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In computing and systems design, a loosely coupled system is one
If systems are decoupled in time, it is difficult to also provide transactional integrity; additional coordination protocols are required. Data replication across different systems provides loose coupling (in availability), but creates issues in maintaining data consistency (data synchronization).
Four types of autonomy which promote loose coupling, are: reference autonomy, time autonomy, format autonomy, and platform autonomy.F. Leymann Loose Coupling and Architectural Implications , ESOCC 2016 keynote
Loose coupling is an architectural principle and design goal in service-oriented architectures. Eleven forms of loose coupling and their tight coupling counterparts are listed in:N. Josuttis, SOA in Practice. O'Reilly, 2007, .
Event-driven architecture also aims at promoting loose coupling. How EDA extends SOA and why it is important Jack van Hoof
Loose coupling between program components can be enhanced by using standard data types in parameters. Passing customized data types or objects requires both components to have knowledge of the custom data definition.
Loose coupling of services can be enhanced by reducing the information passed into a service to the key data. For example, a service that sends a letter is most reusable when just the customer identifier is passed and the customer address is obtained within the service. This decouples services because services do not need to be called in a specific order (e.g. GetCustomerAddress, SendLetter).
An example of tight coupling is when a dependent class contains a pointer directly to a concrete class which provides the required behavior. The dependency cannot be substituted, or its "signature" changed, without requiring a change to the dependent class. Loose coupling occurs when the dependent class contains a pointer only to an interface, which can then be implemented by one or many concrete classes. This is known as dependency inversion. The dependent class's dependency is to a "contract" specified by the interface; a defined list of methods and/or properties that implementing classes must provide. Any class that implements the interface can thus satisfy the dependency of a dependent class without having to change the class. This allows for extensibility in software design. A new class implementing an interface can be written to replace a current dependency in some or all situations, without requiring a change to the dependent class; the new and old classes can be interchanged freely. Strong coupling does not allow this.
This is a UML diagram illustrating an example of loose coupling between a dependent class and a set of concrete classes, which provide the required behavior:
For comparison, this diagram illustrates the alternative design with strong coupling between the dependent class and a provider:
For example, in an object-oriented language, when a function of an object is referenced as an object (freeing it from having any knowledge of its enclosing host object) the new function object can be passed, stored, and called at a later time. Recipient objects (to whom these functional objects are given) can safely execute (call) the contained function at their own convenience without any direct knowledge of the enclosing host object. In this way, a program can execute chains or groups of functional objects, while safely decoupled from having any direct reference to the enclosing host object.
Phone numbers are an excellent analog and can easily illustrate the degree of this decoupling.
For example, some entity provides another with a phone number to get a particular job done. When the number is called, the calling entity is effectively saying, "Please do this job for me." The decoupling or loose coupling is immediately apparent. The entity receiving the number may have no knowledge of where the number came from (e.g. a reference to the supplier of the number). On the other side, the caller is decoupled from specific knowledge of who they are calling, where they are, and knowing how the receiver of the call operates internally.
Carrying the example a step further, the caller might say to the receiver of the call, "Please do this job for me. Call me back at this number when you are finished." The 'number' being offered to the receiver is referred to as a "Call-back". Again, the loose coupling or decoupled nature of this functional object is apparent. The receiver of the call-back is unaware of what or who is being called. It only knows that it can make the call and decides for itself when to call. In reality, the call-back may not even be to the one who provided the call-back in the first place. This level of indirection is what makes function objects an excellent technology for achieving loosely coupled programs.
Communication between loosely coupled components may be based on a flora of mechanisms, like the mentioned asynchronous communication style or the synchronous message passing style
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