docs/guide/javascript/schema-evolution.md

title: Schema Evolution sidebar_position: 60 id: schema_evolution license: | Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

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Schema evolution lets different versions of your service exchange messages safely — a v2 writer can produce a message a v1 reader still understands, and vice versa.

Two Modes

Schema-consistent mode (default): more compact, but both sides must have exactly the same schema. Good when all services update together.
Compatible mode: writes extra field metadata so readers can skip unknown fields and tolerate missing ones. Good for independent deployments or rolling upgrades.

Enable Compatible Mode

const fory = new Fory({ compatible: true });

Use this when:

services deploy schema changes independently
older readers may see newer payloads
newer readers may see older payloads from before a field was added

Example

Writer schema:

const writerType = Type.struct(
  { typeId: 1001 },
  {
    name: Type.string(),
    age: Type.int32(),
  },
);

Reader schema with fewer fields:

const readerType = Type.struct(
  { typeId: 1001 },
  {
    name: Type.string(),
  },
);

With compatible: true, the reader ignores fields it does not know about, and fills unknown fields with default values.

Opting Out of Evolution for One Struct

You can disable evolution metadata for a specific struct even inside a compatible: true instance:

const fixedType = Type.struct(
  { typeId: 1002, evolving: false },
  {
    name: Type.string(),
  },
);

evolving: false produces smaller messages for that struct, but both the writer and reader must agree on this setting. If one side writes with evolving: false and the other reads expecting compatible metadata, deserialization will fail.

When to Use Each Mode

	Schema-consistent	Compatible
Services always update together	✔ best choice	works, but wasteful
Independent deployments	will break	✔ best choice
Smallest possible messages	✔	slightly larger
Rolling upgrades	risky	✔ safe

Cross-Language Requirement