title: Azure Event Hub message retention doesn't seem to work
body: <p>We include dates in the events sent to our hub. Whenever I connect a new Azure Function to our Event Hub with a new consumer group, it seems to receive all events ever sent to the hub. This is somewhat expected, however I set the Message Retention on the hub to 1 day, so I expected at most to receive one day worth of events for the new consumer, but it seems to receive <em>all</em> events, even months old events, based on the date within the message, and lots more events than we generate over a day.</p>
  
  <p>Based on this page:</p>
  
  <p><a href="https://blogs.msdn.microsoft.com/servicebus/2015/03/09/data-retention-in-event-hubs/" rel="nofollow noreferrer">https://blogs.msdn.microsoft.com/servicebus/2015/03/09/data-retention-in-event-hubs/</a></p>
  
  <p>It seems like maybe this retention period is somewhat irrelevant, or misleading. If the "container" hasn't filled up yet, it could contain messages forever. If, for example, the container has a limit of 1000 messages before the event hub looks at it, but it takes a year to generate 1000 messages, does that mean any new consumer could get year-old messages, even with a 1-day "retention period"?</p>
  
  <p>When the container does hit the limit of 1000 messages, are the messages older than 1 day discarded and the messages newer than 1 day ago (within the retention period) retained? Or is the whole container discarded?</p>
  
  <p>From looking at our test and prod environments it seems like this container fits at least 50000 messages (or equivalent size).</p>
  
  <p>Is a checkpoint the only way to limit this initial influx of messages for a new consumer group?</p>