Ensuring All Proto Fields Are Set

The Problem

With proto3, every field is optional. This can be a problem if you want to ensure you always set certain fields.

For example, if we have an RPC to send a message defined as follows:

syntax = "proto3";

package com.my.pkg;

service MessageService {
  rpc SendMessage(MessageRequest) returns (MessageResponse);
}

message MessageRequest {
  string body = 1;
  string from = 2;
  string to = 3;
}

message MessageResponse {
  string id = 1;
}

And we call it using Java:

import com.my.pkg.MessageRequest;

public class Main {
    public static void main(String[] args) {
        var client = new Client();
        var msg = MessageRequest.newBuilder()
            .setBody("hello world")
            .setFrom("+15085550000")
            .build();
        var res = client.sendMessage(msg);
        System.out.println("res", res);
    }
}

We then compile our code, run our linters, which both pass without any problems.

Then when we run it, we’ll end up with an error because we forgot to set the to field on the message!

Bummer.

Fixing it manually

One option is to write some code to validate the message before we send it over the wire.

import com.my.pkg.MessageRequest;

public class Main {
    public static void main(String[] args) {
        var client = new Client();
        var msg = MessageRequest.newBuilder()
            .setBody("hello world")
            .setFrom("+15085550000")
            .build();

        for (var field : msg.getDescriptorForType().getFields()) {
            if (!msg.hasField(field)) {
                throw new RuntimeException("expected field to be set");
            }
        }

        var res = client.sendMessage(msg);
        System.out.println("res", res);
    }
}

This sort of works, we could also even use Mockito and ArgumentCaptor to avoid running the check at runtime, but this still doesn’t handle cases like repeated fields or nested messages.

Fixing it with Protovalidate

Instead of inventing our own validation logic on top of protobufs, we could use the existing protovalidate package.

So instead our proto and code would look as follows:

syntax = "proto3";

package com.my.pkg;

service MessageService {
  rpc SendMessage(MessageRequest) returns (MessageResponse);
}

message MessageRequest {
  string body = 1 [(buf.validate.field).required = true];
  string from = 2 [(buf.validate.field).string = {
    min_len: 1
    max_len: 15
  }];
  string to = 3 [(buf.validate.field).string = {
    min_len: 1
    max_len: 15
  }];
}

message MessageResponse {
  string id = 1;
}

package com.my.pkg;

import build.buf.protovalidate.results.ValidationException;

import com.my.pkg.MessageRequest;

public class Main {
    public static void main(String[] args) {
        var client = new Client();
        var msg = MessageRequest.newBuilder()
            .setBody("hello world")
            .setFrom("+15085550000")
            .build();

        var validator = new Validator();
        try {
            var result = validator.validate(msg);
            if (!result.violations.isEmpty()) {
                System.out.println(result.toString());
                System.exit(1);
            }
        } catch (ValidationException e) {
            System.out.println("Validation failed: " + e.getMessage());
            System.exit(1);
        }

        var res = client.sendMessage(msg);
        System.out.println("res " + res.getId());
    }
}

Conclusion

Check out protovalidate to help validate your protos!