Friday, September 9, 2011

Cross-Cutting / AoP Transactions

I previously posted some notes on the Unit of Work implementation using Aspect Oriented Programming and how it relates to the application style (atomic vs persistent).

I have encountered another issue that has added what I think may be the final nail in the coffin for this design. The issue this time is handling Database Concurrency when using the Unit of Work, and the obvious issue is that the DbContext 'save' occurs outside the scope of the method that performed the changes, which means that method cannot define any custom concurrency handlers.

An example is as follows:

My core "Server" needs to update the users' Action Points (AP) every hour.

This process cannot Fail due to concurrency issues – it must retry (with
newly calculated values) if an error occurs else the user will either not get
their AP (or will end up with too many)

  • A user has 20AP

  • The server begins the process to add 10 AP, loading the user as a 20AP user

  • The user 'spends' 3AP – total AP = 17

  • The server 'Adds' 10 AP – total is 20 + 10 = 30AP

In the above scenario, the user has magically performed an action at no cost,
since the two processes occurred in parallel on the database.

If we simply throw a DB Concurrency error, the server process would have failed, so the +10AP would never occur and the user misses out on their hourly allocation, also a very bad thing.

What we ideally want to do is to catch the Concurrency error in the Service Method that adds the 10AP, recalculate what the new AP should be, and save the new AP count. However using the Unit of Work method the Service Method cannot handle this, it would have to be handled in the consumer of the service method. For User Actions this may be appropriate (we provide a warning and tell them to try again), but for non-interactive and business critical actions we want to handle this within the service, which means we cannot use the Unit of Work attribute to handle the context saving.

At the moment I do not have a way of solving this using the Unit of Work attribute interception, but there may be a way to instantiate appropriate handler chains for specific concurrency issues.
For example you may be able to specify a handler for concurrency issues on a particular Entity (User for example) and/or a specific field in an entity (AP), which will consistently handle concurrency issues on that type. This way if you encounter a concurrency error on the AP field, you always recalculate and try again, but if you encounter a concurrency error on a less important field you can throw an exception and have the user try again.

You could also potentially build up an Action chain where each service method you call determines whether a concurrency error can just throw the error back to the user, or whether to handle it in a custom manner (and build up the action handlers as appropriate)

I will have to investigate this, the second option might be an impressively complex way to resolve this issue.

Obviously the simple option is to make every service method completely self-contained and handle everything specific to the service method, which will work, but defeats the purpose of having a framework do the manual labour.

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