MPD uses a flat file database to maintain the basic music file information when it is not running. Once the daemon has been started, the database is kept completely in-memory and no hard disk access is necessary to look up or search for local audio files. Generally, music files must be located in a sub-directory of the music directory and are only added to the database when the update command is sent to the server. Playback of arbitrary files is allowed but only for local clients which are connected to the server via a Unix Domain Socket. MPD does not provide a built-in tag editor; this functionality is handled by clients or external programs, though 3rd party patches do exist to add this functionality to the server.