The mod_counter module is designed to allow a sort of "locking" to be enforced when the same file is being uploaded or downloaded by multiple clients at the same time.

The mod_counter works by creating a SysV semaphore for a file being read/written, and placing a number of reader or writer "counters" in that semaphore. When the configured maximum number of counters is reached, the FTP command which seeks to add another reader/writer counter will be denied. This allows site to configure the maximum number of clients which can be reading/writing any file at one time.