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The On Demand Operating Environment
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Services communicate with each other by exchanging structured information --
messages or documents (sometimes called business objects). Their capabilities
are defined by interfaces declaring messages they can produce or consume, policy
annotations declaring quality of service required or provided, and choreography
annotations declaring behavioral constraints that must be respected in service
interactions. The actual implementation is hidden from the service requester,
thus SOAs are a convenient way to achieve application integration by allowing
new and existing applications to be quickly combined into new contexts.
The On Demand Operating Environment is based upon the concepts of Service Oriented Architecture (SOA). SOA views every application or resource as a service implementing a specific, identifiable set of (business) functions. In addition to the business functions, services in an on demand environment might also implement management interfaces to participate in the broader configuration, operation, and monitoring of the environment. This article provides an introduction to the On Demand Operating Environment.
Existing applications are "adapted" to service declarations. An
adapter follows the WebSphere® Business Integrator model, for example. The
adapter implements the service interface and transforms messages into operation
on the existing application.
All interactions between services flow through the Enterprise Service Bus (ESB).
This does not mean, however, that all interactions require network communication
and XML messages. The ESB provides services with the "service"
conceptual model, while allowing for optimized communication and encodings of
messages. In extreme cases, the interaction between two services might bind to a
local program call.
Matching of service requesters to providers can be done very early, prior to
deploy time, or very late through dynamic discovery mechanisms.
SOAs require standards for the definition of services and their capabilities and
interactions. The growing acceptance of XML as a standard representation of
structured information and of Web services standards (often called WS-*
standards) have greatly facilitated the adoption of this architectural approach.
The conceptual model of SOA applies to the virtualization of both business
functions and physical infrastructure. It spans the construction of applications
as well as their deployment and management. Clients (users or businesses) only
see a collection of business services and are interested in their quality of
service, but the On Demand operating environment shields them from the details
of application assembly and service delivery.
The next section explores the architecture for the On Demand operating
environment and its guiding principles in more detail.
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