docs/guide/javascript/schema-evolution.md

---
title: Schema Evolution
sidebar_position: 60
id: schema_evolution
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  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
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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

```ts
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:

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

Reader schema with fewer fields:

```ts
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:

```ts
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

Compatible mode only protects you from schema differences in the _fields_ of a type. You still need the same type identity (same numeric ID or same `namespace + typeName`) on every side. See [Cross-Language](cross-language.md).

## Related Topics

- [Type Registration](type-registration.md)
- [Cross-Language](cross-language.md)