While identifying services which one is better – top-down and bottom-up methodologies encourages re-use and maintenance?
Since the top-down approach is business-driven it can be practical to separate the different concerns of business and IT on different plans, providing a common ground in between. So in most situations it the most appropriate if you want to improve reuse and ROI in the medium/longterm.
How can you achieve loose coupling in a SOA ?
One strategy for achieving loose coupling is to use the service interface (the WSDL for a SOAP Web Service) to limit this dependency, hiding the service implementation from the consumer. Loose coupling can be addressed by encapsulating the service functionalities in a manner that limits the impact of changes to the implementation on the service interface. However, at some point you will need to change the interface and manage versioning without impacting service consumers, in addition to managing multiple security constraints, multiple transports, and other considerations
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.