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Auto retry concurrent commands with ncqrs

Marco Franssen

Marco Franssen /

2 min read287 words

Cover Image for Auto retry concurrent commands with ncqrs

In a previous post I showed you some pseudo code Gregory Young mentioned in his DDD CQRS course I attended in Krakow, Poland. In this course Greg made clear to us locking of databases isn't necessary. He showed us some pseudo code how to easily write a merge handler to handle all concurrency conflicts.

In my current project, based on the ncqrs-framework I implemented a simpler version of this merge handler which only retries each command if a ConcurrencyException occurs. To achieve this you can easily build a wrapper around the ncqrs commandservice.

public class SafeCommandService : CommandService
{
    public override void Execute(ICommand command)
    {
        try
        {
            base.Execute(command);
        }
        catch(ConcurrencyException ex)
        {
            Execute(command);
            //Log retry executed
        }
    }
}

You can choose to implement this in a WCF service or whatever you want. Every command that fails will be retried until it succeeds. I have tested this with hundred async JavaScript calls which send a real simple command (PrintLabelCommand). My domain only processes a LabelPrintedEvent. When sending these calls to the server my log tells me there are only 8 retries average. This is acceptable in our situation. If you have more complex scenario's you will probably need a more advanced setup like the pseudocode in this blogpost shows you.

I'm still searching for a better name for my class, because SafeCommandExecutor isn't the best name I think. So let me know if you have a better one. I hope this article will be useful for you guys. As always, please share…

If you improved my code let me know, so I can learn from it.

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