zookeeper源码分析之五服务端(集群leader)处理请求流程

leader的实现类为LeaderZooKeeperServer，它间接继承自标准ZookeeperServer。它规定了请求到达leader时需要经历的路径：

PrepRequestProcessor -> ProposalRequestProcessor ->CommitProcessor -> Leader.ToBeAppliedRequestProcessor ->FinalRequestProcessor

具体情况可以参看代码：

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

        setupContainerManager();
    }

让我们一步步分析这些RP都做了什么工作？其中PrepRequestProcessor、FinalRequestProcessor已经在上篇文章中做了分析：

zookeeper源码分析之四服务端(单机)处理请求流程

那我们就开始余下的RP吧

1. ProposalRequestProcessor

这个RP仅仅将请求转发到AckRequestProcessor和SyncRequestProcessor上，看具体代码：

public void processRequest(Request request) throws RequestProcessorException {
        // LOG.warn("Ack>>> cxid = " + request.cxid + " type = " +
        // request.type + " id = " + request.sessionId);
        // request.addRQRec(">prop");

        /* In the following IF-THEN-ELSE block, we process syncs on the leader.
         * If the sync is coming from a follower, then the follower
         * handler adds it to syncHandler. Otherwise, if it is a client of
         * the leader that issued the sync command, then syncHandler won't
         * contain the handler. In this case, we add it to syncHandler, and
         * call processRequest on the next processor.
         */

        if (request instanceof LearnerSyncRequest){
            zks.getLeader().processSync((LearnerSyncRequest)request);
        } else {
            nextProcessor.processRequest(request);
            if (request.getHdr() != null) {
                // We need to sync and get consensus on any transactions
                try {
                    zks.getLeader().propose(request);
                } catch (XidRolloverException e) {
                    throw new RequestProcessorException(e.getMessage(), e);
                }
                syncProcessor.processRequest(request);
            }
        }
    }

SyncRequestProcessor 我们已经在上文中进行了分析，这里就不在赘述了，那就看看AckRequestProcessor的工作是什么吧？

AckRequestProcessor仅仅将发送过来的请求作为ACk转发给leader。代码见明细：

    /**
     * Forward the request as an ACK to the leader
     */
    public void processRequest(Request request) {
        QuorumPeer self = leader.self;
        if(self != null)
            leader.processAck(self.getId(), request.zxid, null);
        else
            LOG.error("Null QuorumPeer");
    }

leader处理请求如下所示：

 /**
     * Keep a count of acks that are received by the leader for a particular
     * proposal
     *
     * @param zxid, the zxid of the proposal sent out
     * @param sid, the id of the server that sent the ack
     * @param followerAddr
     */
    synchronized public void processAck(long sid, long zxid, SocketAddress followerAddr) {
        if (!allowedToCommit) return; // last op committed was a leader change - from now on
                                     // the new leader should commit
        if (LOG.isTraceEnabled()) {
            LOG.trace("Ack zxid: 0x{}", Long.toHexString(zxid));
            for (Proposal p : outstandingProposals.values()) {
                long packetZxid = p.packet.getZxid();
                LOG.trace("outstanding proposal: 0x{}",
                        Long.toHexString(packetZxid));
            }
            LOG.trace("outstanding proposals all");
        }

        if ((zxid & 0xffffffffL) == 0) {
            /*
             * We no longer process NEWLEADER ack with this method. However,
             * the learner sends an ack back to the leader after it gets
             * UPTODATE, so we just ignore the message.
             */
            return;
        }

        if (outstandingProposals.size() == 0) {
            if (LOG.isDebugEnabled()) {
                LOG.debug("outstanding is 0");
            }
            return;
        }
        if (lastCommitted >= zxid) {
            if (LOG.isDebugEnabled()) {
                LOG.debug("proposal has already been committed, pzxid: 0x{} zxid: 0x{}",
                        Long.toHexString(lastCommitted), Long.toHexString(zxid));
            }
            // The proposal has already been committed
            return;
        }
        Proposal p = outstandingProposals.get(zxid);
        if (p == null) {
            LOG.warn("Trying to commit future proposal: zxid 0x{} from {}",
                    Long.toHexString(zxid), followerAddr);
            return;
        }

        p.addAck(sid);
        /*if (LOG.isDebugEnabled()) {
            LOG.debug("Count for zxid: 0x{} is {}",
                    Long.toHexString(zxid), p.ackSet.size());
        }*/

        boolean hasCommitted = tryToCommit(p, zxid, followerAddr);

