This is a pertinent question because there is still a communications gap between the “business” and “IT” in many (especially large) organizations and as a result SOA initiatives are often driven from the IT side of the fence with little support or even understanding from the business.
There is a fundamental “chicken and egg” problem with SOA. It won’t gain support from the business unless a business case can be made…but conversely SOA payoffs are elusive without business support. Faced with this situation, many IT architects who want to develop an SOA approach are forced to try to conjure the egg out of thin air and hope that a chicken will hatch. The danger with this approach is that rather than gain an SOA you may end up with just a bunch of web services.
SOA is all about providing services to support the processes of the business – typically across organizational silos. There is a need to understand all the processes and their requirements – now and for the future. This visibility needs a vantage point “above the fray”. A point from where you can see the processes in their entirety. Without this vantage point you risk building the wrong services, or building services for the wrong processes or not anticipating future process changes. Moreover this vantage point must be experienced from the point of view
of the business – not just IT. So the question for an SOA initiative is “who in your organization has this vantage?” You may be lucky and have complete visibility from the IT cockpit, but that is rare. In general no one person has this vantage and it generally resides across different roles and people. Hence the need for some governance process to capture that vantage into a SOA plan.
So let’s suppose you have this vantage and want to execute on an SOA initiative. SOA is primarily about sharing – shared services built on shared resources using shared standards. Without sharing you wont get the SOA pay-off. But sharing requires communication and cooperation. Leverage is required to reconcile conflicting priorities in terms of requirements, schedule, funding, and resources. There may be some organizations where all these conflicts can be reconciled within the IT department, but I haven’t seen any yet. Typically resolution is required at an organizational level above the participants – that is the prime purpose of organizational hirearchy. Hence you need the appropriate leverage to get everyone moving down the SOA path.
While the type of issues your SOA initiative will face are fairly general, the specific issues and their resolution will be germane to your organization. This means that the place you will achieve the right level of vantage and leverage will be different for your organization. You might be lucky enough to work in an IT department that has the appropriate leverage and vantage, but that is rare. More likely you will have to go higher up to get these facilities for a successful SOA. This is the essence of the “top-down” approach to SOA.
I guess the “middle-out” approach might be characterised by “figure out how much leverage and vantage you have access to an scope your initiative from there.”