Proto2 and proto3 are wire compatible. The same construct in proto2 and
proto3 will have the same binary representation. If your proto only uses
features available in both proto2 and proto3, systems built with proto2
should be able to communicate with systems built with proto3 without any
problem (it's also true vice versa).

On Tue, Mar 24, 2015 at 11:12 AM Mike Heffner <[email protected]> wrote:

> Hi,
>
> I haven't been able to find any documentation that explicitly states
> whether the wire (binary) format of Protobuf v2 messages are compatible
> with protobuf messages generated with v3 (using `syntax=proto2`). The only
> documentation seems to mention API differences between the two library
> versions.
>
> We use protobuf-java and would like to transition from PB v2->v3, but only
> for new message types. We would like systems that use protobuf-java v2 and
> protoc v2 generated messages to be able to communicate with systems using
> protobuf-java v3 and protoc v3 generated messages (and vice versa). It
> stands to reason that messages that use `syntax=proto3` are not guaranteed
> to work in this fashion, but I'm curious about messages marked as proto2.
>
>
> Thanks,
>
> Mike
>
>
>
>
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