Chapter 22: Schema Evolution and Backward Compatibility

Golden Rules of Proto Evolution

• NEVER change a field number
• NEVER reuse a deleted field number
• NEVER change a field type (int32 → string)
• OK to add new fields (old clients ignore them)
• OK to remove fields (old clients see default value)
• OK to rename fields (wire format uses numbers, not names)

Reserved Fields

message User {
  reserved 3, 7;              // these numbers can never be reused
  reserved "old_field";       // this name can never be reused
  string name = 1;
  string email = 2;
  // field 3 was "phone" — removed, reserved forever
}

Backward vs Forward Compatibility

Scenario	What Happens
New client, old server	Server ignores unknown fields (forward compatible)
Old client, new server	Client sees default values for new fields (backward compatible)

Mark as Complete