Logical Application component is an encapsulation of application functionality that is independent of a particular implementation. As such, it is a replaceable part of a system. It encapsulates its contents and exposes its functionality through a set of interfaces. An application component performs one or more application functions. It may collaborate with other application components through the application interfaces of those components.
Component-based software engineering (CBSE), also called components-based development (CBD), is a branch of software engineering that emphasizes the separation of concerns with respect to the wide-ranging functionality available throughout a given software system. It is a reuse-based approach to defining, implementing and composing loosely coupled independent components into systems. This practice aims to bring about an equally wide-ranging degree of benefits in both the short-term and the long-term for the software itself and for organizations that sponsor such software.
Software engineering practitioners regard components as part of the starting platform for service-orientation. Components play this role, for example, in web services, and more recently, in service-oriented architectures (SOA), whereby a component is converted by the web service into a service and subsequently inherits further characteristics beyond that of an ordinary component.
Definition and characteristics of components
All system processes are placed into separate components so that all of the data and functions inside each component are semantically related (just as with the contents of classes). Because of this principle, it is often said that components are modular and cohesive.
With regard to system-wide co-ordination, components communicate with each other via interfaces. When a component offers services to the rest of the system, it adopts a provided interface that specifies the services that other components can utilize, and how they can do so. This interface can be seen as a signature of the component - the client does not need to know about the inner workings of the component (implementation) in order to make use of it. This principle results in components referred to as encapsulated. The UML illustrations within this article represent provided interfaces by a lollipop-symbol attached to the outer edge of the component.
However, when a component needs to use another component in order to function, it adopts a used interface that specifies the services that it needs. In the UML illustrations in this article, used interfaces are represented by an open socket symbol attached to the outer edge of the component.
Another important attribute of components is that they are substitutable, so that a component can replace another (at design time or run-time), if the successor component meets the requirements of the initial component (expressed via the interfaces). Consequently, components can be replaced with either an updated version or an alternative without breaking the system in which the component operates.
As a rule of thumb for engineers substituting components, component B can immediately replace component A, if component B provides at least what component A provided and uses no more than what component A used.
Software components often take the form of objects (not classes) or collections of objects (from object-oriented programming), in some binary or textual form, adhering to some interface description language (IDL) so that the component may exist autonomously from other components in a computer. In other words, a component acts without changing its source code. Although, the behavior of the component's source code may change based on the application's extensibility, provided by its writer.
When a component is to be accessed or shared across execution contexts or network links, techniques such as serialization or marshalling are often employed to deliver the component to its destination.
Reusability is an important characteristic of a high-quality software component. Programmers should design and implement software components in such a way that many different programs can reuse them. Furthermore, component-based usability testing should be considered when software components directly interact with users.
- fully documented
- thoroughly tested
- robust - with comprehensive input-validity checking
- able to pass back appropriate error messages or return codes
- designed with an awareness that it will be put to unforeseen uses
In the 1960s, programmers built scientific subroutine libraries that were reusable in a broad array of engineering and scientific applications. Though these subroutine libraries reused well-defined algorithms in an effective manner, they had a limited domain of application. Commercial sites routinely created application programs from reusable modules written in assembly language, COBOL, PL/1 and other second- and third-generation languages using both system and user application libraries.
As of 2010, modern reusable components encapsulate both data structures and the algorithms that are applied to the data structures. Component-based software engineering builds on prior theories of software objects, software architectures, software frameworks and software design patterns, and the extensive theory of object-oriented programming and the object-oriented design of all these. It claims that software components, like the idea of hardware components, used for example in telecommunications, can ultimately be made interchangeable and reliable. On the other hand, it is argued that it is a mistake to focus on independent components rather than the framework (without which they would not exist).
The idea that software should be componentized - built from prefabricated components - first became prominent with Douglas McIlroy's address at the NATO conference on software engineering in Garmisch, Germany, 1968, titled Mass Produced Software Components. The conference set out to counter the so-called software crisis. McIlroy's subsequent inclusion of pipes and filters into the Unix operating system was the first implementation of an infrastructure for this idea.
