Postgresql data backup

To whomsoever it may concern,

Iam involved with a team that is developing a java
based application on RedHat Linux 6.2 version.We are
using PostgreSQL
version 7.0.2 as the database.At present Iam
developing a backup & restore module with a GUI  for
the user to interact.
       I have a problem.I need to take a backup only
after ensuring that all clients connected to my
database are disconnected.
Modifying the access control privileges using the file
pg_hba.conf only restricts new connections or sessions
to my database.
Please mail me ur replies on the following :
1).I would like to know how to stop users who have
already opened up sessions with my database without
stopping the postmaster
service.
2).I would also like to know whether the command
pg_dump prevents any user from writing or modifying my
database until
th process is completed. If not,will it ensure
transactional integrity until the backup process is
completed ?

      I request a solution at the earliest.
     Thanks.




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