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.