Brad Cox of Stepstone largely defined the modern concept of a software component. He called them Software ICs and set out to create an infrastructure and market for these components by inventing the Objective-C programming language. (He summarizes this view in his book Object-Oriented Programming - An Evolutionary Approach 1986.)
The software components are used in two different contexts and two kinds: i) using components as parts to build a single executable, or ii) each executable is treated as a component in a distributed environment, where components collaborate with each other using internet or intranet communication protocols for IPC (Inter Process Communications). The above belongs to former kind, while the below belongs to later kind.
IBM led the path with their System Object Model (SOM) in the early 1990s. As a reaction, Microsoft paved the way for actual deployment of component software with Object linking and embedding (OLE) and Component Object Model (COM). As of 2010 many successful software component models exist.
A computer running several software components is often called an application server. This combination of application servers and software components is usually called distributed computing. Typical real-world application of this is in, e.g., financial applications or business software.
During the last decades, researchers and practitioners have proposed several component models with different characteristics. A classification of the existing component models is given in. Examples of component models are: Enterprise JavaBeans (EJB) model, Component Object Model (COM) model, .NET model, X-MAN component model, and Common Object Request Broker Architecture (CORBA) component model.
- Business object technologies
- Component-based software frameworks for specific domains
- Component-oriented programming
- Bundles as defined by the OSGi Service Platform
- Component web platform for modular js, css, and other assets
- Component Object Model (OCX/ActiveX/COM) and DCOM from Microsoft
- TASCS - SciDAC Center for Technology for Advanced Scientific Component Software
- Eiffel programming language
- Enterprise JavaBeans from Sun Microsystems (now Oracle)
- Flow-based programming
- Fractal component model from ObjectWeb
- component framework for Midgard and PHP
- Oberon, Component Pascal, and BlackBox Component Builder
- rCOS method of component-based model driven design from UNU-IIST
- SOFA component system from ObjectWeb
System.ComponentModelnamespace in Microsoft .NET
- Unity developed by Unity Technologies
- Unreal Engine developed by Epic Games
- UNO from the OpenOffice.org office suite
- VCL and CLX from Borland and similar free LCL library.
- XPCOM from Mozilla Foundation
- Compound document technologies
- Distributed computing software components
- .NET Remoting from Microsoft
- 9P distributed protocol developed for Plan 9, and used by Inferno and other systems.
- CORBA and the CORBA Component Model from the Object Management Group
- D-Bus from the freedesktop.org organization
- DCOM and later versions of COM (and COM+) from Microsoft
- DSOM and SOM from IBM (now scrapped)
- Ice from ZeroC
- Java EE from Sun
-  from SICS
- Universal Network Objects (UNO) from OpenOffice.org
- Web services
- Zope from
- AXCIOMA (the component framework for distributed, real-time, and embedded systems) by Remedy IT
- COHORTE the cross-platform runtime for executing and managing robust and reliable distributed Service-oriented Component-based applications, by isandlaTech
- DX-MAN Service Model
- Generic programming emphasizes separation of algorithms from data representation
- Interface description languages (IDLs)
- Open Service Interface Definitions (OSIDs)
- Part of both COM and CORBA
- SIDL - Scientific Interface Definition Language
- SOAP IDL from World Wide Web Consortium (W3C)
- XML-RPC, the predecessor of SOAP
- Inversion of Control (IoC) and Plain Old C++/Java Object (POCO/POJO) component frameworks
- Pipes and filters
- Unix operating system
- Business logic
- Modular programming
- Service Component Architecture (SCA)
- Software Communications Architecture (JTRS SCA)
- Third-party software component
- Web service
- Web components
- Foukalas et al "Protocol Reconfiguration Using Component-Based Design"
- Wallace, Bruce (May 19, 2010). "A hole for every component, and every component in its hole". Existential Programming.
There is no such thing as a Component
- McIlroy, Malcolm Douglas (January 1969). "Mass produced software components" (PDF). Software Engineering: Report of a conference sponsored by the NATO Science Committee, Garmisch, Germany, 7-11 Oct. 1968. Scientific Affairs Division, NATO. p. 79.
