How do you ensure consistent client reads in an eventual consistent system?

Question

I\'m digging into CQRS and I am looking for articles on how to solve client reads in an eventual consistent system. Consider for example a web shop where users can add items to

Answer 1

I'd recommend you have a look at the Microsoft Patterns & Practices team's guidance on CQRS. Although this is still work-in-progress they have given one solution to the issue you've raised.

Their approach for commands requiring feedback is to submit the command asynchronously, redirect to another controller action and then poll the read model for the expected change or a time-out occurs. This is using the Post-Redirect-Get pattern which works better with the browser's forward and back navigation buttons, and gives the infrastructure more time to process the command before the MVC controller starts polling.

Example code from the RegistrationController using ASP.NET MVC 4 asynchronous controllers.

[HttpGet]
[OutputCache(Duration = 0, NoStore = true)]
public Task SpecifyRegistrantAndPaymentDetails(Guid orderId, int orderVersion)
{
    return this.WaitUntilOrderIsPriced(orderId, orderVersion)
        .ContinueWith(

        ...

    );
}

...

private Task WaitUntilOrderIsPriced(Guid orderId, int lastOrderVersion)
{
    return
        TimerTaskFactory.StartNew(
            () => this.orderDao.FindPricedOrder(orderId),
            order => order != null && order.OrderVersion > lastOrderVersion,
            PricedOrderPollPeriodInMilliseconds,
            DateTime.Now.AddSeconds(PricedOrderWaitTimeoutInSeconds));
}

I'd probably use AJAX polling instead of having a blocked web request at the server.