/// <summary>
/// Returns the next batch of events in the stream or null if no new
/// batches are currently available.
/// </summary>
/// <exception cref="System.IO.IOException">
/// because of network error or edit log
/// corruption. Also possible if JournalNodes are unresponsive in the
/// QJM setting (even one unresponsive JournalNode is enough in rare cases),
/// so catching this exception and retrying at least a few times is
/// recommended.
/// </exception>
/// <exception cref="Org.Apache.Hadoop.Hdfs.Inotify.MissingEventsException">
/// if we cannot return the next batch in the
/// stream because the data for the events (and possibly some subsequent
/// events) has been deleted (generally because this stream is a very large
/// number of transactions behind the current state of the NameNode). It is
/// safe to continue reading from the stream after this exception is thrown
/// The next available batch of events will be returned.
/// </exception>
public virtual EventBatch Poll()
{
TraceScope scope = Trace.StartSpan("inotifyPoll", traceSampler);
try
{
// need to keep retrying until the NN sends us the latest committed txid
if (lastReadTxid == -1)
{
Log.Debug("poll(): lastReadTxid is -1, reading current txid from NN");
lastReadTxid = namenode.GetCurrentEditLogTxid();
return(null);
}
if (!it.HasNext())
{
EventBatchList el = namenode.GetEditsFromTxid(lastReadTxid + 1);
if (el.GetLastTxid() != -1)
{
// we only want to set syncTxid when we were actually able to read some
// edits on the NN -- otherwise it will seem like edits are being
// generated faster than we can read them when the problem is really
// that we are temporarily unable to read edits
syncTxid = el.GetSyncTxid();
it = el.GetBatches().GetEnumerator();
long formerLastReadTxid = lastReadTxid;
lastReadTxid = el.GetLastTxid();
if (el.GetFirstTxid() != formerLastReadTxid + 1)
{
throw new MissingEventsException(formerLastReadTxid + 1, el.GetFirstTxid());
}
}
else
{
Log.Debug("poll(): read no edits from the NN when requesting edits " + "after txid {}"
, lastReadTxid);
return(null);
}
}
if (it.HasNext())
{
// can be empty if el.getLastTxid != -1 but none of the
// newly seen edit log ops actually got converted to events
return(it.Next());
}
else
{
return(null);
}
}
finally
{
scope.Close();
}
}
/// <exception cref="System.IO.IOException"/>
internal DFSInotifyEventInputStream(Sampler <object> traceSampler, ClientProtocol
namenode)
: this(traceSampler, namenode, namenode.GetCurrentEditLogTxid())
{
}
