In my opinion events are just as important in a SOA as services. I think it’s not possible to achieve a “real” SOA without addressing events separately, just as we do for services. It can be somewhat confusing that events have there own acronym emphasizing their importance, namely Event Driven Architecture (EDA). Some coin this as the upgraded version of SOA, or SOA 2. How we love 3 (or 4 or 5) letter acronyms :-) Most of these different acronyms only add to the confusion, so I prefer sticking to SOA and emphasize events as integral part of it.
So why are events important?
A service-oriented approach should result in business-value (ROI), otherwise SOA only introduces technical complexity through additional middleware. One of the ways to achieve this is through business-IT alignment. Events play a major role in both of these worlds (business and IT).
Businesses deal with (and are causing) events all the time: a customer moving to a different address, a new purchase order, receiving an invoice from a supplier, sending a bill to a partner, and so on. Entities in a business’ ecosystem such as employees, partners, suppliers and customers all react to these events. They initiate new processes, perform activities, propagate events, and so on. It is therefore logical to incorporate events when modeling business processes, which are at the heart of SOA. Just as in all modellng practices, one wants to model and focus on important aspects. In case of SOA this includes processes, services and events.
From a technical point of view events can achieve asynchronicity and de-coupling. By using publish/subscribe and queueing mechanisms, software components are not required to know of each others existence. They simply subscribe to a topic and act based on received (and subscribed to) events. The other way around, if components have some (intermediate) result or state, they can share this with the rest of the world by publishing an event and then forget about it. These components don’t need to know what other components are interested in this information. Of course you’ll need some glue (i.e. middleware) to implement this.
So what does this mean concretely in a SOA-project? For one thing, besides trying to identify what types of services you have (business services, composite technical services, etc.), also investigate what types of events can be identified. There should be an event-registry besides a service-registry. This does not need to be a full-blown tool at first but can simply be documented using Excel in the beginning. Also define the relationship between processes, services and events. What services publish what type of events? What processes are initiated by what events? What running processes are influenced by what events?