How to represent that one of two Thrift arguments is required?

Let's say I have an API to fetch a user that can take a user's id or name as an argument. My Thrift definition might look like this.

struct GetUserRequest {
  1: optional string user_id;
  2: optional string user_name;
}

service MyService {
  GetUserResponse get_user(1: GetUserRequest request);
}

but my concern is to show that GetUserRequest can take a user_id OR a user_name, but it doesn't need both fields, but it also needs one. Is there a good way to represent this intent in Thrift?

You can use union:

union GetUserRequest {
  1: string user_id
  2: string user_name
}