Concurrency Control

[Anon]

Can anyone help?!? I'm developing a guestbook using flat text file. In this guestbook, guest can add, modify or delete entries. Now, the problem came when 2 or more guest access a particular entry. Let say one guest wants to modify the entry, and at the same instance, another user wants to delete it. Therefore, the is a concurrency problem. I know using a rdbms backend can solve this, but can using flat text file do this? Anyone up for the challenge??

[Anon]

what about sessions technique PHPLIB provides? or why not use cookies.

[Anon]

I have considered these alternatives. But there are some things I'm not sure about.

The way I figure, the only way to have concurrency control is locking particular entries using shared or exclusive locks, like used in SQL Servers. However, the should be some mechanism that makes sure locks are not held for too long. Can session technique control the amount of time a lock is held? Furthermore, since the internet is basically connectionless network, can the session technique detect users which have terminated their session?