The servers in the Usenet community are independent in the sense that each gets to decide what portions of the Usenet they will serve to their users and how long postings will last on the server. For example, some servers may leave out the adult groups. Because postings have to be transmitted from computer to computer, a given posting takes some time to be available on all servers. Further, they may disappear from active groups on busy servers in a matter of days or weeks.