zookeeper代码阅读-主要代码逻辑
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zookeeper集群有两种工作状态，一种是恢复状态（选leader状态），一种是广播状态（正常工作状态）。好比公司的管理一样，高层开会后决定职责任务，然后各部门负责人专职专管。
以下代码的循环体就是集群工作的循环状态。可以看到每个机器都会保存自己现在的状态，enum类型参数，总共有四种状态LOOKING, FOLLOWING, LEADING, OBSERVING。这四种状态也就对应四种角色，之前也介绍了机器角色的功能，这里不多说。
以下代码来自QuarumPeer.class 和 FastLeadElection。QuarumPeer是集群启动状态的核心代码。这里先介绍比较核心的 部分。 以下代码是机器工作时候的主循环代码，可以看到在机器保存running的时候，他会不停的根据自己的状态去做该状态应该做的事情。FastLeadElection是当机器处于looking状态时候要做的事情，会同步leader或者和其它机器共同选取leader。
updateThreadName(); LOG.debug("Starting quorum peer"); try { jmxQuorumBean = new QuorumBean(this); MBeanRegistry.getInstance().register(jmxQuorumBean, null); for(QuorumServer s: getView().values()){ ZKMBeanInfo p; if (getId() == s.id) { p = jmxLocalPeerBean = new LocalPeerBean(this); try { MBeanRegistry.getInstance().register(p, jmxQuorumBean); } catch (Exception e) { LOG.warn("Failed to register with JMX", e); jmxLocalPeerBean = null; } } else { RemotePeerBean rBean = new RemotePeerBean(s); try { MBeanRegistry.getInstance().register(rBean, jmxQuorumBean); jmxRemotePeerBean.put(s.id, rBean); } catch (Exception e) { LOG.warn("Failed to register with JMX", e); } } } } catch (Exception e) { LOG.warn("Failed to register with JMX", e); jmxQuorumBean = null; } try { /* * Main loop */ while (running) { switch (getPeerState()) { case LOOKING: LOG.info("LOOKING"); if (Boolean.getBoolean("readonlymode.enabled")) { LOG.info("Attempting to start ReadOnlyZooKeeperServer"); // Create read-only server but don't start it immediately final ReadOnlyZooKeeperServer roZk = new ReadOnlyZooKeeperServer(logFactory, this, this.zkDb); // Instead of starting roZk immediately, wait some grace // period before we decide we're partitioned. // // Thread is used here because otherwise it would require // changes in each of election strategy classes which is // unnecessary code coupling. Thread roZkMgr = new Thread() { public void run() { try { // lower-bound grace period to 2 secs sleep(Math.max(2000, tickTime)); if (ServerState.LOOKING.equals(getPeerState())) { roZk.startup(); } } catch (InterruptedException e) { LOG.info("Interrupted while attempting to start ReadOnlyZooKeeperServer, not started"); } catch (Exception e) { LOG.error("FAILED to start ReadOnlyZooKeeperServer", e); } } }; try { roZkMgr.start(); reconfigFlagClear(); if (shuttingDownLE) { shuttingDownLE = false; startLeaderElection(); } setCurrentVote(makeLEStrategy().lookForLeader()); } catch (Exception e) { LOG.warn("Unexpected exception", e); setPeerState(ServerState.LOOKING); } finally { // If the thread is in the the grace period, interrupt // to come out of waiting. roZkMgr.interrupt(); roZk.shutdown(); } } else { try { reconfigFlagClear(); if (shuttingDownLE) { shuttingDownLE = false; startLeaderElection(); } setCurrentVote(makeLEStrategy().lookForLeader()); } catch (Exception e) { LOG.warn("Unexpected exception", e); setPeerState(ServerState.LOOKING); } } break; case OBSERVING: try { LOG.info("OBSERVING"); setObserver(makeObserver(logFactory)); observer.observeLeader(); } catch (Exception e) { LOG.warn("Unexpected exception",e ); } finally { observer.shutdown(); setObserver(null); updateServerState(); } break; case FOLLOWING: try { LOG.info("FOLLOWING"); setFollower(makeFollower(logFactory)); follower.followLeader(); } catch (Exception e) { LOG.warn("Unexpected exception",e); } finally { follower.shutdown(); setFollower(null); updateServerState(); } break; case LEADING: LOG.info("LEADING"); try { setLeader(makeLeader(logFactory)); leader.lead(); setLeader(null); } catch (Exception e) { LOG.warn("Unexpected exception",e); } finally { if (leader != null) { leader.shutdown("Forcing shutdown"); setLeader(null); } updateServerState(); } break; } start_fle = Time.currentElapsedTime(); } } finally { LOG.warn("QuorumPeer main thread exited"); MBeanRegistry instance = MBeanRegistry.getInstance(); instance.unregister(jmxQuorumBean); instance.unregister(jmxLocalPeerBean); for (RemotePeerBean remotePeerBean : jmxRemotePeerBean.values()) { instance.unregister(remotePeerBean); } jmxQuorumBean = null; jmxLocalPeerBean = null; jmxRemotePeerBean = null; } } public Vote lookForLeader() throws InterruptedException { try { self.jmxLeaderElectionBean = new LeaderElectionBean(); MBeanRegistry.getInstance().register( self.jmxLeaderElectionBean, self.jmxLocalPeerBean); } catch (Exception e) { LOG.warn("Failed to register with JMX", e); self.jmxLeaderElectionBean = null; } if (self.start_fle == 0) { self.start_fle = Time.currentElapsedTime(); } try { //准备选举工作 HashMap<Long, Vote> recvset = new HashMap<Long, Vote>(); HashMap<Long, Vote> outofelection = new HashMap<Long, Vote>(); int notTimeout = finalizeWait; synchronized(this){ logicalclock.incrementAndGet(); updateProposal(getInitId(), getInitLastLoggedZxid(), getPeerEpoch()); } LOG.info("New election. My id = " + self.getId() + ", proposed zxid=0x" + Long.toHexString(proposedZxid)); //发出通知 sendNotifications(); /* * Loop in which we exchange notifications until we find a leader */ //这个循环一直进行直到找到leader while ((self.getPeerState() == ServerState.LOOKING) && (!stop)){ /* * Remove next notification from queue, times out after 2 times * the termination time */ //接收其它机器到来的通知 Notification n = recvqueue.poll(notTimeout, TimeUnit.MILLISECONDS); /* * Sends more notifications if haven't received enough. * Otherwise processes new notification. */ if(n == null){ //没有收到反馈就接着发出通知 if(manager.haveDelivered()){ sendNotifications(); } else { manager.connectAll(); } /* * Exponential backoff */ int tmpTimeOut = notTimeout*2; notTimeout = (tmpTimeOut < maxNotificationInterval? tmpTimeOut : maxNotificationInterval); LOG.info("Notification time out: " + notTimeout); } //合法性检验 else if (self.getCurrentAndNextConfigVoters().contains(n.sid)) { /* * Only proceed if the vote comes from a replica in the current or next * voting view. */ switch (n.state) { case LOOKING: if (getInitLastLoggedZxid() == -1) { LOG.debug("Ignoring notification as our zxid is -1"); break; } if (n.zxid == -1) { LOG.debug("Ignoring notification from member with -1 zxid" + n.sid); break; } // If notification > current, replace and send messages out if (n.electionEpoch > logicalclock.get()) { logicalclock.set(n.electionEpoch); recvset.clear(); if(totalOrderPredicate(n.leader, n.zxid, n.peerEpoch, getInitId(), getInitLastLoggedZxid(), getPeerEpoch())) { updateProposal(n.leader, n.zxid, n.peerEpoch); } else { updateProposal(getInitId(), getInitLastLoggedZxid(), getPeerEpoch()); } sendNotifications(); } else if (n.electionEpoch < logicalclock.get()) { if(LOG.isDebugEnabled()){ LOG.debug("Notification election epoch is smaller than logicalclock. n.electionEpoch = 0x" + Long.toHexString(n.electionEpoch) + ", logicalclock=0x" + Long.toHexString(logicalclock.get())); } break; } else if (totalOrderPredicate(n.leader, n.zxid, n.peerEpoch, proposedLeader, proposedZxid, proposedEpoch)) { updateProposal(n.leader, n.zxid, n.peerEpoch); sendNotifications(); } if(LOG.isDebugEnabled()){ LOG.debug("Adding vote: from=" + n.sid + ", proposed leader=" + n.leader + ", proposed zxid=0x" + Long.toHexString(n.zxid) + ", proposed election epoch=0x" + Long.toHexString(n.electionEpoch)); } recvset.put(n.sid, new Vote(n.leader, n.zxid, n.electionEpoch, n.peerEpoch)); if (termPredicate(recvset, new Vote(proposedLeader, proposedZxid, logicalclock.get(), proposedEpoch))) { // Verify if there is any change in the proposed leader while((n = recvqueue.poll(finalizeWait, TimeUnit.MILLISECONDS)) != null){ if(totalOrderPredicate(n.leader, n.zxid, n.peerEpoch, proposedLeader, proposedZxid, proposedEpoch)){ recvqueue.put(n); break; } } /* * This predicate is true once we don't read any new * relevant message from the reception queue */ if (n == null) { self.setPeerState((proposedLeader == self.getId()) ? ServerState.LEADING: learningState()); Vote endVote = new Vote(proposedLeader, proposedZxid, proposedEpoch); leaveInstance(endVote); return endVote; } } break; case OBSERVING: LOG.debug("Notification from observer: " + n.sid); break; case FOLLOWING: case LEADING: /* * Consider all notifications from the same epoch * together. */ if(n.electionEpoch == logicalclock.get()){ recvset.put(n.sid, new Vote(n.leader, n.zxid, n.electionEpoch, n.peerEpoch)); if(termPredicate(recvset, new Vote(n.leader, n.zxid, n.electionEpoch, n.peerEpoch, n.state)) && checkLeader(outofelection, n.leader, n.electionEpoch)) { self.setPeerState((n.leader == self.getId()) ? ServerState.LEADING: learningState()); Vote endVote = new Vote(n.leader, n.zxid, n.peerEpoch); leaveInstance(endVote); return endVote; } } /* * Before joining an established ensemble, verify that * a majority are following the same leader. * Only peer epoch is used to check that the votes come * from the same ensemble. This is because there is at * least one corner case in which the ensemble can be * created with inconsistent zxid and election epoch * info. However, given that only one ensemble can be * running at a single point in time and that each * epoch is used only once, using only the epoch to * compare the votes is sufficient. * * @see https://issues.apache.org/jira/browse/ZOOKEEPER-1732 */ outofelection.put(n.sid, new Vote(n.leader, IGNOREVALUE, IGNOREVALUE, n.peerEpoch, n.state)); if (termPredicate(outofelection, new Vote(n.leader, IGNOREVALUE, IGNOREVALUE, n.peerEpoch, n.state)) && checkLeader(outofelection, n.leader, IGNOREVALUE)) { synchronized(this){ logicalclock.set(n.electionEpoch); self.setPeerState((n.leader == self.getId()) ? ServerState.LEADING: learningState()); } Vote endVote = new Vote(n.leader, n.zxid, n.peerEpoch); leaveInstance(endVote); return endVote; } break; default: LOG.warn("Notification state unrecoginized: " + n.state + " (n.state), " + n.sid + " (n.sid)"); break; } } else { LOG.warn("Ignoring notification from non-cluster member " + n.sid); } } return null; } finally { try { if(self.jmxLeaderElectionBean != null){ MBeanRegistry.getInstance().unregister( self.jmxLeaderElectionBean); } } catch (Exception e) { LOG.warn("Failed to unregister with JMX", e); } self.jmxLeaderElectionBean = null; } }英文注释写的比较清晰，不太需要加注释了。接下来会关注QuorumPeer的对外服务，以及机器间通信相关代码
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