[matsemann]

> Even if we ignore poison messages, strict ordering on its own isn't that easy to pull off. Namely, you're limited to a single consumer and can only fetch one message at a time.

Yup. If you want to be sure, you need to persist which message for each entity X you have already processed and ignore older ones. And also make sure you handle race conditions where both messages are handled almost at the same time at two different consumers, by using a lock in a db or so. Which both are annoying, ideally I could just process messages without any care.

Spent all day trying to architecture this for a new queue where ordering matters but we need lots and lots of consumeres working at the same time prefetching messages. My case is actually a bit similar to the one in the article about a "stream of vehicle positions". I only really care about the latest one. Problem is it's hard to know which is the latest one without having a db and check if I've already processed something more recent. Any other ideas on how to efficiently solve this? As in, strategies to handle messages arriving out of order, so I can avoid that as a requirement.

[nickkell]

Have you considered frameworks like Orleans? Presumably you would have an individual actor “grain” for each vehicle and it would serialise the messages to it for you.

The state of the vehicle is kept in memory for some time.