Replication in mysql with many servers There are 3 shops.  In each of these is a mysql server.  I want all servers to synchronize their date if at least one server changes the date. I read a bit of literature and realized that I need master master replication. However, I must take into account the fact that one or more of the databases can work a couple of hours in offline mode (e.g. shut down Internet).  How will this work in this case? Will there be duplication of column with Primary key? if so then do I need to take this into account? 

Advise me, please. I'm new to mysql.
If your Servers get out of sync they play back the events from the binlog, so as long you have all statements writen into your binlog, the master/slave is able to catch up. I had to catch up 12days without any problems, but in a cluster this is not what you want to happen.
And be careful when using "unsafe" statements, this are statements which the server can't reproduce on your master/slave setup.

Mysql Article on safe/unsafe statements - http://dev.mysql.com/doc/refman/5.1/en/replication-rbr-safe-unsafe.html