Wire Types Recap

The wire format intentionally LACKS schema info. To parse, you need the .proto.

Forward compatibility:

• Old code reading new data: unknown fields preserved + skipped.
• Each wire type knows how to skip itself:
  • Varint: read until top bit clear.
  • Fixed: read N bytes.
  • Length-delimited: read length, skip that many bytes.

Backward compatibility:

• New code reading old data: missing fields default-filled.

Field type changes:

• int32 ↔ int64 ↔ uint32 ↔ uint64 ↔ bool: COMPATIBLE (varint).
• sint32 ↔ sint64: compatible (zigzag).
• string ↔ bytes: compatible (length-delimited).
• fixed32 ↔ sfixed32: compatible (4 bytes).
• fixed64 ↔ sfixed64: compatible (8 bytes).
• Cannot change wire type.
• Cannot reuse a removed field number for different type.

Reserved tags / names:

message User {
    reserved 5, 6, 9 to 11;
    reserved "old_name", "another";

    string name = 1;
    ...
}

Prevents accidental reuse. Compiler errors if conflicting.

Field renaming:

• Wire format unchanged (uses tag, not name).
• Code generators may break if field accessed by name.
• Reflection users beware.