Monday, 22 December 2008

VISUG Event : Entity Framework & WCF


Kurt Claeys gave a presentation about using the Entity Framework (EF) in a distributed scenario (Service oriented with WCF, Tight coupled Client-server) for the Visug , the Belgian Visual Studio user group.

His presentation was based on his personal research into the subject so he didn't gave immediately all the answers. Instead he showed us via concrete examples which difficulties he encountered during his endeavor.

The reason for going N-tier and the way to achieve this were out of scope for the session but decoupling was the key concept he used to convey this requirement.

After a very quick introduction into WCF and the EF, Kurt showed us the challenges you are confronted with if going this route and how WCF, EF or something else could provide a solution: Serialization of object graphs, contract sharing , … But the biggest hurdle in the current implementation seems to be change tracking.

In EF, a special mechanism called the ObjectContext, keeps track of all the things you do with the entity instances you retrieve through the EF infrastructure. This means the entity identification , the relationships ….and also the changes you make to the entity instances or the relations between them. Without the objectContext the EF infrastructure cannot create the necessary insert/delete/update statements for your data store.

Now a typical Service-oriented WCF service (cfr. 4 tenets of SOA ) is stateless in nature. This means that an operation that enables to retrieve an object graph through EF registers this action with the EF objectContext , but due to the nature of the service operation, the objectContext doesn’t stay around.

Kurt showed in some examples what this means for us as a developer . He also tried to come up with solution candidates to the problems like for example re-fetching the entity again and using special functionality from the objectContext the apply the changes. It worked but only for a single entity instance without relations. If you would like to go further …you had to do it yourself. Another solution candidate was to re-attach the object and apply all the changes to the re-attached object . If you had relations….you get the picture: DIY was the common factor in the candidate solutions.

There are some efforts in the EF community to give support of object change tracking in a distributed scenario but some of them are not active anymore and some are more research like efforts to look for solution.

Kurt concluded his presentation with what is coming in Vnext of EF , as told on PDC 2008.

What I remember is that if you’re going to use EF in a distributed scenario you should clearly do your homework first and recognize the potential difficulties you about to face in your situation and come up with a strategy to solve them.

Feel free to check the Visug site if you want to browse through the presentation. Maybe Kurt will also post his examples so you can try them out?

What are your experiences with EF?

Thanks for reading,

Best regards,