What is composition of a Service ?

Composition is the process by which services are combined to produce composite   composite services. A composite application consists of the aggregation of services to produce an enterprise portal or enterprise process. A composite service consists of an aggregation of services that produces another reusable service. It’s just like combining electronic components to create a computer motherboard, and then using that motherboard in a computer. Think of the motherboard as a reusable composite service that is a component of the

How do I integrate my Legacy applications with SOA?

Legacy applications are frequently at the core of your IT environment. With the right skills and tools, you need to identify discrete elements within your legacy applications and wrap; them in standards-based interfaces and use them as services within your SOA.

How does the ESB fits in this picture?

The Enterprise Service Bus is a core element of any SOA. ESBs provide the "any to any" connectivity between services within your own company, and beyond your business to connect to your trading partners. But SOA does not stop at just implementing an ESB. Depending on what your goals are, you may want to use an ESB to connect other services within your SOA such as information services, interaction services and business process management services. Additionally, you will need to consider development services and IT service management services. The SOA reference architecture can help you lay out an SOA environment that meets your needs and priorities. The ESB is part of this reference architecture and provides the backbone of an SOA but it should not be considered an SOA by itself.
Are web-services SOA ?
SOA is a thinking, it’s an architectural concept and web service is one of the technical approach to complete it. Web services are the preferred standards to achieve SOA.
. In SOA we need the services to be loosely coupled. A web service communicates using SOAP protocol which is XML based which is very loosely coupled. It answers the what part of the service.
. SOA services should be able to describe themselves. WSDL describes how we can access the service.
. SOA services are located in a directory. UDDI describes where we can get the web service. This nothing but implementation of SOA registry.

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