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

Post-Redirect-Get

You're hoping that the save command executes on time before Get is called. What if the command takes 10 seconds to complete in the back end but Get is called in 1 second?

Local Storage

With storing the result of the command on the client while the command goes off to execute, you're assuming that the command will go through without errors. What if the back-end runs into an error while processing the command? Then what you have locally isn't consistent.

Polling

Polling seems to be the option that is actually in line with eventual consistency; you're not faking or assuming. Your polling mechanism can be an asynchronous as a part of your page, e.g. shopping cart page component polls until it gets an update without refreshing the page.

Callbacks

You could introduce something like web hooks to make a call back to the client if the client is capable of receiving such. By providing a correlation Id once the command is accepted by the back-end, once the command has finished processing, the back-end can notify the front end of the command's status along with the correlation Id on whether the command went through successfully or not. There is no need for any kind of polling with this approach.