Rules for evolving union members

[Andrew Sun]

Suppose I have a schema that looks like this:

struct Foo {

    union {

        bar @0 :Void;

        baz @1 :UInt64;

        # ...

    }

    # ...

}

Now suppose I want to add information to union member bar, or I want to add more fields to baz. What options do I have to do this in a forwards- and backwards-compatible manner?

Would this be the "correct" way to evolve the schema?

struct Foo {

    union {

        bar :group {

            unused @0 :Void;

            foo @2 :UInt32;

            bar @3 :OtherStruct;

            # ...

        }

        baz :group {

            a @1 :UInt64;

            b @4 :Text;

            # ...

        }

        # ...

    }

    # ...

}

[Kenton Varda]

Hi Andrew,

Yes, you got it right. Annoyingly you will have to keep that unused Void in there, but at least it doesn't take any space on the wire.

-Kenton

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