Middleware Matters

Dan Pritchett takes issue with Pete Lacey's characterization in his InfoQ interview of web services being too tightly coupled. Dan's issue seems to be that REST is more tightly coupled than its proponents would have you believe. He writes:

The simple facts though is that REST doesn't make this any better. Resources have formats (schemas if XML). They are located by URI's which is a contract for where they can be found. Change the resource format in an incompatible way and clients break. Move the resource to a new location and the clients break. Contracts are contracts, break them and you break the software.

The "move the resource" issue is bogus, given that you can just redirect clients (e.g., HTTP 301). But more generally, coupling isn't so black-and-white. It's not an on-or-off switch. Rather, coupling is measured in degrees. REST advocates claim that it can allow for more loosely coupled systems than SOA or WS-*. And I agree with them.

Take for example REST's uniform interface constraint. If you consider the client/service interaction equation to consist of factors for interface coupling, data coupling, and semantic coupling, REST's uniform interface constraint effectively removes the entire interface coupling factor. SOA and WS-*, OTOH, actually promote interface coupling as a strength.

As I finish writing this, I see that Pete has already responded to Dan, and he's right on target, as usual.