Question about proto message - breaking change behavior

[Aggarwal Sre]

Hi All,

I recently observed some behavior w.r.t proto message - marshalling/unmarshalling. Wanted to know if there is a better way to determine the contract break specifically for code written in C#?

Client (Version 1) and server (Version 1) were using different versions of a message with some *breaking change*. My understanding *was* in case of a contract break we will see some sort of exception but turns out the field (field3) was just an empty object [Not null]. In our scenario it is possible to have an empty object for that field, so checking null or empty object won't help either.

Is there a guideline to determine a contract break at run time? Or do we need to somehow ensure in our code change review process or build process that ordinals are not changed?

Version 1

Message A {

string field1 = 1;

uint64 filed2 = 2;

map<string, int> field3 = 3;

}

Version 2

Message A {

string field1 = 1;

map<string, int> field3 = 2;

}

Regards,

Vivek 

[Terry Wilson]

I can't comment on the C# specifics, but when dealing with protobuf changes, one should never reuse the ordinal numbers of deleted fields. If you don't need a field anymore, you should reserve the field number. You can find some documentation on this over here.

A better approach here for your version 2 would be:

message A {

  reserved 2;

  string field1 = 1;

  map<string, int> field3 = 3;

}

This will avoid problems with coding/decoding the message, which I suspect you are experiencing here.

Hope that helps,

Terry