Push vs Pull Architecture

You follow 500 accounts on a social app. You open the app, and your feed appears in less than a second.

Where did that feed come from? There are only two possible answers. Either the system built it just now, by fetching recent posts from all 500 accounts. Or the feed was already prepared, because each post was copied into your feed when it was written.

This is the choice between pull and push. It appears in feeds, message queues, monitoring, and caches. This lesson explains both approaches, why large feed systems combine them, and how to choose.