title: RDS - max replica lag when using read replica
body: <p>I am using RDS's read replica mechanism for a schema update to a very large Mysql table.
  I ran an <code>Alter</code> command which locks the table for a long period of time (more than 24 hours).
  In that period of time my read replica was not getting updated and I noticed the <code>Replica lag</code> value was slowly increasing.</p>
  
  <p>When the table update was complete I saw that the <code>Replica lag</code> was slowly decreasing until the read replica finally caught up with the original DB.</p>
  
  <p>While my <code>Alter</code> command was running, I did a small experiment and occasionally updated a specific row so I can follow it on my read replica. My experiment showed that the updates to this specific row indeed eventually happened also in the read replica (after the table was unlocked).</p>
  
  <p>Based on the above experiment result I assume all updates that were blocked while my read replica was updating eventually were also performed on my replicated DB after the table modification but it would be hard to prove something like that for such a big table and such a long period of time.</p>
  
  <p>I couldn't find any official documentation on how this mechanism works and I was wondering where exactly all these updates are buffered and what would be the limit of this buffer (e.g. when will I start loosing changes that occured on my master DB)?</p>