How do I represent a UUID in a protobuf message?

Question

I want to attach a UUID to a field in my protobuf User message example.

message User {
  // field containing id as UUID type
  required string email;
  optional string name;
}

I know that protobuf messages do not yet support the UUID type. I've read that the best approach is to have a UUID message type.

So I'm guessing my User message would import my UUID message proto definition and use it as a field type like so:

import "myproject/UUID.proto";

message User {
  required UUID id;
  required string email;
  optional string name;
}

My question is, how will the UUID message look like, and how will I encode/decode it? I'm aiming for Java/Scala and C# compatibility.

Answer 1

You should probably use string or bytes to represent a UUID. Use string if it is most convenient to keep the UUID in human-readable format (e.g. "de305d54-75b4-431b-adb2-eb6b9e546014") or use bytes if you are storing the 128-bit value raw. (If you aren't sure, you probably want string.)

Wrapping the value in a message type called UUID can be helpful to make the code more self-documenting but will have some performance overhead and isn't strictly required. If you want to do this, define the type like:

message UUID {
  required string value = 1;
}

or:

message UUID {
  required bytes value = 1;
}

Answer 2

If anything, you want to use string to avoid problems with endianness. Note that a UUID and a MS GUID that have the same string representation (and therefore are the same "id") have, however, different byte-stream order (big-endian vs little-endian). If you use bytes in the protocol to communicate between Java using UUID and C# using System.Guid, you could end up with flipped IDs.