zookeeper（15）源码分析-服务器（2）

LearnerZooKeeperServer是所有Follower和Observer的父类，在LearnerZooKeeperServer里有2个重要的属性：
//提交请求处理器
protected CommitProcessor commitProcessor;
//同步处理器
protected SyncRequestProcessor syncProcessor;

FollowerZooKeeperServer和ObserverZooKeeperServer都继承了LearnerZooKeeperServer服务器。

1、FollowerZooKeeperServer

1.1、类属性

    //待同步的请求
    ConcurrentLinkedQueue<Request> pendingSyncs;
    //待处理的事务请求
    LinkedBlockingQueue<Request> pendingTxns = new LinkedBlockingQueue<Request>();

1.2、核心函数

1.2.1、setupRequestProcessors

构建请求处理链，FollowerZooKeeperServer的请求处理链是：
FollowerRequestProcessor -> CommitProcessor ->FinalRequestProcessor

@Override
    protected void setupRequestProcessors() {
        //最后的处理器
        RequestProcessor finalProcessor = new FinalRequestProcessor(this);
        //第二个处理器
        commitProcessor = new CommitProcessor(finalProcessor,
                Long.toString(getServerId()), true, getZooKeeperServerListener());
        commitProcessor.start();
        //第一个请求处理器FollowerRequestProcessor
        firstProcessor = new FollowerRequestProcessor(this, commitProcessor);
        ((FollowerRequestProcessor) firstProcessor).start();
        syncProcessor = new SyncRequestProcessor(this,
                new SendAckRequestProcessor((Learner)getFollower()));
        syncProcessor.start();
    }

1.2.2、logRequest

该函数将请求进行记录（放入到对应的队列中），等待处理。

public void logRequest(TxnHeader hdr, Record txn) {
        Request request = new Request(hdr.getClientId(), hdr.getCxid(), hdr.getType(), hdr, txn, hdr.getZxid());
        //zxid不等于0，说明此服务器已经处理过请求
        if ((request.zxid & 0xffffffffL) != 0) {
            // 将该请求放入pendingTxns中，等待事务处理
            pendingTxns.add(request);
        }
        // 使用SyncRequestProcessor处理请求(其会将请求放在队列中，异步进行处理)
        syncProcessor.proce***equest(request);
    }

1.2.3、commit

函数会提交zxid对应的请求（pendingTxns的队首元素），其首先会判断队首请求对应的zxid是否为传入的zxid，然后再进行移除和提交（放在committedRequests队列中）。

public void commit(long zxid) {
        // 没有还在等待处理的事务
        if (pendingTxns.size() == 0) {
            LOG.warn("Committing " + Long.toHexString(zxid)
                    + " without seeing txn");
            return;
        }
        // 队首元素的zxid
        long firstElementZxid = pendingTxns.element().zxid;
        // 如果队首元素的zxid不等于需要提交的zxid，则退出程序
        if (firstElementZxid != zxid) {
            LOG.error("Committing zxid 0x" + Long.toHexString(zxid)
                    + " but next pending txn 0x"
                    + Long.toHexString(firstElementZxid));
            System.exit(12);
        }
        // 从待处理事务请求队列中移除队首请求
        Request request = pendingTxns.remove();
        // 提交该请求
        commitProcessor.commit(request);
    }

2、ObserverZooKeeperServer

2.1、类属性

// 同步处理器是否可用，系统参数控制
private boolean syncRequestProcessorEnabled = this.self.getSyncEnabled();
// 待同步请求队列
ConcurrentLinkedQueue<Request> pendingSyncs = 
        new ConcurrentLinkedQueue<Request>();

2.2、核心方法

2.2.1、setupRequestProcessors

构建请求处理链，ObserverZooKeeperServer的请求处理链是：ObserverRequestProcessor->CommitProcessor->FinalRequestProcessor，可能会存在SyncRequestProcessor。

@Override
    protected void setupRequestProcessors() {
        // We might consider changing the processor behaviour of
        // Observers to, for example, remove the disk sync requirements.
        // Currently, they behave almost exactly the same as followers.
        RequestProcessor finalProcessor = new FinalRequestProcessor(this);
        commitProcessor = new CommitProcessor(finalProcessor, Long.toString(getServerId()), true, getZooKeeperServerListener());
        commitProcessor.start();
        firstProcessor = new ObserverRequestProcessor(this, commitProcessor);
        ((ObserverRequestProcessor) firstProcessor).start();

        /*
         * Observer should write to disk, so that the it won't request
         * too old txn from the leader which may lead to getting an entire
         * snapshot.
         *
         * However, this may degrade performance as it has to write to disk
         * and do periodic snapshot which may double the memory requirements
         */
             //是否使用同步处理器，看系统参数配置，会影响性能
        if (syncRequestProcessorEnabled) {
            syncProcessor = new SyncRequestProcessor(this, null);
            syncProcessor.start();
        }
    }

2.2.2、commitRequest

同步处理器可用，则使用同步处理器进行处理（放入同步处理器的queuedRequests队列中），然后提交请求（放入提交请求处理器的committedRequests队列中）

public void commitRequest(Request request) {
        if (syncRequestProcessorEnabled) {
            // Write to txnlog and take periodic snapshot
                        //写事务日志，并定期快照
            syncProcessor.proce***equest(request);
        }
        commitProcessor.commit(request);
    }