Push vs Pull Architecture

You follow 500 people. You open the app, and your feed appears in under a second.

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

That is the push versus pull decision. It is one of the most common trade-offs in a system design interview.

Pull means the consumer asks for data when it needs it. Push means the producer sends data out as soon as it has it.