We have a Data Access service in our SOA WCF system. This service is responsible for doing CRUD (create, update, delete) operations on “system wide” database tables, and is also the source of this data for queries. Any other service in the system wanting to access the tables under the contol of the DAS have to go to the DAS to get it or modify it. We use Entity Framework and built our own POCO state tracking system for this DAS.
We have other tables in our database that belong to single services and store data only for their own use, ie state information they can access if they crash and resume or recording of business information. We have a rule any one table cannot be accessed by more than one service: so data needed by multiple services ends up in the DAS.
Truth is I have never really understood why a Data Access Service is a good idea as opposed to just accessing tables directly. It seems to be to be slower, our DAS is not transactional as it cannot send back a POCO graph for database update (only single POCOS at a time) and we have issues also where the DAS is actually a client to another service which needs data from it…circular dependancy.
Why bother with a DAS? Why is a DAS so important when it comes to SOA? What am I missing here? Single point of control?
Is it also an SOA design flaw that not all tables are part of a DAS and that some services have their own “private” tables?
Any discussion about this welcome.
You’re correct in thinking that this is the proper way to do things, and you’re also correct that it slows things down and can occasionally be cumbersome. SOA necessarily trades off some efficiency in exchange for ensuring single points of control for all data associated with a service. In fact, even the idea of having a “common DAS” service is slightly smelly in some SOA circles.
By centralizing all CRUD operations to one service in an SOA application, you can ensure data integrity and that business rules are being acted upon properly. To give an example, think of an entity you’d like to store that has some business rules associated with it that are difficult to approach from a pure SQL perspective – for example, let’s say a table that stores file references, and create / update services that ensure that these files exist.
With SOA and a single access point to those tables, you can code the logic into the create / update methods and be reasonably assured that the data you’re recieving from the service is valid – i.e. the files referenced exist. If anyone was capable of writing to these tables or retrieving data from them, no such assurance would exist – even if you’re calling the service yourself, you don’t know what other programmers, through malice or just plan forgetfulness, forgot to implement that critical business rule. This leads to defensive programming where every bit of client code is ensuring business logic independently, and ultimately a tangled mess of business logic scattered throughout your application.
Another benefit is scalability and maintanability. Let’s say one of your services is accessing a huge chunk of data. With SOA, everything is “black-boxed” so that your client code doesn’t have much knowledge of how the data is ultimately obtained. You could change your RDBMS, partition tables, or implement caching, and make that all invisible to the client code calling it – ensuring your painful updates only need to be made in one place. With database code scattered throughout your app, this sort of upgrade becomes extremely painful.