The problem here as I understood it is more general and orthogonal to what the Parse, Don't Validate article is talking about. 1. A field being "required", is

The problem here as I understood it is more general and orthogonal to what the Parse, Don't Validate article is talking about. 1. A field being "required", is not a property of the field itself, but of the construct holding the field. JSON-Schema does this correctly, by letting you define an array of required fields on the schema of an object instead of it being a property of a specific field. 2. Consumers, not producers should decide what their assumptions are. Producers should decide what their guarantees are. 3. Everyone, including the parts in the middle (here it's a message bus) should only state assumptions that they actually need in order to function. Use different schemas for different assumptions (which is easier to do if the "required" assumption is a property of a construct and not a field). The example in the article you mentioned illustrates nicely how these three principles are broken by "required" (or maybe how Protocol Buffers are used in general).