The Swiftotter Magento developer Study guide states concerning Observers:
Event data should be able to be stored and then run in a queue at a later time.
Is this correct? Do I really have to assume that my events observers are ran in a asynchronous way at a later time?
Depends how you look at it. Magento is (MySQL) transactional which means things are getting queued whilst tables are locked. They are then processed in a first come first serves basis when locks are released. Observers would also work on this basis. I don't believe community edition has anything more complex then that. I've read about enterprise edition having what's essentially a worker queue. But I think in this context that's what they mean.
I also found this now in the dev docs:
https://devdocs.magento.com/guides/v2.1/coding-standards/technical-guidelines.html#14-events
14.1. All values (including objects) passed to an event MUST NOT be modified in the event observer. Instead, plugins SHOULD BE used for modifying the input or output of a function.
So I think if we follow this rule, events could really be executed asynchronously.
