Interoperability is Hard

True interoperability between independent software products is hard. There are multiple levels on which you need to guarantee compatibility (read: agreement among all parties) in order for any deep interoperability to work correctly, including: transport, schema, semantics, ownership, and identity.

A lot of so-called interoperability works fairly well by severely limiting one or more of these aspects (most often identity and ownership), which is fine. However, most of the conversations I hear tend to revolve around the transport, paying minimal attention to schema, and almost none to semantics.

I think this is because 95% of your implementation time tends to get eaten up by the transport, which fools you into thinking it's the hardest part. In fact, semantics is often the hardest part, but that's all done in design, which can easily get ignored. The problem is, a transport defect can be found and fixed in one product, whereas a semantic defect usually causes you to need to change the schema, which affects all products (and therefore usually doesn't get fixed, leading to poor interoperability).

Therefore good interoperability needs the meaning of your data understood and agreed upon by all parties before you settle on the schema. This turns out to be quite hard to do. It's pretty difficult even if you're in control of all the moving pieces.

I'd like to think there's a way to decouple parts of interoperability to be able to iterate on standards after they are entrenched, but I haven't found it yet.