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See also: Why to distinguish between SOA and EDA
Moving toward on-demand business
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.
SOA, false promise?
This requires loosely coupled application components to be able to easily put the scissors in the organization structure without disturbing the supporting IT-systems. However, the synchronous command-and-control nature of SOA is a way of tightly coupling application components which doesn’t allow for this kind of flexibility. SOA may be loosely coupled in the technical domain, where common web services technology is used, but it certainly is not in the functional domain where SOA is associated with ‘calling’ foreign (reusable) services and eliminating data redundancy. The availability of services and stored data can be vanished after an act of outsourcing, which may lead to costly consequences and high risks. This has all to do with creating dependencies with SOA. The promise of SOA delivering loose coupling, which typically is asynchronous, could at the functional level be stated as a false promise.
IT-flexibility versus organization flexibility
Of course the use of SOA has benefits. Agility in application construction by using shareable components in a well defined functional decomposition and using standards based technology can have benefits in time to market with concern to the delivery of the application if adequate tools are used. And also restructuring the application in following business process redesign can be of much less effort than in the ‘traditional’ way. The IT-department may gain benefits, which will lead to lower IT-costs for the business and faster delivery.
At the moment SOA is positioned in the market mostly as a type command-and-control architecture at the higher granularity levels of functional decomposition. To achieve loose coupling and autonomy in the context of the previous mentioned organizational evolutions EDA (even-driven architecture) is a more appropriate style of architecture at this level of granularity. EDA provides flexibility directly to the organization itself. With EDA an organization can reorganize without affecting application constructions. Changing the structure of the organization without changing the applications is a promise of EDA. So we are talking about quite a different magnitude of agility.
But why is SOA promoted at this level of granularity? The causality of four aspects is in charge. Firstly, SOA by most people is thought of in terms of web services. Secondly, the performance of web services technology at the moment is not appropriate at the lower levels of granularity. Thirdly, web services originate from the request-and-reply pattern, and so are associated with command-and-control solutions. Fourthly, the event-driven model is not well known; people tend to look for solutions in well known domains. Unfortunately the command-and-control pattern is not appropriate at this level. SOA based on synchronous web services might be a good idea in the middle layers of functional decomposition. Here the command-and-control type of interaction may be required and may be in good balance with web services performance. But you have to explicitly investigate it during design phase. So finding the correct context where an SOA might be appropriate is not a trivial job.
When to use SOA and when to use EDA?
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
- 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
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.
Redundancy: strong design
Loose coupling means independency. Loosely coupled components do not rely on each other. Not even on each others stored data. Each loosely coupled environment maintains its own copy of (a relevant subset of) the data and services. In loosely coupled environments redundancy must not be seen as poor design, but as strong design. Banning redundancy across decoupling borders makes the coupling more tightly. Maintaining redundancy across the decoupling borders makes the loose coupling more robust. EDA is perfectly suited by its nature to support automatic data synchronization mechanisms in redundant environments while maintaining loose coupling.
Within a reuse domain (see figure 1) a finer grained EDA may be implemented. The more fine-grained EDA, the more flexible the IT-systems are, but also to smaller the reuse domains will be.
If using web services technology (SOAP) at those points of decoupling combined with a common infrastructure (Enterprise Service Bus), it is possible to easily connect heterogeneous systems. Downstream systems are not only SOAs, but also SOAP-wrapped legacy systems, commercial of the shelf software (COTS), ERP systems and gateways to external systems. Figure 2 visualizes an EDA.
Components are no longer directly coupled, but connected via decoupling points (events).
To implement EDA with web services technology today, an additional SOAP-aware message queuing infrastructure is required, whereas this is not the case for a web services based SOA implementation. SOA in its basic form can be implemented solely by web services over existing network infrastructures like the HTTP-layer. This is where the current ‘SOA-hype’ originated from and this also might be the reason why SOA is overwhelming EDA. Current ESB (Enterprise Service Bus) infrastructures provide a way of message queuing combined with web services technology. That is why the use of an ESB infrastructure is very appropriate to implement EDA and SOA solutions and to combine both styles of architecture.
Evolving web services standards like WS-Eventing, WS-Notification, WS-MetadataExchange, WS-ReliableMessaging, WS-Security, WS-Choreography, and lots more, combined with the emerging SOAP-aware infrastructure components like network devices and operating systems will in future provide much of the ESB-functionality which at the moment we have to obtain from ESB-vendors.
SOA and EDA implementations must be regarded in the context of Business Process Management (BPM). Modern BPM-tools are based on BPEL (Business Process Execution Language). The current BPEL implementation focuses strongly on the command-and-control model, the orchestration of services, and so on SOA. Beside orchestration BPEL - to a certain extend - also supports workflow, a kind of choreography, which goes in the direction of EDA. BPEL, however, has a procedural nature and runtime implementations need a controlling BPEL-engine. This is not a problem in case of SOA, but to achieve the aimed loose coupling of EDA it is. Good support for EDA would rather be a declarative model than a procedural model. A model where the designer – simply said – can connect events to publishers and subscribers by a point-and-click mechanism. Runtime implementations should be independent of a controlling engine, but rather be based on the earlier mentioned web services standards. It is obvious that solutions evolve in this direction. At this moment in time it is appropriate to use SOAP over JMS or other SOAP-based alternatives provided by the ESB. By creating current solutions based on commonly recognized, understood and implemented web services technology the systems will be robust in following technological, economic and organizational (r)evolutions in the future.