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? Either the system built it just now by fetching from all 500 accounts, or it was already sitting there because every one of those 500 posts was copied into your feed the moment it was written.

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

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