Going from proto3 to proto2 should be fine. There are some slight
differences but I can't think of any major problems. The only thing that
comes to mind is that proto2 handles unknown enum values a little bit
differently from proto3. I doubt that would be a problem but if you want to
be extra cautious you could double-check that you're not storing any
unknown enum values.

However, there is no need to downgrade to version 2.6.1 and if anything
that would only introduce bugs and make the code slower. The proto2
semantics are still fully supported in all versions going forward, so all
you have to do is put syntax = "proto2"; at the top of your .proto files.
You can stick with 3.6.1 or even upgrade to any newer version.

On Thu, Mar 14, 2019 at 10:21 AM Jason Huang <[email protected]>
wrote:

> i chose proto3 for cache (serialize) for my application , and it have run
> for several month . but now i wanna change my mind for proto2 , because the
> hasField is really needed .
>
> the problem is , there're still lots of data in the cache which are
> serialized with proto3 , if i can't deserialize with them with proto2 . it
> will be unacceptable .
>
> my question is , is it safe to switch from proto3 to proto2 ? i made some
> test i could work in some case . but i'm afraid this is not full coverage
> test .
>
> my current proto3 version is 3.6.1 , and the proto2 version i want to use
> is 2.6.1
>
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