Joe quotes John Hagel's definition of loose coupling, which refers to reduced interdependencies between modules or components, and consequently reduced interoperability risk. Hagel clearly intends this definition to apply to dependencies between business units, not just technical artefacts. I think this is fine as far as it goes, but is not precise enough to my taste.
In his post The Developer's View of SOA: Just adding complexity?, Ron Ten-Hove (Sun Microsystems) defines loose coupling in terms of knowledge - "the minimization of the "knowledge" a service consumer has of a service provider it is is using, and vice versa".
(It is surely possible to link these defining notions - interoperability and interdependency, risk and knowledge - at a deep level, but I'm not going to attempt it right now.)
I want to have a notion of loose coupling that applies to sociotechnical systems of systems - and therefore needs to cover organizational interdependencies and interoperability as well as technical. I have previously proposed a definition of Loose Coupling based on Karl Weick's classic paper on loosely coupled organizations.
The trouble with proclaiming the wonders of loose coupling is that it sounds as if tight coupling was just a consequence of stupid design and/or stupid technology. It fails to acknowledge that there are sometimes legitimate reasons for tight coupling.
Ron Ten-Hove puts forward a more sophisticated argument for loose coupling. He acknowledges the advantages of what he calls a mixed service model, namely that "it allows for creation of a component model that combines close-coupling and loose-coupling in a uniform fashion". But he also talks about the disadvantages of this model, in terms of reduced SOA benefits and increased developer complexity, at least with the current technology.
Loose coupling is great, but it is not a free lunch. It is not simply a bottom-up consequence of the right design on the right platform. Sometimes loose coupling requires a top-down forcing-apart. I think the correct word for this top-down forcing-apart is deconfliction, although when I use this word it causes some of my colleagues to shudder in mock horror.
Deconfliction is a word used in military circles, to refer to the active principle of making one unit independent of another, and this will often include the provision of redundant supplies and resources, or a tolerance of reduced utilization of some central resources. Deconfliction is a top-down design choice.
Deconfliction is an explicit acceptance of the costs of loose coupling, as well as the benefits. Sometimes the deconflicted solution is not the most efficient in terms of the economics of scale, but it is the most effective in terms of flexibility and interoperability. This is the kind of trade-off that military planners are constantly addressing.
[See earlier post on Deconfliction and Interoperability]
Sometimes coupling is itself a consequence of scale. At low volumes, a system may be able to operate effectively in asynchronous mode. At high volumes, the same system may have to switch to a more synchronous mode. If an airport gets two incoming flights per hour, then the utilization of the runway is extremely low and planes hardly ever need to wait. But if the airport gets two incoming flights per minute, then the runway becomes a scarce resource demanding tight scheduling, and planes are regularly forced to wait for a take-off or landing slot. Systems can become more complex simply as a consequence of a change in scale.
(See my earlier comments on the relationship between scale and enterprise architecture: Lightweight Enterprise.)
In technical systems, loose coupling carries an overhead - not just an operational overhead, but a design and governance overhead. Small-grained services may give you greater decoupling, but only if you have the management capability to coordinate them effectively. In sociotechnical systems, fragmentation may impair the effectiveness of the whole, unless there is appropriate collaboration.
In summary, I don't see loose coupling as a principle of SOA. I prefer to think of it as a design choice. I think it's great that SOA technology give us better choices, but I want these choices to be taken intelligently rather than according to some fixed rules. SOA entails just-enough loose coupling with just-enough coordination. What is important is getting the balance right.