Azure Transient Fault Handling and Entity Framework Tracking Issues.

If you’ve ever worked in a multi-tenant environment with shared resources you’ll know of the many pains that can come along with it. Resource sharing always ends up leading to contention and some of the time this will mean that you won’t be able to get access to the resources you want. For cloud services this is par for the course as since you’re always accessing shared services and so any application you build on these kinds of platforms has to take this into consideration lets your application spend an eternity crashing from random connection drop outs. Thankfully Microsoft has provided a few frameworks which will handle these situations for you, especially in the case of Azure SQL.

NuGet Install Enterprise Library Transient Fault Handling FrameworkThe Transient Fault Handling Application Block (or Topaz, which is a lot better in my view) gives you access to a number of classes which take out a lot of the pain when dealing with the transient errors you get when using Azure services. Of those the most useful one I’ve found is the RetryPolicy which when instantiated as SqlAzureTransientErrorDetectionStrategy allows you to simply wrap your database transactions with a little bit of code in order to make them resistant to the pitfalls of Microsoft’s cloud SQL service. For the most part it works well as prior to using it I’d get literally hundreds of unhandled exception messages per day. It doesn’t catch everything however so you will still need to handle some connection errors but it does a good job of eliminating the majority of them.

Currently however there’s no native support for it in Entity Framework (Microsoft’s data persistence framework) and this means you have to do a little wrangling in order to get it to work. This StackOverflow question outlines the problem and there’s a couple solutions on there which all work however I went for the simple route of instantiating a RetryPolicy and then just wrapping all my queries with ExecuteAction. As far as I could tell this all works fine and is the supported way of using EF with Topaz at least until 1.6 comes out which will have in built support for connection resiliency.

However when using Topaz in this way it seems that it mucks with entity tracking, causing returned objects to not be tracked in the normal way. I discovered this after I noticed many records not getting updated even though manually working through the data showed that they should be showing different values. As far as I can tell if you wrap an EF query with a RetryPolicy the entity ends up not being tracked and you will need to .Attach() to it prior to making any changes. If you’ve used EF before then you’ll see why this is strange as you usually don’t have to do that unless you’ve deliberately detached the entity or recreated the context. So as far as I can see there must be something in Topaz that causes it to become detached requiring you to reattach it if you want to persist your changes using Context.SaveChanges().

I haven’t tested any of the other methods of using Topaz with EF so it’s entirely possible there’s a way to get the entity tracked properly without having to attach to it after performing the query. Whether they work or not will be an exercise left for the reader as I’m not particularly interested in testing it, at least not just after I got it all working again. By the looks of it though a RC version of EF 6 might not be too far away, so this issue probably won’t remain one for long.

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