Loose coupling has more up front costs than tight coupling. The assumption is, over the life of a component (or service) that the loose-coupling device will pay for itself.
A common belief is that software engineers should design services to be 'loosely coupled'. That is, they should verify that the service can speak the 'least common denominator' and do it in a manner that mitigates the impact of location: attempt to minimize the impacts associated with distributed computing (location transparency).
In the physical world, we use devices to decouple entities. Instead of soldering two components together, we connect each component to a plug and then use the plugs to connect the items together. This is necessary in the physical world.
In the logical world, we have the opportunity to attach a 'plug' to the end of a component or a service. And just like in the physical world, it costs more money to buy and attach the plug. But, the plug should make the component more reusable. That is the investment.
Imagine an electrical component that is attached to a printed circuit board. The component has nice plastic plug coming off of it enabling other components to easily connect to it. In addition, the wire lead on the component is exposed and it is mounted in such a way that additional wires can be soldered directly to it. In essence, the component was designed with loose coupling in mind (the plug) but the instantiation (or the use) of the component allowed the engineer to bypass the plug by directly connecting to the wire lead.
Here, the component has both 'component design-time' and 'assembly design-time' properties. The 'component design-time' properties describe the features that the component has; in our case, we are interested in the plug at the end. The 'assembly design-time' deals with the decisions that the engineer makes at design time when they assemble multiple components together (e.g., does the engineer use the plug or solder directly to the wire?)
When we discuss loose coupling or tight coupling, we must remember that there are different issues to consider:
1. The level of investment that was placed on enabling a component to be loosely coupled (e.g., it comes with a plug attached). [POTENTIAL COUPLING]
2. The decisions that the component assembler made when combining the piece components (e.g., they used the plugs everywhere vs. soldered them together everywhere vs. some combination) [ASSEMBLY COUPLING]
3. The option of delaying the assembly interface mechanism decisions to runtime. [DYNAMIC COUPLING]
A significant amount of literature is available on making components/services reusable (creating high potential decoupled devices); thus I won't revisit. Less is written on Assembly Coupling, but I'm still happy with my, "Service Coupling Index" as a foundation. However, very little information is available on the concept of Dynamic Coupling.
In many ways, Dynamic Coupling contradicts the 'loosely coupled' manifest. Rather than stating that something must be implemented in a certain way (asynchronous, document centric, XML based, etc.), it states that the component need only have Decoupling Potential AND that the component be aware of any Dynamic Decoupling Device.
I like to think of the Dynamic Decoupling Device as a "magic printed circuit board". Here, the assembler inserts components into the board. But rather than soldering the components together or using plugs to connect them, the assembler will never execute the actual assembly mechanism. Instead, the assembler tells the printed circuit board which components need to be connected to each other. The "magic printed circuit board" is now responsible for querying the components and determining the best mechanism to connect the piece parts together. We have moved from explicitly stating the connection mechanism to only stating the intent.
In the physical world, creating the "magical printed circuit board" is... well, challenging. In the logical world it is quite easy. At the core, it means:
- We use policies to describe capabilities and interfaces.
- We publish the policies in a consistent manner.
- We perform protocol negotiation at runtime.
- We build components with high decoupling potential.
- We build components that were designed to connect to Dynamic Decoupling Devices.
- We quit hardcoding our assembly mechanisms.
- We mandate the use of Dynamic Decoupling in assembly.
If an architecture is indeed defined by the constraints rather than the features, than the aforementioned may serve as an architectural style that facilitates reuse without giving up performance.