        // If p is a reconfiguration, multiple other operations may be ready to be committed,
        // since operations wait for different sets of acks.
       // Currently we only permit one outstanding reconfiguration at a time
       // such that the reconfiguration and subsequent outstanding ops proposed while the reconfig is
       // pending all wait for a quorum of old and new config, so its not possible to get enough acks
       // for an operation without getting enough acks for preceding ops. But in the future if multiple
       // concurrent reconfigs are allowed, this can happen and then we need to check whether some pending
        // ops may already have enough acks and can be committed, which is what this code does.

        if (hasCommitted && p.request!=null && p.request.getHdr().getType() == OpCode.reconfig){
               long curZxid = zxid;
           while (allowedToCommit && hasCommitted && p!=null){
               curZxid++;
               p = outstandingProposals.get(curZxid);
               if (p !=null) hasCommitted = tryToCommit(p, curZxid, null);
           }
        }
    }
    

调用实现，最终由CommitProcessor 接着处理请求：

 /**
     * @return True if committed, otherwise false.
     * @param a proposal p
     **/
    synchronized public boolean tryToCommit(Proposal p, long zxid, SocketAddress followerAddr) {
       // make sure that ops are committed in order. With reconfigurations it is now possible
       // that different operations wait for different sets of acks, and we still want to enforce
       // that they are committed in order. Currently we only permit one outstanding reconfiguration
       // such that the reconfiguration and subsequent outstanding ops proposed while the reconfig is
       // pending all wait for a quorum of old and new config, so its not possible to get enough acks
       // for an operation without getting enough acks for preceding ops. But in the future if multiple
       // concurrent reconfigs are allowed, this can happen.
       if (outstandingProposals.containsKey(zxid - 1)) return false;

       // getting a quorum from all necessary configurations
        if (!p.hasAllQuorums()) {
           return false;
        }

        // commit proposals in order
        if (zxid != lastCommitted+1) {
           LOG.warn("Commiting zxid 0x" + Long.toHexString(zxid)
                    + " from " + followerAddr + " not first!");
            LOG.warn("First is "
                    + (lastCommitted+1));
        }     

        // in order to be committed, a proposal must be accepted by a quorum              

        outstandingProposals.remove(zxid);

        if (p.request != null) {
             toBeApplied.add(p);
        }

        if (p.request == null) {
            LOG.warn("Going to commmit null: " + p);
        } else if (p.request.getHdr().getType() == OpCode.reconfig) {
            LOG.debug("Committing a reconfiguration! " + outstandingProposals.size()); 

            //if this server is voter in new config with the same quorum address,
            //then it will remain the leader
            //otherwise an up-to-date follower will be designated as leader. This saves
            //leader election time, unless the designated leader fails
            Long designatedLeader = getDesignatedLeader(p, zxid);
            //LOG.warn("designated leader is: " + designatedLeader);

            QuorumVerifier newQV = p.qvAcksetPairs.get(p.qvAcksetPairs.size()-1).getQuorumVerifier();

            self.processReconfig(newQV, designatedLeader, zk.getZxid(), true);

            if (designatedLeader != self.getId()) {
                allowedToCommit = false;
            }

            // we're sending the designated leader, and if the leader is changing the followers are
            // responsible for closing the connection - this way we are sure that at least a majority of them
            // receive the commit message.
            commitAndActivate(zxid, designatedLeader);
            informAndActivate(p, designatedLeader);
            //turnOffFollowers();
        } else {
            commit(zxid);
            inform(p);
        }
        zk.commitProcessor.commit(p.request);
        if(pendingSyncs.containsKey(zxid)){
            for(LearnerSyncRequest r: pendingSyncs.remove(zxid)) {
                sendSync(r);
            }
        } 

        return  true;
    }

该程序第一步是发送一个请求到Quorum的所有成员

    /**
     * Create a commit packet and send it to all the members of the quorum
     *
     * @param zxid
     */
    public void commit(long zxid) {
        synchronized(this){
            lastCommitted = zxid;
        }
        QuorumPacket qp = new QuorumPacket(Leader.COMMIT, zxid, null, null);
        sendPacket(qp);
    }

发送报文如下：

    /**
     * send a packet to all the followers ready to follow
     *
     * @param qp
     *                the packet to be sent
     */
    void sendPacket(QuorumPacket qp) {
        synchronized (forwardingFollowers) {
            for (LearnerHandler f : forwardingFollowers) {
                f.queuePacket(qp);
            }
        }
    }

第二步是通知Observer

    /**
     * Create an inform packet and send it to all observers.
     * @param zxid
     * @param proposal
     */
    public void inform(Proposal proposal) {
        QuorumPacket qp = new QuorumPacket(Leader.INFORM, proposal.request.zxid,
                                            proposal.packet.getData(), null);
        sendObserverPacket(qp);
    }

