Proto2 vs Proto3: Key Differences

Proto2 vs Proto3 Comparison

Proto3 simplified the protobuf language by removing several proto2 features and changing default behaviors. Understanding the differences is essential when working with legacy schemas or migrating between versions.

// Proto2 syntax
syntax = "proto2";

message UserV2 {
  required int32 id = 1;
  required string name = 2;
  optional string email = 3;
  optional int32 age = 4 [default = 0];
  repeated string tags = 5;

  extensions 100 to 199;
}

// Proto3 syntax
// syntax = "proto3";
//
// message UserV3 {
//   int32 id = 1;
//   string name = 2;
//   string email = 3;
//   optional int32 age = 4;
//   repeated string tags = 5;
// }

Feature Comparison

Feature	Proto2	Proto3
Field presence	required, optional, implicit	Implicit (all optional), explicit optional keyword
Default values	Custom defaults allowed	Fixed defaults only (0, false, "")
required keyword	Supported	Removed (considered harmful)
Unknown fields	Discarded by default	Preserved by default
Enum zero value	Not required	Must have zero value
Extensions	Supported	Removed (use Any instead)
Groups	Supported (deprecated)	Removed
Map fields	Not available	Added
JSON mapping	Basic	Full standard mapping

Why required Was Removed

The required keyword in proto2 was identified as the largest source of backward-compatibility bugs. Once a field is marked required, it can never be removed without breaking all existing parsers. Proto3 treats all fields as optional, making schema evolution safer.

Field Presence in Proto3

In proto3, scalar fields do not track whether they were explicitly set. A field with value 0 could be "set to zero" or "not set at all." To distinguish these cases:

• Use wrapper types (google.protobuf.Int32Value)
• Use the optional keyword (added back in proto3.15+)
• Use a boolean companion field (e.g., bool has_age = 5;)

Migration Strategy

When migrating from proto2 to proto3:

1. Remove all required labels
2. Remove custom default values
3. Remove extensions (replace with google.protobuf.Any or oneof)
4. Ensure all enums have a zero value
5. Test with existing serialized data to verify compatibility