Hi all, While I was trying to understand how schema evolution works in Avro, I noticed that the schema resolution logic ( https://avro.apache.org/docs/1.8.1/spec.html#Schema+Resolution) is implemented in two different places in Avro: in ResolvingGrammarGenerator and in SchemaCompatibility.
Could someone please explain the differences between ResolvingGrammarGenerator, SchemaCompatibility, and SchemaValidator? It seems that SchemaValidator is used to check the compatibility between the reader and the writer schema, which sounds exactly what SchemaCompatibility does, why does it use ResolvingGrammarGenerator instead of SchemaCompatibility? Why is the same schema resolution logic implemented twice (if I understood correctly)? I also have a question related to unions and schema resolution. In the patch for AVRO-1933 Anders commented out a case for unions: union with an int branch is not compatible with a long union reader. After a brief investigation I noticed, that ResolvingGrammarGenerator#bestBranch doesn't allow int -> float promotion in union branches. Is this a bug? Should we allow this promotion at all, since like Anders mentioned in AVRO-1931 this is a lossy conversion? What do you think? Thanks, Nandor
