i have designed a protocol and programmed a simple clinet and server for on-line music subscription system.no of users are 5-6 n no. of tracks are 7-8. this is how it works:

after connecting to server client asks for user ID and Password. server opens a text file containing all id and password (passwords.txt). if the id and password match client provides 3 options, playlist, get track, and exit, for the user.
when playlist selected, client requests a playlist, server opens the file 'playlist.txt' reads all data and stores it in one array and sens it to client to be displayed for user.

when get track is selected, client asks for a code for track (code is available in playlist). server opens the playlist.txt file again, checks the code and when it finds the match stores the line of file related to that code into a string and sends it to cliant to be displayed.

now i need to suggest improvements to this design for when the number of users increase to thousand and tracks to few hundred. what kinf of things do i have to look into?

how can i prevent deadlock? what security issues need to be looked into? basically what do u think is required for this system to be commercially usable? nothing too detailed any suggestions with a brief description would be very help full!

thank you