The recommended way to solve this problem is to use 
google.protobuf.FieldMask. The client should just send a request with 1 or 
2 fields plus the FieldMask, and server will perform partial update on it. 
protobuf will provide better support on FieldMask, which the server code 
can utilize to perform the partial update.

On Tuesday, April 7, 2015 at 5:09:41 PM UTC-7, Jonathan Briggs wrote:
> I just ran into this while trying to write a gRPC service that can handle 
> forward and back compatibility.
> So say that a new service is running with new fields in its proto3 message 
> and it returns this to an older client. The older client is supposed to 
> receive it, modify one or two fields and then send it back to update the 
> data.
> It seems that with proto3 the new fields are just being erased when the 
> older client sends back an older message.
> What is the solution to this?
> On Wednesday, December 10, 2014 at 9:51:01 PM UTC-7, Feng Xiao wrote:
>> 2. Removal of unknown fields.

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