# Evolving Schemas Safely — gRPC

Source: https://www.skillbyai.com/en/grpc/p-evolve

> Backward and forward compatibility.

## Rules for changing messages

Protobuf supports compatible evolution if you follow the rules. **Safe**: adding new fields with new numbers (old code ignores unknown fields), adding enum values (with care for old clients), adding new RPC methods. **Unsafe**: changing a field's number or type, reusing a deleted field's number or name, renaming packages or services. When removing a field, mark its number and name `reserved` so they are never reused. Tools such as `buf breaking` check changes against the previous version automatically.

## Removing a field safely

Reserve the old number and name.

```protobuf
message Order {
  reserved 6;                 // was coupon_code
  reserved "coupon_code";
  string id = 1;
  string customer_id = 2;
  repeated OrderItem items = 3;
  int64 total_cents = 4;
  string currency = 5;
  repeated string promotion_ids = 7;   // new field, new number
}
```

## Version packages for breaking changes

Put breaking redesigns in a new package such as shop.v2 and run both versions during migration.

**Quiz:** Which protobuf change is safe?

- [ ] Changing a field from string to int64
- [ ] Changing an existing field's number
- [ ] Reusing a deleted field's number
- [x] Adding a new field with a new number

*Answer:* Adding a new field with a new number. Unknown fields are ignored by old code.