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An enterprise service bus (ESB) implements a communication system between mutually interacting software applications in a service-oriented architecture (SOA). It implements a software architecture as depicted on the right. As it implements a software architecture for distributed computing, it implements a special variant of the more general client-server model therefore also. Whereas in general any application using ESB can behave as server or client in turns. ESB promotes agility and flexibility with regard to high protocol-level communication between applications. The primary goal of the high protocol-level communication is enterprise application integration (EAI) of heterogeneous and complex landscapes.
Contents
Overview
The concept is analogous to the bus concept found in computer hardware architecture combined with the modular and concurrent design of high-performance computer operating systems. The motivation was to find a standard, structured, and general purpose concept for describing implementation of loosely coupled software components (called services) that are expected to be independently deployed, running, heterogeneous, and disparate within a network. ESB is also a common implementation pattern for service-oriented architecture.
Working
An ESB transports the design concept of modern operating systems to networks of disparate and independent computers. Like concurrent operating systems an ESB caters for commodity services in addition to adoption, translation and routing of a client request to the appropriate answering service.
The primary duties of an ESB are:
Ambiguous use of the term ESB in commerce
There is no global standard for enterprise service bus concepts or implementations. Most providers of message-oriented middleware have adopted the enterprise service bus concept as de facto standard for a service-oriented architecture. The implementations of ESB use event-driven and standards-based message-oriented middleware in combination with message queues as technology frameworks. However, some software manufacturers relabel their existing middleware and communication solutions as ESB without adopting the crucial aspect of a bus concept.
History
The first published usage of the term "enterprise service bus" is attributed to Roy W. Schulte from the Gartner Group 2002 and the book The Enterprise Service Bus by David Chappell.
In fact, the term "bus" was created in the 1980s by Teknekron Software Systems. Frustrated by how software seemed to always under-deliver, while hardware was always on time and under budget, Vivek Ranadivé set out to build software based on the premise of a "Software Bus" (which later became known as "The Information Bus" or TIB), where a "bus" is the standard data highway by which various elements—such as a computer system such as the CPU, the memory, the I/O devices, etc.—communicate. This concept would allow for the "tight" coupling of applications.
In 1986 Teknekron Corporation embarked on a consulting project with Goldman Sachs to redefine the "trading floor of the future" applying this approach. In 1987 the first TIB—for the integration and delivery of market data such as stock quotes, news, and other financial information—went live at Fidelity, followed by First Interstate Bank, then Salomon, eventually digitizing all of Wall Street. Teknekron was later acquired by Reuters in 1994 to expand its use of the Information Bus in the financial services markets. In January 1997, Ranadivé founded Tibco Software Inc. to create and market software for use in the integration of business applications outside the financial services sector. In 1998 Tibco Software released TIB/ActiveEnterprise suite. In July 1999 Tibco went public on the NASDAQ Stock Market under the ticker symbol TIBX. Tibco stands for The Information Bus Company.
ESB as software
The ESB is implemented in software that operates between the business applications, and enables communication among them. Ideally, the ESB should be able to replace all direct contact with the applications on the bus, so that all communication takes place via the ESB. To achieve this objective, the ESB must encapsulate the functionality offered by its component applications in a meaningful way. This typically occurs through the use of an enterprise message model. The message model defines a standard set of messages that the ESB transmits and receives. When the ESB receives a message, it routes the message to the appropriate application. Often, because that application evolved without the same message model, the ESB has to transform the message into a format that the application can interpret. A software adapter fulfills the task of effecting these transformations, analogously to a physical adapter.
ESBs rely on accurately constructing the enterprise message model and properly designing the functionality offered by applications. If the message model does not completely encapsulate the application functionality, then other applications that desire that functionality may have to bypass the bus, and invoke the mismatched applications directly. Doing so violates the principles of the ESB model, and negates many of the advantages of using this architecture.
The beauty of the ESB lies in its platform-agnostic nature and the ability to integrate with anything at any condition. It is important that Application Lifecycle Management vendors truly apply all the ESB capabilities in their integration products while adopting SOA. Therefore, the challenges and opportunities for EAI vendors are to provide an integration solution that is low-cost, easily configurable, intuitive, user-friendly, and open to any tools customers choose.
Characteristics
Most observers accept certain core capabilities as functions of an ESB:
² While process choreography supports implementation of complex business processes that require coordination of multiple business services (usually using BPEL), service orchestration enables coordination of multiple implementation services (most suitably exposed as an aggregate service) to serve individual requests.
These solutions often focus on low-level ESB functions, such as connectivity, routing and transformation, and require coding or scripting to implement orchestration. Developers operating at a project or tactical level, e.g., just trying to fix a problem, often gravitate toward lightweight service bus technologies, but there is often ongoing tension between these initiatives and an enterprise architecture whose goal it is to optimize infrastructure across multiple projects.
If the message broker, the ESB software, translates a message from one format to another, then as with any translation, there is the issue of semantics of the message. For example, a record can be translated from JSON to XML, but the same set of fields can be interpreted differently by different applications, specially in the case of the various corner cases that are usually known only to developers that have extensive experience with the application that is connected to the ESB. For the known corner cases the number of tests that cover all corner cases increases exponentially with every application that is connected to the ESB, because every ESB-connected application must be tested against every other application that is connected to the ESB.
Key benefits
Key disadvantages
Existing ESB products
A more complete overview can also be found in the Comparison of business integration software article.