From SOA and EDA:

“Organizations tend to change their structure frequently. The evolving focus on service orientation and globalization will enforce this trend. The world is preparing for network oriented business structures with independent autonomous service providers and service consumers. Parts of the business process will be outsourced to external partners. Departments and business units are transformed to service providers. These service providers no longer focus only internally on the organization, but they are seeking for external markets to offer their services. Everything is moving toward on-demand business where service providers react to impulses – events – from the environment. To excel in a competitive market a high level of autonomy is required, including the freedom to select the appropriate supporting IT-systems. This increasing degree of separation creates a need for loose coupling between application components to be able to have the supported business processes bend unimpeded with the continuously changing composition of the organization structure. To achieve this agility the supporting applications must be agnostic to organizational changes like reshuffling responsibilities and roles, outsourcing or insourcing, splitting up departments or the whole company, fusions and all kinds of other reorganizations. Business processes must not be limited by the supporting IT-systems to smoothly follow all of these organizational changes. If, for instance, part of the process will be outsourced to an external partner, a part of the supporting IT-system will be cut off. The remaining part of the system must start communicating with the external partner. The system must not collapse neither is it desirable that adaption to the new situation costs a lot of money or time. The same is applicable in case of changing partners or in case of insourcing external tasks.”

“In contrast to SOA, EDA provides a way of loose coupling. EDA is not a synchronous command-and-control type of pattern, but just the contrary: an asynchronous publish-and-subscribe type of pattern. The publisher is completely unaware of the subscriber and vice versa; components are loosely coupled in the sense that they only share the semantics of the message.

If you are seeking to support strong cohesion in the business processes, situations where all process steps are under one control, SOA is the way to go. The command-and-control style of SOA – in general – is applicable to:

• Vertical interaction between the hierarchical layers of functional decomposition
• Functional request-and-reply processes such as man-machine dialogues; the user waits for an answer
• Processes with a transactional nature which require commit and rollback facilities
• Data enrichment in a message to be published to bring the message to its full content in a formal format

If you are seeking to support independency between business process steps, EDA is the way to go. This style of architecture is appropriate in federated and autonomous processing environments. Recognizable situations where EDA might be applicable are:

• Horizontal communication between tiers in a process chain
• Workflow type of processes
• Processes that cross recognizable functional organization borders, external (B2B) as well as internal

To find points of decoupling look for parts of the business process of which you are sure they always will stay together in one organizational unit (strong cohesion, atomic business function). In this way a functional composition of the business will arise. The borderlines between the business functions are the points of decoupling. If atomic transactions cross the decoupling borders, then implement compensating rollback transactions at these points.
Striving for loose coupling – and so for flexibility and agility – always is a good idea at all levels of granularity. So a rule of thumb might be: use loose coupling whenever possible and only use command-and-control if required. All of this with respect to the functional dimension for Both EDA and SOA. Of course these principles always must be challenged with performance aspects like required and feasible response times.”