Whats the best way to deprecate a field in Protocol Buffer v3, reserved vs deprecated=true?

Question

When using Protocol Buffers version 3, from what I can see, there are 2 ways you can deprecate a field:

Either using the deprecate field using tags:

message MyMessage {
    string deprecated_my_field = 1 [deprecated=true];
}

Or creating a reserved field ID:

message MyMessage {
    reserved 1; // Deprecated "my_field"
}

I am leaning towards reserved since then no one will be able to use the field.

Is this a bad idea?

Answer 1

Both will work; the first option keeps the definition in the model, so it can still be accessed and queried - but it may generate a build warning when available (for clients who update from the schema). This may have useful applications in some cases.

The second option completely removes the field from the model, so any existing usages will break at build when clients update from the schema. Again, this could be good or bad, depending on how "done" you are with the field.

In some cases, the softer "mark it as a build warning, but allow access" may be preferred; in others, the hard "that doesn't exist!" approach is cleaner. It is subjective.

Either will fundamentally get the job done.