# SchemaRegistryException: 模式不向后兼容——版本3中添加了没有默认值的字段'phone_number'

- **ID:** `data/avro-schema-registry-backward-transitive-failure`
- **领域:** data
- **类别:** schema_error
- **错误码:** `42201`
- **验证级别:** ai_generated
- **修复率:** 90%

## 根因

Avro模式演化要求在启用向后兼容性时新字段必须有默认值；添加没有默认值的字段会破坏读取旧数据的消费者。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| Confluent Schema Registry 7.5.0 | active | — | — |
| Apache Avro 1.11.3 | active | — | — |
| Kafka 3.6.0 | active | — | — |

## 解决方案

1. ```
   Update the Avro schema to add the field with a default value: `{"name": "phone_number", "type": ["null", "string"], "default": null}` and register as version 4 with backward compatibility
   ```
2. ```
   If the field must be required, change compatibility to FORWARD_TRANSITIVE, register the new schema, then update consumers to handle the new field before switching back to BACKWARD
   ```
3. ```
   Use Schema Registry REST API to set compatibility to BACKWARD_TRANSITIVE on the subject, then re-register: `curl -X PUT -H "Content-Type: application/json" --data '{"compatibility": "BACKWARD_TRANSITIVE"}' http://localhost:8081/config/<subject>`
   ```

## 无效尝试

- **** — Schema Registry enforces compatibility globally; deleting a version may break existing producers/consumers and cause data loss if they reference that version ID. (80% 失败率)
- **** — Disabling compatibility entirely removes all checks, allowing any schema change, which can cause silent data corruption for downstream consumers that expect a specific structure. (75% 失败率)
- **** — The producer will fail to serialize because the schema must match the registered schema exactly (Avro writer schema validation). (100% 失败率)