> Adding a new type to a union? Breaking change. Adding a new element to an enum? Breaking change. This is incorrect. I've personally gone from type X to union

> Adding a new type to a union? Breaking change. Adding a new element to an enum? Breaking change. This is incorrect. I've personally gone from type X to union {X,Y} in C++, you just need to use the resolving decoder that takes both schemas. Enums can also be resolved based on names. Check out the schema evolution rules here: See https://avro.apache.org/docs/current/spec.html#Schema+Resolution https://avro.apache.org/docs/current/spec.html#Schema+Resolu... Of all the binary serialization formats of this ilk (proto, thrift, flatbuffers etc), Avro definitely has the most schema flexibility.