A recent question mentioned the term canonical schema in a context of microservices architecture. After reading the Wikipedia article, as well as one of the answers to the question, I still don't understand what canonical schema is about. I get that it's a way to decouple the microservices by doing some magic with the data model, but I'm lost when it comes to the concrete application of the pattern. Other resources are talking about standardized information sets, which make things only more cryptic.
Imagine the microservice A is consuming messages from the microservice B through a message queue service. Let's say those JSON messages contain information about the availability of the products in a warehouse. While A doesn't have to know anything about the existence or location of B, I imagine that it still needs to know:
That the messages are formatted using JSON. If B suddenly starts to format messages in XML, I can hardly see how A would magically adapt itself, unless it was specifically programmed to deal with both JSON and XML messages.
The actual data model, limited to the part used by A. If A simply needs the product ID and the availability, A may not bother to know that the JSON message also contains the product full name, or the location within the warehouse. But it has to know that the product ID is stored in the field
/product/idand formatted as a GUID, and that the quantity is stored in
/quantityand formatted as a number. Again, if B switches to
long-based IDs for the products, A won't be able to deal with it, unless the programmer had this potential format change in mind.
So, what this design pattern is about, and how is it used in practice? Given my example with the services A and B, what would happen if canonical schema pattern is applied?
Maybe it's all about A reading the schema of B and adapting dynamically to it? So it's exactly like Swagger, and also like reading WSDL on runtime and determining how a SOAP service should be called, is it?