Do you recall any pattern which could be use to leverage loose coupling ?

The Mediation pattern, using an enterprise service bus (ESB), will help in achieving this.
Mediation will take loose coupling to the highest level. It will establish independence between consumers and providers on all levels, including message formats, message types (including SOAP, REST, XML, binary) and transport protocols (including HTTP, HTTPS, JMS).
Architecturally speaking this means the separation of concerns between consumers and providers on the transport, message type, and message format levels.

The Service of a SOA should be engineered as stateless or stateful ?

Service should be stateless. It may have a context within its stateless execution, but it will not have an intermediary state waiting for an event or a call-back. The retention of state-related data must not extend beyond a request/response on a service. This is because state management consumes a lot of resources, and this can affect the scalability and availability that are required for a reusable service.

What is composition of a Service ?

Composition is the process by which services are combined to produce composite applications or 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 computer, and of the computer as the composite application.

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.

What are the common pitfalls of SOA ?

One of the most common pitfalls is to view SOA as an end, rather than a means to an end. Developers who focus on building an SOA solution rather than solving a specific business problem are more likely to create complex, unmanageable, and unnecessary interconnections between IT resources.

Another common pitfall is to try to solve multiple problems at once, rather than solving small pieces of the problem. Taking a top-down approach—starting with major organization-wide infrastructure investments—often fails either to show results in a relevant timeframe or to offer a compelling return on investment. 

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