The strategy illustrated at the end still ignores the learning of the FAQ. What if, let's say, one microservice is responsible for calculating the x coordinate

The strategy illustrated at the end still ignores the learning of the FAQ. What if, let's say, one microservice is responsible for calculating the x coordinate but another microservice is responsible for calculating the y coordinate? The former microservice could rightly fill in only the x coordinate and ask the latter microservice to fill in the y coordinate. Then you start to make compromises such as defining an IncompleteCoordinate message that requires x. But what if later you calculate y first and use that to calculate x? Etc. Etc. I find it more convenient to make everything optional at the wire format level and explicitly validate what is required at the point of use, not at the point of parse.