发送observer程序如下：

    /**
     * send a packet to all observers
     */
    void sendObserverPacket(QuorumPacket qp) {
        for (LearnerHandler f : getObservingLearners()) {
            f.queuePacket(qp);
        }
    }

第三步到

 zk.commitProcessor.commit(p.request);

2. CommitProcessor

CommitProcessor是多线程的，线程间通信通过queue，atomic和wait/notifyAll同步。CommitProcessor扮演一个网关角色，允许请求到剩下的处理管道。在同一瞬间，它支持多个读请求而仅支持一个写请求，这是为了保证写请求在事务中的顺序。

1个commit处理主线程，它监控请求队列，并将请求分发到工作线程，分发过程基于sessionId，这样特定session的读写请求通常分发到同一个线程，因而可以保证运行的顺序。

　　0~N个工作进程，他们在请求上运行剩下的请求处理管道。如果配置为0个工作线程，主commit线程将会直接运行管道。

　　经典(默认)线程数是：在32核的机器上，一个commit处理线程和32个工作线程。

多线程的限制：

　　每个session的请求处理必须是顺序的。

　　写请求处理必须按照zxid顺序。

　　必须保证一个session内不会出现写条件竞争，条件竞争可能导致另外一个session的读请求触发监控。

当前实现解决第三个限制，仅仅通过不允许在写请求时允许读进程的处理。

 @Override
    public void run() {
        Request request;
        try {
            while (!stopped) {
                synchronized(this) {
                    while (
                        !stopped &&
                        ((queuedRequests.isEmpty() || isWaitingForCommit() || isProcessingCommit()) &&
                         (committedRequests.isEmpty() || isProcessingRequest()))) {
                        wait();
                    }
                }

                /*
                 * Processing queuedRequests: Process the next requests until we
                 * find one for which we need to wait for a commit. We cannot
                 * process a read request while we are processing write request.
                 */
                while (!stopped && !isWaitingForCommit() &&
                       !isProcessingCommit() &&
                       (request = queuedRequests.poll()) != null) {
                    if (needCommit(request)) {
                        nextPending.set(request);
                    } else {
                        sendToNextProcessor(request);
                    }
                }

                /*
                 * Processing committedRequests: check and see if the commit
                 * came in for the pending request. We can only commit a
                 * request when there is no other request being processed.
                 */
                processCommitted();
            }
        } catch (Throwable e) {
            handleException(this.getName(), e);
        }
        LOG.info("CommitProcessor exited loop!");
    }

主逻辑程序如下：

 /*
     * Separated this method from the main run loop
     * for test purposes (ZOOKEEPER-1863)
     */
    protected void processCommitted() {
        Request request;

        if (!stopped && !isProcessingRequest() &&
                (committedRequests.peek() != null)) {

            /*
             * ZOOKEEPER-1863: continue only if there is no new request
             * waiting in queuedRequests or it is waiting for a
             * commit.
             */
            if ( !isWaitingForCommit() && !queuedRequests.isEmpty()) {
                return;
            }
            request = committedRequests.poll();

            /*
             * We match with nextPending so that we can move to the
             * next request when it is committed. We also want to
             * use nextPending because it has the cnxn member set
             * properly.
             */
            Request pending = nextPending.get();
            if (pending != null &&
                pending.sessionId == request.sessionId &&
                pending.cxid == request.cxid) {
                // we want to send our version of the request.
                // the pointer to the connection in the request
                pending.setHdr(request.getHdr());
                pending.setTxn(request.getTxn());
                pending.zxid = request.zxid;
                // Set currentlyCommitting so we will block until this
                // completes. Cleared by CommitWorkRequest after
                // nextProcessor returns.
                currentlyCommitting.set(pending);
                nextPending.set(null);
                sendToNextProcessor(pending);
            } else {
                // this request came from someone else so just
                // send the commit packet
                currentlyCommitting.set(request);
                sendToNextProcessor(request);
            }
        }
    }

启动多线程处理程序

    /**
     * Schedule final request processing; if a worker thread pool is not being
     * used, processing is done directly by this thread.
     */
    private void sendToNextProcessor(Request request) {
        numRequestsProcessing.incrementAndGet();
        workerPool.schedule(new CommitWorkRequest(request), request.sessionId);
    }

真实逻辑是

 /**
     * Schedule work to be done by the thread assigned to this id. Thread
     * assignment is a single mod operation on the number of threads.  If a
     * worker thread pool is not being used, work is done directly by
     * this thread.
     */
    public void schedule(WorkRequest workRequest, long id) {
        if (stopped) {
            workRequest.cleanup();
            return;
        }

