follower在初始化的时候初始化调用链

    @Override
    protected void setupRequestProcessors() {
        RequestProcessor finalProcessor = new FinalRequestProcessor(this);
        commitProcessor = new CommitProcessor(finalProcessor,
                Long.toString(getServerId()), true,
                getZooKeeperServerListener());
        commitProcessor.start();
        firstProcessor = new FollowerRequestProcessor(this, commitProcessor);
        ((FollowerRequestProcessor) firstProcessor).start();
        syncProcessor = new SyncRequestProcessor(this,
                new SendAckRequestProcessor((Learner)getFollower()));
        syncProcessor.start();
    }

FollowerRequestProcessor-》CommitProcessor-》FinalRequestProcessor

SyncRequestProcessor-》SendAckRequestProcessor

在看看leader的调用链

    @Override
    protected void setupRequestProcessors() {
        RequestProcessor finalProcessor = new FinalRequestProcessor(this);
        RequestProcessor toBeAppliedProcessor = new Leader.ToBeAppliedRequestProcessor(
                finalProcessor, getLeader().toBeApplied);
        commitProcessor = new CommitProcessor(toBeAppliedProcessor,
                Long.toString(getServerId()), false,
                getZooKeeperServerListener());
        commitProcessor.start();
        ProposalRequestProcessor proposalProcessor = new ProposalRequestProcessor(this,
                commitProcessor);
        proposalProcessor.initialize();
        firstProcessor = new PrepRequestProcessor(this, proposalProcessor);
        ((PrepRequestProcessor)firstProcessor).start();
    }

PrepRequestProcessor-》ProposalRequestProcessor-》CommitProcessor-》ToBeAppliedRequestProcessor-》FinalRequestProcessor

ProposalRequestProcessor-》SyncRequestProcessor-》AckRequestProcessor

zookeeper什么时候会调用到这个处理链呢
以follower为例

                while (self.isRunning()) {
                    readPacket(qp);
                    processPacket(qp);
                }

follower在初始化好后会在这儿循环
在看下processPacket方法

    protected void processPacket(QuorumPacket qp) throws IOException{
        switch (qp.getType()) {
        case Leader.PING:            
            ping(qp);            
            break;
        case Leader.PROPOSAL:            
            TxnHeader hdr = new TxnHeader();
            Record txn = SerializeUtils.deserializeTxn(qp.getData(), hdr);
            if (hdr.getZxid() != lastQueued + 1) {
                LOG.warn("Got zxid 0x"
                        + Long.toHexString(hdr.getZxid())
                        + " expected 0x"
                        + Long.toHexString(lastQueued + 1));
            }
            lastQueued = hdr.getZxid();
            fzk.logRequest(hdr, txn);
            break;
        case Leader.COMMIT:
            fzk.commit(qp.getZxid());
            break;
        case Leader.UPTODATE:
            LOG.error("Received an UPTODATE message after Follower started");
            break;
        case Leader.REVALIDATE:
            revalidate(qp);
            break;
        case Leader.SYNC:
            fzk.sync();
            break;
        }
    }

如果客户端提交写请求，follower会先将事务请求提交给leader，leader会发起一个投票，Leader.PROPOSAL就是接受到投票命令，当过半机器同意后，leader在下发一个提交请求，对应的就是Leader.COMMIT。
而commit动作就是将事物处理放在一个队列中，同时唤醒CommitProcessor起的一个线程来处理。

    synchronized public void commit(Request request) {
        if (!finished) {
            if (request == null) {
                LOG.warn("Committed a null!",
                         new Exception("committing a null! "));
                return;
            }
            if (LOG.isDebugEnabled()) {
                LOG.debug("Committing request:: " + request);
            }
            committedRequests.add(request);
            notifyAll();
        }
    }

在最后一个处理链条FinalRequestProcessor上，实际就是将数据写入到内存中

               rc = zks.processTxn(hdr, txn);