The way Twitter works is through Scala Actors which are essentially mailboxes of functions. Addresses can send and receive asynchronous messages. For fault tolerance and address or servers can be made to be fault tolerant by providing distributed redundant services if they were programmed that way.
If I assigned your address Elon, to take out the garbage but for some reason it doesn't get done, then I can go to a different "place" and ask for the job to be done. There is usually some type of directory or orchestration service involved. Hope that helps and have a Merry Christmas by realizing Christ is the Light of Self Sacrifice.