What's the difference between interoperability and integration?
I found this question in "Outstanding questions regarding BPEL and ESB" on the Richard Brown's Gendal World blog. Richard Brown is an ISSW coworker of mine in the UK and, from what I hear, quite smart. (Then again, he says he reads my blog. But maybe he's still smart anyway!) His blog is really good, BTW. Richard gets the question from James McGovern in a post with the same title, "Outstanding questions regarding BPEL and ESB."
So, what's the difference? Wikipedia says "Interoperability: the capability of different programs to exchange data via a common set of business procedures, and to read and write the same file formats and use the same protocols" and "Integration allows data from one device or software to be read or manipulated by another, resulting in ease of use." Yuck, those aren't much help.
To me, interoperability means that two (or more) systems work together unchanged even though they weren't necessarily designed to work together. Integration means that you've written some custom code to connect two (or more) systems together. So integrating two systems which are already interoperable is trivial; you just configure them to know about each other. Integrating non-interoperable systems takes more work.
The beauty of interoperability is that two systems developed completely independently can still work together. Magic? No, standards (or at least specifications, open or otherwise); see Open Standards in Everyday Life. Consider a Web services consumer that wants to invoke a particular WSDL, and a provider that implements the same WSDL; they'll work together, even if they were implemented independently. Why? Because they agree on the same WSDL (which may have come from a third party) and a protocol (such as SOAP over HTTP) discovered in the binding. How does the consumer discover the provider? Some registry, perhaps one that implements UDDI (which sucks, BTW). So SOAP, HTTP, WSDL, UDDI--all that good WS-I stuff--make Web services interoperable.
Another example I like is the "X/Open Distributed Transaction Processing (DTP) model" (aka the XA spec); see "Configuring and using XA distributed transactions in WebSphere Studio." With it, a transaction manager by one vendor can use resource managers by other vendors. Even though they weren't all written for each other, they still work together because they follow the same spec. They're interoperable.
Now consider two systems that weren't designed to be interoperable, or perhaps interoperable but with different specs. This requires integration. The integration code--could be Java, Message Broker, etc.; I co-authored a whole book on this--takes the interface one system expects and converts it to the one the other system provides. This is why WPS has stuff like Interface Maps and Business Object Maps.
So, you want interoperable systems; integrating them is simple. Otherwise, you have to integrate them yourself.