A method and apparatus for operating a plurality of computers is disclosed. A file is maintained on a selected computer of the plurality of computers. An open request for the file is sent by a first computer of the plurality of computers to the selected computer, the open request including a first token. The file is opened for the first computer in response to the open request. An open request is sent for the file by a second computer to the selected computer, the open request including a second token. The first token is compared with the second token. If the first token and the second token match, opening the file for the second computer, and if the first token and the second token do not match, not opening the file for the second computer.