computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...

and

systems design Systems design interfaces, and data for an electronic control system to satisfy specified requirements. System design could be seen as the application of system theory to product development. There is some overlap with the disciplines of system an ...

, a loosely coupled system is one # in which components are weakly associated (have breakable relationships) with each other, and thus changes in one component least affect existence or performance of another component. # in which each of its components has, or makes use of, little or no knowledge of the definitions of other separate components. Subareas include the

coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...

of classes, interfaces, data, and services. Loose coupling is the opposite of tight coupling.

Advantages and disadvantages

Components in a loosely coupled system can be replaced with alternative implementations that provide the same services. Components in a loosely coupled system are less constrained to the same platform,

language Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languages are the primary means by which humans communicate, and may be conveyed through a variety of ...

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

, or build environment. If systems are decoupled in time, it is difficult to also provide

transactional integrity Transaction or transactional may refer to: Commerce *Financial transaction, an agreement, communication, or movement carried out between a buyer and a seller to exchange an asset for payment *Debits and credits in a Double-entry bookkeeping sys ...

; additional coordination protocols are required.

Data replication Replication in computing involves sharing information so as to ensure consistency between redundant resources, such as software or hardware components, to improve reliability, fault-tolerance, or accessibility. Terminology Replication in com ...

across different systems provides loose coupling (in availability), but creates issues in maintaining

consistency In classical deductive logic, a consistent theory is one that does not lead to a logical contradiction. The lack of contradiction can be defined in either semantic or syntactic terms. The semantic definition states that a theory is consistent ...

( data synchronization).

In integration

Loose coupling in broader

distributed system A distributed system is a system whose components are located on different networked computers, which communicate and coordinate their actions by passing messages to one another from any system. Distributed computing is a field of computer sci ...

design is achieved by the use of transactions, queues provided by message-oriented middleware, and interoperability standards. Four types of autonomy, which promote loose coupling, are: ''

reference autonomy Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''name'' ...

'', ''time autonomy'', ''format autonomy'', and ''platform autonomy''. Loose coupling is an architectural principle and design goal in

service-oriented architecture In software engineering, service-oriented architecture (SOA) is an architectural style that focuses on discrete services instead of a monolithic design. By consequence, it is also applied in the field of software design where services are provided ...

s; eleven forms of loose coupling and their tight coupling counterparts are listed in: * physical connections via mediator, *

asynchronous communication In telecommunications, asynchronous communication is transmission of data, generally without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream. Any timing required to recover data f ...

style, * simple common types only in

data model A data model is an abstract model that organizes elements of data and standardizes how they relate to one another and to the properties of real-world entities. For instance, a data model may specify that the data element representing a car be c ...

, * weak type system, * data-centric and self-contained messages, * distributed control of process logic, * dynamic binding (of service consumers and providers), * platform independence, * business-level compensation rather than system-level transactions, * deployment at different times, * implicit upgrades in versioning. Enterprise Service Bus (ESB) middleware was invented to achieve loose coupling in multiple dimensions; however, overengineered and mispositioned ESBs can also have the contrary effect and create undesired tight coupling and a central architectural hotspot.

Event-driven architecture Event-driven architecture (EDA) is a software architecture paradigm promoting the production, detection, consumption of, and reaction to events. Overview An ''event'' can be defined as "a significant change in state". For example, when a consume ...

also aims at promoting loose coupling.How EDA extends SOA and why it is important
Jack van Hoof

Methods for decreasing coupling

Loose coupling of interfaces can be enhanced by publishing data in a standard format (such as

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. ...

JSON JSON (JavaScript Object Notation, pronounced ; also ) is an open standard file format and data interchange format that uses human-readable text to store and transmit data objects consisting of attribute–value pairs and arrays (or other s ...

). 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).

In programming

Coupling refers to the degree of direct knowledge that one component has of another. Loose coupling in computing is interpreted as encapsulation vs. non-encapsulation. An example of tight coupling occurs 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. 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 The Unified Modeling Language (UML) is a general-purpose, developmental modeling language in the field of software engineering that is intended to provide a standard way to visualize the design of a system. The creation of UML was originally ...

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:

Other forms

Computer programming languages having notions of either functions as the core module (see

Functional programming In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions that ...

) or functions as objects provide excellent examples of loosely coupled programming. Functional languages have patterns of

Continuation In computer science, a continuation is an abstract representation of the control state of a computer program. A continuation implements ( reifies) the program control state, i.e. the continuation is a data structure that represents the computat ...

s, Closure, or generators. See Clojure and

Lisp A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech. Types * A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lispi ...

as examples of function programming languages. Object-oriented languages like

Smalltalk Smalltalk is an object-oriented, dynamically typed reflective programming language. It was designed and created in part for educational use, specifically for constructionist learning, at the Learning Research Group (LRG) of Xerox PARC by ...

and

Ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapp ...

have code blocks, whereas

Eiffel Eiffel may refer to: Places * Eiffel Peak, a summit in Alberta, Canada * Champ de Mars – Tour Eiffel station, Paris, France; a transit station Structures * Eiffel Tower, in Paris, France, designed by Gustave Eiffel * Eiffel Bridge, Ungheni, ...

has agents. The basic idea is to objectify (encapsulate as an object) a function independent of any other enclosing concept (e.g. decoupling an object function from any direct knowledge of the enclosing object). See

First-class function In computer science, a programming language is said to have first-class functions if it treats functions as first-class citizens. This means the language supports passing functions as arguments to other functions, returning them as the values from ...

for further insight into functions as objects, which qualifies as one form of first-class function. So, 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 call 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 to call 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

style or the

synchronous message passing In computer science, message passing is a technique for invoking behavior (i.e., running a program) on a computer. The invoking program sends a message to a process (which may be an actor or object) and relies on that process and its supporting i ...

style

Measuring data element coupling

The degree of the loose coupling can be measured by noting the number of changes in

data element In metadata, the term data element is an atomic unit of data that has precise meaning or precise semantics. A data element has: # An identification such as a data element name # A clear data element definition # One or more representation terms ...

s that could occur in the sending or receiving systems and determining if the computers would still continue communicating correctly. These changes include items such as: # Adding new data elements to messages # Changing the order of data elements # Changing the names of data elements # Changing the structures of data elements # Omitting data elements

References

{{Software quality Enterprise application integration Service-oriented (business computing) Programming principles Software quality