Why not Shared Services?
As the name indicates these Services are created to be shared among multiple web applications. Primarily these services would provide Search, Excel Services, Profile Synchronization, Audiences and My Sites etc. For instance if you want to have ‘Search’ enabled on you web application you have to subscribe to all the other services too, it is in fact all or nothing scenario. A perfect analogy would be a combo in fast food restaurant you have to have your fries and drink along with your burger. As explained in the below diagrams ‘Share Service 1’ is share among multiple web applications. ‘Web Application 1’ would need only ‘Search’ but it is served with the rest of the redundant services too. On the other hand for example due to the sensitive nature of the data if a web application is required to have its own ‘Search’ you still have to create new ‘Shared Service’ which will feed the Web application with all the other redundant services. The downside to this approach is some of these services would require a stand alone database which has to be created and populate for no reason.
Why Service Applications?
Unlike Shared Services, Service Applications can subscribe only to the required service as show below. That is one reason why this is referred as Service A la carte and a good marketing strategy by Microsoft.
Key Advantages of Service Applications over Shared Services:
- As each service can be subscribed individually redundant databases need not be created to support unwanted services
- The architecture is loosely coupled any third party service applications can be easily integrated.
- Cross farm communication is possible via Service Application proxy and location easily using Service URI