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上一篇分析到了 fastForwardFromEdits 类，本次分析syncWithLeader方法
源码分析
首先定位到Learner类，因为syncWithLeader方法的代码过长，一步步分析。
QuorumPacket ack = new QuorumPacket(Leader.ACK, 0, null, null);
QuorumPacket qp = new QuorumPacket();
long newEpoch = ZxidUtils.getEpochFromZxid(newLeaderZxid);
boolean snapshotNeeded = true;
readPacket(qp);
本段是接受leader发来的数据
if (qp.getType() == Leader.DIFF) {
LOG.info("Getting a diff from the leader 0x{}", Long.toHexString(qp.getZxid()));
snapshotNeeded = false;
}
如果有一个follower宕机，将会落后leader一部分数据，只需要重启然后去同步宕机之后的数据即可
else if (qp.getType() == Leader.SNAP) {
LOG.info("Getting a snapshot from leader 0x" + Long.toHexString(qp.getZxid()));
// The leader is going to dump the database
// clear our own database and read
zk.getZKDatabase().clear();
zk.getZKDatabase().deserializeSnapshot(leaderIs);
String signature = leaderIs.readString("signature");
if (!signature.equals("BenWasHere")) {
LOG.error("Missing signature. Got " + signature);
throw new IOException("Missing signature");
}
zk.getZKDatabase().setlastProcessedZxid(qp.getZxid());
}
如果新加入了follower或者observer，由于他们是空白数据，所以需要进行全部数据的同步，将leader中的快照全部同步过来。
else if (qp.getType() == Leader.TRUNC) {
//we need to truncate the log to the lastzxid of the leader
LOG.warn("Truncating log to get in sync with the leader 0x"
+ Long.toHexString(qp.getZxid()));
boolean truncated=zk.getZKDatabase().truncateLog(qp.getZxid());
if (!truncated) {
// not able to truncate the log
LOG.error("Not able to truncate the log "
+ Long.toHexString(qp.getZxid()));
System.exit(13);
}
zk.getZKDatabase().setlastProcessedZxid(qp.getZxid());
}
else {
LOG.error("Got unexpected packet from leader "
+ qp.getType() + " exiting ... " );
System.exit(13);
}
zk服务器原来是leader，然后有客户端发送过来数据，写入本地日志文件，还没来得及发送给follower，此时leader宕机了，然后原有的集群中某个follower会被选为leader，此时相当于就丢掉了一条数据。
当宕机的leader重启之后，会作为一个follower加入集群中，此时和leader进行同步，因为此时的leader中是没有这条数据的，所以会把宕机之前最后的一条数据给删除掉。再继续从leader中同步因为宕机而丢失的数据
接下来是一个while循环
case Leader.PROPOSAL:
PacketInFlight pif = new PacketInFlight();
pif.hdr = new TxnHeader();
pif.rec = SerializeUtils.deserializeTxn(qp.getData(), pif.hdr);
if (pif.hdr.getZxid() != lastQueued + 1) {
LOG.warn("Got zxid 0x"
+ Long.toHexString(pif.hdr.getZxid())
+ " expected 0x"
+ Long.toHexString(lastQueued + 1));
}
lastQueued = pif.hdr.getZxid();
packetsNotCommitted.add(pif);
break;
对于从leader传来proposals阶段的数据，放到packetsNotCommitted集合中
case Leader.COMMIT:
if (!writeToTxnLog) {
pif = packetsNotCommitted.peekFirst();
if (pif.hdr.getZxid() != qp.getZxid()) {
LOG.warn("Committing " + qp.getZxid() + ", but next proposal is " + pif.hdr.getZxid());
} else {
zk.processTxn(pif.hdr, pif.rec);
packetsNotCommitted.remove();
}
} else {
packetsCommitted.add(qp.getZxid());
}
break;
如果是commit 的数据，判断是已经commit过的数据还是proposal阶段的数据
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case Leader.INFORM:
/*
* Only observer get this type of packet. We treat this
* as receiving PROPOSAL and COMMMIT.
*/
PacketInFlight packet = new PacketInFlight();
packet.hdr = new TxnHeader();
packet.rec = SerializeUtils.deserializeTxn(qp.getData(), packet.hdr);
// Log warning message if txn comes out-of-order
if (packet.hdr.getZxid() != lastQueued + 1) {
LOG.warn("Got zxid 0x"
+ Long.toHexString(packet.hdr.getZxid())
+ " expected 0x"
+ Long.toHexString(lastQueued + 1));
}
lastQueued = packet.hdr.getZxid();
if (!writeToTxnLog) {
// Apply to db directly if we haven't taken the snapshot
zk.processTxn(packet.hdr, packet.rec);
} else {
packetsNotCommitted.add(packet);
packetsCommitted.add(qp.getZxid());
}
break;
如果是OBSERVER的情况，仅仅通知observer
case Leader.UPTODATE:
if (isPreZAB1_0) {
zk.takeSnapshot();
self.setCurrentEpoch(newEpoch);
}
self.cnxnFactory.setZooKeeperServer(zk);
break outerLoop;
如果同步完成后，跳出循环
case Leader.NEWLEADER: // Getting NEWLEADER here instead of in discovery
// means this is Zab 1.0
// Create updatingEpoch file and remove it after current
// epoch is set. QuorumPeer.loadDataBase() uses this file to
// detect the case where the server was terminated after
// taking a snapshot but before setting the current epoch.
File updating = new File(self.getTxnFactory().getSnapDir(),
QuorumPeer.UPDATING_EPOCH_FILENAME);
if (!updating.exists() && !updating.createNewFile()) {
throw new IOException("Failed to create " +
updating.toString());
}
if (snapshotNeeded) {
zk.takeSnapshot();
}
self.setCurrentEpoch(newEpoch);
if (!updating.delete()) {
throw new IOException("Failed to delete " +
updating.toString());
}
writeToTxnLog = true; //Anything after this needs to go to the transaction log, not applied directly in memory
isPreZAB1_0 = false;
writePacket(new QuorumPacket(Leader.ACK, newLeaderZxid, null, null), true);
break;
}
}
}
发送一个NEWLEADER通知，本地把内存中的数据生成一个快照文件保存起来