// TODO: we should collect any "ties" and pass the URL for all of them
// when syncing, so we can tolerate failure during recovery better.
Entry<AsyncLogger, PrepareRecoveryResponseProto> bestEntry = Collections.max(
prepareResponses.entrySet(), SegmentRecoveryComparator.INSTANCE);
AsyncLogger bestLogger = bestEntry.getKey();
PrepareRecoveryResponseProto bestResponse = bestEntry.getValue();
// Log the above decision, check invariants.
if (bestResponse.hasAcceptedInEpoch()) {
LOG.info("Using already-accepted recovery for segment " +
"starting at txid " + segmentTxId + ": " +
bestEntry);
} else if (bestResponse.hasSegmentState()) {
LOG.info("Using longest log: " + bestEntry);
} else {
// None of the responses to prepareRecovery() had a segment at the given
// txid. This can happen for example in the following situation:
// - 3 JNs: JN1, JN2, JN3
// - writer starts segment 101 on JN1, then crashes before
// writing to JN2 and JN3
// - during newEpoch(), we saw the segment on JN1 and decide to
// recover segment 101
// - before prepare(), JN1 crashes, and we only talk to JN2 and JN3,
// neither of which has any entry for this log.
// In this case, it is allowed to do nothing for recovery, since the
// segment wasn't started on a quorum of nodes.
// Sanity check: we should only get here if none of the responses had
// a log. This should be a postcondition of the recovery comparator,
// but a bug in the comparator might cause us to get here.
for (PrepareRecoveryResponseProto resp : prepareResponses.values()) {
assert !resp.hasSegmentState() :
"One of the loggers had a response, but no best logger " +
"was found.";
}
LOG.info("None of the responders had a log to recover: " +
QuorumCall.mapToString(prepareResponses));
return;
}
SegmentStateProto logToSync = bestResponse.getSegmentState();
assert segmentTxId == logToSync.getStartTxId();
// Sanity check: none of the loggers should be aware of a higher
// txid than the txid we intend to truncate to
for (Map.Entry<AsyncLogger, PrepareRecoveryResponseProto> e :
prepareResponses.entrySet()) {
AsyncLogger logger = e.getKey();
PrepareRecoveryResponseProto resp = e.getValue();
if (resp.hasLastCommittedTxId() &&
resp.getLastCommittedTxId() > logToSync.getEndTxId()) {
throw new AssertionError("Decided to synchronize log to " + logToSync +
" but logger " + logger + " had seen txid " +
resp.getLastCommittedTxId() + " committed");
}
}
URL syncFromUrl = bestLogger.buildURLToFetchLogs(segmentTxId, 0);