution in streaming systems?

Schema evolution in streaming systems allows data schemas to change over time while maintaining compatibility between producers and consumers. In streaming architectures like Apache Kafka or Apache Pulsar, data is continuously produced and consumed, making it impractical to stop the system for schema updates. Schema evolution handles this by ensuring new schema versions can coexist with older ones. For example, if a producer starts sending data with an added field, consumers using an older schema should still process the data without errors, ignoring the new field. This is achieved through compatibility modes like backward, forward, and full compatibility, which define how schemas can evolve without breaking existing applications.

To implement schema evolution, streaming systems often use schema registries and serialization formats that support versioning. A schema registry (e.g., Confluent Schema Registry, Apicurio) stores schema versions and enforces compatibility rules. When a producer sends data, it references the schema version in the registry, and consumers fetch the appropriate schema to deserialize the data. Formats like Avro, Protobuf, and JSON Schema provide built-in support for schema evolution. For instance, Avro allows adding or removing fields with default values, while Protobuf uses field numbers and optional/required flags to manage changes. A common example is adding a non-required field (e.g., a “middle_name” field in a user profile): producers can include it, and consumers without the updated schema simply skip it.

However, schema evolution requires careful planning. Breaking changes, like renaming a field without backward compatibility, can cause consumer failures. Teams should test schema changes in staging environments, use automated compatibility checks, and document version histories. For example, if a field’s data type changes from an integer to a string, a forward-compatible schema might retain the integer field while introducing a new string field, allowing consumers to migrate gradually. Monitoring tools can alert developers to schema mismatches or consumer lag during transitions. By combining registry tools, compatible serialization formats, and clear versioning policies, streaming systems can evolve schemas safely without disrupting real-time data flows.