[sbov]

Your solution seems to assume only one thing will be accessing what is being mutated at once. If another thread comes in and gets a cart (e.g. maybe the user reloads the page) and they aren't waiting on the operation to be processed anymore. If you remove it from the queue after a few seconds of failure then fine. But if the point is "self healing" it presumably hangs around for a while.

You have to deal with this to some extent in any webapp that has more than 1 OS thread or process. But if you're keeping actions around for minutes or hours instead of second you're going to have to account for a lot of weird stuff you normally wouldn't.

If you really wanted something like this, I would think you would want a concept of "stale" data and up-to-date data. If a process is OK with stale data, the service can just return whatever it sees. But if a process isn't OK with it (like, say, checkout), you probably need to wait on the queue to finish processing.

And since the front end may care about these states, you probably need to expose this concept to clients. It seems like a client should be able to know if it's serving stale data so you can warn the user.

Maybe I'm mistaken.