title: Chronicle queue events listener
body: <p>1) In <a href="https://github.com/OpenHFT/Chronicle-Queue" rel="nofollow noreferrer">chronicle queue v4</a> most <a href="https://github.com/OpenHFT/Chronicle-Queue/tree/master/src/test/java/net/openhft/chronicle/queue" rel="nofollow noreferrer">test patterns</a> show some form of <code>DocumentContext.isPresent()</code> <a href="https://en.wikipedia.org/wiki/Busy_waiting" rel="nofollow noreferrer">busy state checking</a>, when <code>ExcerptTailer</code> is positioned at the end of the queue and <strong>code is waiting</strong> for new entries to arrive from <code>ExcerptAppender</code>.</p>
  
  <p>2) Is there a built-in chronicle queue mechanism for asynchronous <code>appender -&gt; tailer</code> notifications, such that upon receipt of a notification event, given <code>tailer</code> is guaranteed to have at least one entry posted by <code>appender</code>, <strong>ready for read</strong>?</p>
  
  <p>3) If not, what is <strong>recommended pattern</strong> to implement such <code>event listener</code>? Please share a working example?</p>