Output files can be designated as sticky but not upload-when-present. Sometimes this is done because the file will be used as an input for a later workunit; it may also be done to spread large output data across participant disks, in cases where the data may be too large to store on the project back-end.
To access such a file, the back end inserts a file access request in the database. This specifies a file and a data-server URL. When a host storing the file contacts the scheduling server (which may not happen for a while) the scheduling server forwards the file access request. The host confirms the file transfer when it is completed; the back-end must poll for this event.