- Rainer Niekamp. "Software Component Architecture" (PDF). Gestión de Congresos - CIMNE/Institute for Scientific Computing, TU Braunschweig. p. 4. Retrieved 2011-07-29.
- Raphael Gfeller (December 9, 2008). "Upgrading of component-based application". HSR - Hochschule für Technik Rapperswill. p. 4. Retrieved 2011-07-29.
1990, IBM invents their System Object Model. 1990, as a reaction, Microsoft released OLE 1.0 OLE custom controls (OCX)[permanent dead link]
- Crnkovic, I.; Sentilles, S.; Vulgarakis, A.; Chaudron, M. R. V. (2011). "A Classification Framework for Software Component Models". IEEE Transactions on Software Engineering. 37 (5): 593–615. doi:10.1109/TSE.2010.83. S2CID 15449138.
- Lau, Kung-Kiu; Wang, Zheng (2007). "Software Component Models". IEEE Transactions on Software Engineering. 33 (10): 709–724. doi:10.1109/TSE.2007.70726. ISSN 0098-5589.
- Lau, Kung-Kiu; Velasco Elizondo, Perla; Wang, Zheng (2005). Heineman, George T.; Crnkovic, Ivica; Schmidt, Heinz W.; Stafford, Judith A.; Szyperski, Clemens; Wallnau, Kurt (eds.). "Exogenous Connectors for Software Components". Component-Based Software Engineering. Lecture Notes in Computer Science. Springer Berlin Heidelberg. 3489: 90–106. doi:10.1007/11424529_7. ISBN 9783540320494. S2CID 17971442.
- MASH defines assets as people, property and information and management as monitoring, control and configuration. Presented at the 2013 IEEE IoT conference in Mountain View MASH includes a full IDE, Android client and runtime. "MASH YouTube channel"
- A component-oriented approach is an ideal way to handle the diversity of software in consumer electronics. The Koala model, used for embedded software in TV sets, allows late binding of reusable components with no additional overhead. 
- Component model for embedded devices like TV developed by Philips based on paper by van Ommering, R.: Koala, a Component Model for Consumer Electronics Product Software  Archived 2014-08-09 at the Wayback Machine
- Arad, Cosmin (April 2013). Programming Model and Protocols for Reconfigurable Distributed Systems (PDF). Doctoral Dissertation. Stockholm, Sweden: KTH Royal Institute of Technology. ISBN 978-91-7501-694-8.
- Arellanes, Damian; Lau, Kung-Kiu (2017). "Exogenous Connectors for Hierarchical Service Composition" (PDF). 2017 IEEE 10th Conference on Service-Oriented Computing and Applications (SOCA). Kanazawa: IEEE: 125–132. doi:10.1109/SOCA.2017.25. ISBN 9781538613269. S2CID 31211787.
- Brad J. Cox, Andrew J. Novobilski (1991). Object-Oriented Programming: An Evolutionary Approach. 2nd ed. Addison-Wesley, Reading ISBN 0-201-54834-8
- Bertrand Meyer (1997). Object-Oriented Software Construction. 2nd ed. Prentice Hall.
- George T. Heineman, William T. Councill (2001). Component-Based Software Engineering: Putting the Pieces Together. Addison-Wesley Professional, Reading 2001 ISBN 0-201-70485-4
- Richard Veryard (2001). Component-based business : plug and play. London : Springer. ISBN 1-85233-361-8
- Clemens Szyperski, Dominik Gruntz, Stephan Murer (2002). Component Software: Beyond Object-Oriented Programming. 2nd ed. ACM Press - Pearson Educational, London 2002 ISBN 0-201-74572-0
- Why Software Reuse has Failed and How to Make It Work for You by Douglas C. Schmidt
- What is the True essence and reality of CBD? (Evidence to show existing CBD paradigm is flawed)
- comprehensive list of Component Systems on SourceForge
- Brief Introduction to Real COP (Component Oriented Programming) by Using a small GUI application as an example