> Suppose I'm an application author and I have an integer field called "quantity" which decoded to a 0. How can I tell whether that 0 meant "the quantity was 0

> Suppose I'm an application author and I have an integer field called "quantity" which decoded to a 0. How can I tell whether that 0 meant "the quantity was 0 in the database" or "the quantity field was missing" instead? First, this is clear on the level of wire encoding: either the field has encoded 0 value, or it is simply missing from encoding. Second, in proto2, you actually have has_quantity() method on a proto message, which will tell you whether quantity is missing or set to 0. In proto3, the design decision was that the has_foo() methods are available only on embedded message field, and not available on primitive fields, so you'd have to wrap your int64 in a message wrapper, like e.g. the ones available in google/protobuf/wrappers.proto. The point here (and a common pattern inside google3) is that in your handling code you simply manually check the presence of all required fields: if (!foo.has_quantity()) { return FailedPreconditionError("missing quantity"); }. It is a bit of a hassle, but the benefit is that you have control on where the bug originates and how it is handled in your application layer, as opposed to silently dropping the whole proto message on the floor.