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 their cart. How can you ensure that the client displays items in the cart if the actual processing of the command "AddItemToCart" is done async? I understand the principles of dispatching commands async and updating the read model async based on domain events, but I fail to see how this is handled from the clients perspective.

Answer 1

There are a few different ways of doing it;

Wait at user till consistent

Just poll the server until you get the read model updated. This is similar to what Ben showed.

Ensure consistency through 2PC

You have a queue that supports DTC; and your commands are put there first. They are then; executed, events sent, read model updated; all inside a single transaction. You have not actually gained anything with this method though, so don't do it this way.

Fool the client

Place the read models in local storage at the client and update them when the corresponding event is sent -- but you were expecting this event anyway, so you had already updated the javascript view of the shopping cart.

Answer 2

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<ActionResult> SpecifyRegistrantAndPaymentDetails(Guid orderId, int orderVersion)
{
    return this.WaitUntilOrderIsPriced(orderId, orderVersion)
        .ContinueWith<ActionResult>(

        ...

    );
}

...

private Task<PricedOrder> WaitUntilOrderIsPriced(Guid orderId, int lastOrderVersion)
{
    return
        TimerTaskFactory.StartNew<PricedOrder>(
            () => 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.