        ScheduledWorkRequest scheduledWorkRequest =
            new ScheduledWorkRequest(workRequest);

        // If we have a worker thread pool, use that; otherwise, do the work
        // directly.
        int size = workers.size();
        if (size > 0) {
            try {
                // make sure to map negative ids as well to [0, size-1]
                int workerNum = ((int) (id % size) + size) % size;
                ExecutorService worker = workers.get(workerNum);
                worker.execute(scheduledWorkRequest);
            } catch (RejectedExecutionException e) {
                LOG.warn("ExecutorService rejected execution", e);
                workRequest.cleanup();
            }
        } else {
            // When there is no worker thread pool, do the work directly
            // and wait for its completion
            scheduledWorkRequest.start();
            try {
                scheduledWorkRequest.join();
            } catch (InterruptedException e) {
                LOG.warn("Unexpected exception", e);
                Thread.currentThread().interrupt();
            }
        }
    }

请求处理线程run方法：

       @Override
        public void run() {
            try {
                // Check if stopped while request was on queue
                if (stopped) {
                    workRequest.cleanup();
                    return;
                }
                workRequest.doWork();
            } catch (Exception e) {
                LOG.warn("Unexpected exception", e);
                workRequest.cleanup();
            }
        }

调用commitProcessor的doWork方法

        public void doWork() throws RequestProcessorException {
            try {
                nextProcessor.processRequest(request);
            } finally {
                // If this request is the commit request that was blocking
                // the processor, clear.
                currentlyCommitting.compareAndSet(request, null);

                /*
                 * Decrement outstanding request count. The processor may be
                 * blocked at the moment because it is waiting for the pipeline
                 * to drain. In that case, wake it up if there are pending
                 * requests.
                 */
                if (numRequestsProcessing.decrementAndGet() == 0) {
                    if (!queuedRequests.isEmpty() ||
                        !committedRequests.isEmpty()) {
                        wakeup();
                    }
                }
            }
        }

将请求传递给下一个RP：Leader.ToBeAppliedRequestProcessor

3.Leader.ToBeAppliedRequestProcessor

Leader.ToBeAppliedRequestProcessor仅仅维护一个toBeApplied列表。

 /**
         * This request processor simply maintains the toBeApplied list. For
         * this to work next must be a FinalRequestProcessor and
         * FinalRequestProcessor.processRequest MUST process the request
         * synchronously!
         *
         * @param next
         *                a reference to the FinalRequestProcessor
         */
        ToBeAppliedRequestProcessor(RequestProcessor next, Leader leader) {
            if (!(next instanceof FinalRequestProcessor)) {
                throw new RuntimeException(ToBeAppliedRequestProcessor.class
                        .getName()
                        + " must be connected to "
                        + FinalRequestProcessor.class.getName()
                        + " not "
                        + next.getClass().getName());
            }
            this.leader = leader;
            this.next = next;
        }

        /*
         * (non-Javadoc)
         *
         * @see org.apache.zookeeper.server.RequestProcessor#processRequest(org.apache.zookeeper.server.Request)
         */
        public void processRequest(Request request) throws RequestProcessorException {
            next.processRequest(request);

            // The only requests that should be on toBeApplied are write
            // requests, for which we will have a hdr. We can't simply use
            // request.zxid here because that is set on read requests to equal
            // the zxid of the last write op.
            if (request.getHdr() != null) {
                long zxid = request.getHdr().getZxid();
                Iterator<Proposal> iter = leader.toBeApplied.iterator();
                if (iter.hasNext()) {
                    Proposal p = iter.next();
                    if (p.request != null && p.request.zxid == zxid) {
                        iter.remove();
                        return;
                    }
                }
                LOG.error("Committed request not found on toBeApplied: "
                          + request);
            }
        }

4. FinalRequestProcessor前文已经说明，本文不在赘述。

小结：从上面的分析可以知道，leader处理请求的顺序分别是：PrepRequestProcessor -> ProposalRequestProcessor ->CommitProcessor -> Leader.ToBeAppliedRequestProcessor ->FinalRequestProcessor。

请求先通过PrepRequestProcessor接收请求，并进行包装，然后请求类型的不同，设置同享数据；主要负责通知所有follower和observer；CommitProcessor 启动多线程处理请求；Leader.ToBeAppliedRequestProcessor仅仅维护一个toBeApplied列表；

FinalRequestProcessor来作为消息处理器的终结者，发送响应消息，并触发watcher的处理程序。