what's the right way to do polymorphism with protocol buffers?

Question

I'm trying to long-term serialize a bunch of objects related by a strong class hierarchy in java, and I'd like to use protocol buffers to do it due to their simplicity, performance, and ease of upgrade. However, they don't provide much support for polymorphism. Right now, the way I'm handling it is by having a "one message to rule them all" solution that has a required string uri field that allows me to instantiate the correct type via reflection, then a bunch of optional fields for all the other possible classes I could serialize, only one of which will be used (based on the value of the uri field). Is there a better way to handle polymorphism, or is this as good as I'm going to get?

Answer 1

Have you considered using extensions? You could have your uri field determine the type to use and then just load the appropriate extensions. If you know your fields are mutually exclusive then you could reuse the field id between separate extensions.

You have to handle this all yourself because protocol buffers aren't designed to be self describing beyond a simple list of values. This is touched on in the google techniques page.

Answer 2

Check out Extensions and Nested Extensions for a slightly cleaner way to do this.