title: MySQL dual master replication -- is this scenario safe?
body: <p>I currently have a MySQL dual master replication (A&lt;->B) set up and everything seems to be running swimmingly.  I drew on the basic ideas from <a href="https://web.archive.org/web/20131225154105/http://signalboxes.net/misc/mysql-dual-master-replication/" rel="nofollow noreferrer">here</a> and <a href="https://web.archive.org/web/20120222172836/http://www.hbyconsultancy.com/blog/create-a-master-master-mysql-replication-ubuntu-server-10-04-x64.html" rel="nofollow noreferrer">here</a>.  </p>
  
  <p>Server A is my web server (a VPS).  User interaction with the application leads to updates to several fields in table X (which are replicated to server B).  Server B is the heavy-lifter, where all the big calculations are done.  A cron job on server B regularly adds rows to table X (which are replicated to server A).  </p>
  
  <p>So server A can update (but never add) rows, and server B can add rows.  Server B can also update fields in X, but only <strong>after</strong> the user no longer has the ability to update that row.</p>
  
  <p>What kinds of potential disasters can I expect with this scenario if I go to production with it?  Or does this seem OK?  I'm asking mostly because I'm ignorant about whether <strong>any</strong> simultaneous operation on the table (from either the A copy or the B copy) can cause problems or if it's just operations on <em>the same row</em> that get hairy.</p>