Upgrade compiler and runtime lib from proto2 to proto3 but stay file syntax version on 2

[Jiang Cao]

We had persistent some data using proto2, we plan upgrade the compiler and runtime library to proto3 but keep the proto file syntax on 2. Is it a safe upgrade operation? Is there any risk reading the old binary data? Is it officially supported by protobuf project?  I don't see any doc describing it. 

[sh...@google.com]

I don't think we have separate compiler and runtime versions for proto2 and proto3, unless you meant that proto2 is protobuf 2.x release and proto3 is protobuf 3.x release. In that case, there might be minor breaking changes for the version upgrades you'll need to pay attention. Actually, I recommend you make an upgrade since 2.x release is about nine years ago.

[Jiang Cao]

Thanks for your help with this issue! Yes, I mean protobuf 2.x and protobuf 3.x release. We have lots of binary data persistent by protobuf 2.x and syntax v2. Now we want upgrade to protobuf 3.x  and keep using syntax v2, but we have concern on the existing data. Will protobuf 3.x be compatible with protobuf 2.x binary data?   Is it officially supported by protobuf project? 

[Dr. Holley]

Compiled files produced by protoc executable aren't binary files. They are just plain serializable data structures, not objects, to be usable in the accepted programming language of your choice. Protobuf is designed to be backward compatible, so you can rest assure on that outdated version support.

-Dr. Holley