How to distinguish between multiple messages types in Cap'n Proto?

Question

I'm using Cap'n proto to send and retrieve messages between multiple clients and my websocket server.

Since I have only one websocket channel to send and receive data and various type of messages can be sent, I need a way to distinguish between then when trying to decode it.

How can this be done correctly in Cap'n proto?

I think this question is language agnostic, but if a language is needed, I have my server in Rust and clients in Rust, Go and Javascript, so a solution that works in all these languages would be much appreciated.

Answer 1

The best thing to do is create an outer struct which is a union of all the possible types. For example, if you have types Foo, Bar, and Baz, define a type like:

struct Outer {
  union {
    foo @0 :Foo;
    bar @1 :Bar;
    baz @2 :Baz;
  }
}

How to access a union depends on the language, but usually there is a which() method that returns an enum value specifying which field is filled in, and then you use the regular getter methods for a nested struct.

Note that both ends have to use this Outer type. You can't serialize a message with a root type of Foo and then parse it as an Outer -- the message has to be created with type Outer. In general there is no way to distinguish different Cap'n Proto types based on the bytes alone.