Loosely Coupled System - Coupling is the dependency between interacting components. This dependency can be decomposed into real dependency and artificial dependency. Real dependency is the set of features or services that a components consumes from other components. The real dependency always exists and cannot be reduced.

Artificial dependency is the set of factors that a component has to comply with in order to consume the features or services provided by other components. Typical artificial dependency factors are data format dependency, platform dependency, and API-application program interface dependency. A ideal loosely coupled system is one in which artificial dependency between components has been reduced to the minimum.

Examples For Real & Artificial Dependencies: The electric appliances (e.g. TV, LCD-Clock, CD-player or hair-dryer) run on power. Hence the need for the electric power is real dependency. But most power outlets are designed for specific voltage and have a certain type of power outlet (i.e. socket to plug in).

For example, outlets in the Hotel rooms in many Asian countries supply 220V and have different type of sockets (e.g. size and shape). Hence many appliances made in the USA might not work in Asian, because the appliances are designed for 110V and their plugs do not fit into the outlet (or socket) in Asia. Hence, the appliances need to comply artificial factors, such as, voltage range and shape of the plug.

Likewise, monitory compensation is required to buy goods and services. Hence monitory compensation is real dependency. However, overseas travelers may need to convert their national currency to the local currency (e.g. Euro in Europe, dollars in the USA and Yen in Japan), to buy goods or services.

Here the form of the said monitory compensation must be in one of the acceptable form (e.g. local currency or an international credit card), which is an artificial dependence. Although, gold is universal monitory standard for centuries, most shops won’t accept gold and give you back remaining change in small gold pieces. I wish they do that.

Service provider usually determines factors or form of the artificial dependency. For example, a software component vendor may build and sell a reusable-shopping cart (e.g. An Invoice Table or Spreadsheet) for online mall. It may provide services, such as, add an item to the Invoice (i.e. Shopping cart), when visitor purchases an item from an online catalog. The Invoice needs the item’s data (e.g. Price, Discounts, Description and/or Size), which is a real dependency.

The Shopping cart may define one or more data formats to communicate the data, such as, an XML-Schema and a Data-object (or record/structure). Then, an interacting component (e.g. shopping item in the online-catalog) must send its data in one of the supported formats, to properly add the item to the shopping cart.

For example, if the shopping-item sends all the data pieces, but in a no-compliant data format (e.g. a large string containing name and value pairs separated by semicolon) the service request fails because the shopping cart cannot properly process the data send to it.

If you wish to further explore “loosely coupled” systems and service-oriented architecture, you may search the web to find articles or tutorials, such as: http://webservices.xml.com/pub/a/ws/2003/09/30/soa.html