protobuf oneof backwards compatibility - protocol-buffers

Question

If I had some protobufs created with the following protobuf schema
message Foo {
Bar1 bar_1 = 1;
Bar2 bar_2 = 2;
}
but later on updated the protobuf schema to
message Foo {
oneof foo {
Bar1 bar_1 = 1;
Bar2 bar_2 = 2;
}
}
Will this second version be able to read the protos created with the first version?

Answer 1

I don't think it can.
A Foo message created with the first version of the schema can contain both a bar_1 and a bar_2.
Code generated from the second schema is expecting there to be only a bar_1 or only a bar_2, so regardless of whatever markers GPB puts in its wireformat to denote a oneof, this code wouldn't know what to do with the surplus bar_2.
It's possible that, with the right schema syntax version (is it v3 makes everything optional?) that a message created using code for the first schema that contains only a bar_1 or bar_2 (made possible by the fields being optional) may be parsable by code generated from the second schema. But that would come down to how oneof is treated in the GPB wireformat.
All in all, it's best to not make assumptions about wireformat compatibility between conflicting schemas. It'd be easy to write a small utility to read Foo messages created by the first schema, check to see if there are 2 fields pressent, and create fresh Foo messages under the 2nd schema if there are not (you may have to compile the schema with appropriately diverse namespaces configured). That way you can catch the exceptions (2 fields present), and be sure to have ended up with compatible wireformat data.

Answer 2

Yes, the second version of the protobuf schema should be able to read protobufs created with the first version. When you update a protobuf schema, the changes you make only affect how new protobufs are encoded and decoded. Protobufs that were created with the previous version of the schema will still be encoded and decoded using the old schema. This means that the second version of the schema should still be able to read protobufs created with the first version, even though the schema has changed.
However, it is worth noting that when you make changes to a protobuf schema, you should take care to ensure that the changes are backward-compatible. This means that the new schema should still be able to read protobufs created with the old schema, without losing any information. In the example you provided, the change from the first version of the schema to the second is backward-compatible, so the second version should be able to read protobufs created with the first version. However, if you made a change that was not backward-compatible, the second version of the schema would not be able to read protobufs created with the first version.