Enterprise service bus

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In computing, an enterprise service bus (ESB) refers to a software architecture construct, implemented by technologies found in a category of middleware infrastructure products usually based on standards, that provides foundational services for more complex architectures via an event-driven and standards-based messaging engine (the bus).

An ESB generally provides an abstraction layer on top of an implementation of an enterprise messaging system which allows integration architects to exploit the value of messaging without writing code. Contrary to the more classical enterprise application integration (EAI) approach of a monolithic stack in a hub and spoke architecture, the foundation of an enterprise service bus is built of base functions broken up into their constituent parts, with distributed deployment where needed, working in harmony as necessary.

ESB does not implement a service-oriented architecture (SOA) but provides the features with which one may be implemented. Although a common belief, ESB is not necessarily web-services based^{[citation needed]}. ESB should be standards-based and flexible, supporting many transport mediums. Based on EAI rather than SOA patterns, it tries to remove the coupling between the service called and the transport medium.

Most ESB providers now build ESBs to incorporate SOA principles and increase their sales, e.g. Business Process Execution Language (BPEL).

1 Salient characteristics
2 Key benefits
3 Key disadvantages
4 See also
5 Footnotes
6 Books
7 External links

[edit] Salient characteristics

Although the exact definition of an ESB varies, most agree that the following characteristics are common:

The Enterprise Service Bus is emerging as a service-oriented architecture component that makes large-scale implementation of the SOA principles manageable in a heterogeneous world.
It is usually operating-system and programming-language agnostic; for example, it should enable interoperability between Java and .NET applications.
It uses XML (eXtensible Markup Language) as the standard communication language.
It supports web-services standards.
It supports the creation of real-time business value driven by standards-based middleware.
It supports messaging (synchronous, asynchronous, point-to-point, publish-subscribe).
It includes standards-based adapters (such as J2C/JCA) for supporting integration with legacy systems.
It includes support for service orchestration.
It includes intelligent content-based routing services (itinerary routing).
It includes a modular architecture derived from a component-oriented design.
It includes a standardized security model to authorize, authenticate and audit use of the ESB.
To facilitate the transformation of data formats and values, it includes transformation services (often via XSLT or XQuery) between the format of the sending application and the receiving application.
The core construct is robust, scalable, and extensible.
It includes validation against schemas for sending and receiving messages.
It can uniformly apply business rules, enriching messages from other sources, the splitting and combining of multiple messages and the handling of exceptions.
It can provide a unified abstraction across multiple layers
It can route or transform messages conditionally, based on a non-centralized policy (i.e. no central rules-engine needs to be present).
It is monitored for various SLA (Service Level Agreement) threshold message latencies and other SLA characteristics.
It speeds time-to-value by encouraging a "best-practices" development style.
It (often) facilitates "service classes," responding appropriately to higher and lower priority users.
It supports queuing, holding messages if applications are temporarily unavailable.
It provides a responsive development environment that can reflect changing business strategy on demand.
It is comprised of selectively deployed application adapters in a (geographically) distributed environment.

[edit] Key benefits

Faster and cheaper accommodation of existing systems.
Increased flexibility; easier to change as requirements change.
Standards-based.
Scales from point solutions to enterprise-wide deployment (distributed bus).
More configuration rather than integration coding.
No central rules engine, no central broker.
Incremental changes can be applied with zero down-time; enterprise becomes "refactorable".

[edit] Key disadvantages

Enterprise Message Model is usually mandatory.
Value of the ESB requires many disparate systems to collaborate on message standards.
Without forward planning, the versioning of messages between systems can cause tight coupling instead of the intended loose coupling.
Vendor depending, it requires more hardware to run.
New skills needed to configure ESB.
Extra translation layer when compared to regular messaging solutions.
Rarely realizes ROI (Return On Investment) within first few projects; next few projects generally refine messages and services; the fifth project may begin to realize ROI.^{[citation needed]}
For effective implementation, requires a mature IT governance model (such as ITIL) and a well-defined enterprise strategy to be in place already.

[edit] See also

[edit] Footnotes

An alternative view, particularly for high-performance enterprise service buses, is that "standard" message formats should flow across the bus, not just XML. Generating XML and parsing it can be costly in terms of processing and memory, so high-volume scenarios may not be viable.

[edit] Books

Dave Chappell, "Enterprise Service Bus" (O’Reilly: June 2004, ISBN 0-596-00675-6)

[edit] External links

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