Simultanious access to DB file by several applications

I currently have Python application that deals with DBD XML database.
That application is being used 2 ways:
1. Apache + mod_python + Python application
2. wxPython or console + Python application
Web access is for read-only, so I don't really want to close/open container any request and would just reuse singleton connection object (with opened container in it) in all sessions. That arises the problem, DB is being cached into Apache process memory and does not aware of any changes done by other applications to physical DB file. I wonder if there is a standard way to detect if DB file was updated. The cached DB lives happily in Apache process even if physical file gets deleted.
I assume all connections in one process are reusing the same cached DB (nothing else makes sense) and just flash their changes to DB file time-to-time, but how to collaborate the same DB file between several processes?

The standard mechanism is to use transactional access for sharing among threads and processes. As long as the processes are opening the same environment (directory), and using transactions properly, they'll be synchronized.
Another method, which is now available in 2.3 uses what DB calls "concurrent" applications, which allow multiple readers, but only a single writer. If you don't need transactions for other reasons (such as recovery), this model will work best for you because it's very simple to use.
In both cases, it's important to share the environment.
You should read the Berkeley DB Programmer's Reference:
http://www.oracle.com/technology/documentation/berkeley-db/xml/ref/toc.html
Particularly chapters 8 and 9 that talk about concurrent and transactional applications.
Regards,
George

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