zookeeper（15）源碼分析-服務器（2）

LearnerZooKeeperServer是所有Follower和Observer的父類，在LearnerZooKeeperServer裏有2個重要的屬性：
//提交請求處理器
protected CommitProcessor commitProcessor;
//同步處理器
protected SyncRequestProcessor syncProcessor;

FollowerZooKeeperServer和ObserverZooKeeperServer都繼承了LearnerZooKeeperServer服務器。

1、FollowerZooKeeperServer

1.1、類屬性

    //待同步的請求
    ConcurrentLinkedQueue<Request> pendingSyncs;
    //待處理的事務請求
    LinkedBlockingQueue<Request> pendingTxns = new LinkedBlockingQueue<Request>();

1.2、核心函數

1.2.1、setupRequestProcessors

構建請求處理鏈，FollowerZooKeeperServer的請求處理鏈是：
FollowerRequestProcessor -> CommitProcessor ->FinalRequestProcessor

@Override
    protected void setupRequestProcessors() {
        //最後的處理器
        RequestProcessor finalProcessor = new FinalRequestProcessor(this);
        //第二個處理器
        commitProcessor = new CommitProcessor(finalProcessor,
                Long.toString(getServerId()), true, getZooKeeperServerListener());
        commitProcessor.start();
        //第一個請求處理器FollowerRequestProcessor
        firstProcessor = new FollowerRequestProcessor(this, commitProcessor);
        ((FollowerRequestProcessor) firstProcessor).start();
        syncProcessor = new SyncRequestProcessor(this,
                new SendAckRequestProcessor((Learner)getFollower()));
        syncProcessor.start();
    }

1.2.2、logRequest

該函數將請求進行記錄（放入到對應的隊列中），等待處理。

public void logRequest(TxnHeader hdr, Record txn) {
        Request request = new Request(hdr.getClientId(), hdr.getCxid(), hdr.getType(), hdr, txn, hdr.getZxid());
        //zxid不等於0，說明此服務器已經處理過請求
        if ((request.zxid & 0xffffffffL) != 0) {
            // 將該請求放入pendingTxns中，等待事務處理
            pendingTxns.add(request);
        }
        // 使用SyncRequestProcessor處理請求(其會將請求放在隊列中，異步進行處理)
        syncProcessor.proce***equest(request);
    }

1.2.3、commit

函數會提交zxid對應的請求（pendingTxns的隊首元素），其首先會判斷隊首請求對應的zxid是否爲傳入的zxid，然後再進行移除和提交（放在committedRequests隊列中）。

public void commit(long zxid) {
        // 沒有還在等待處理的事務
        if (pendingTxns.size() == 0) {
            LOG.warn("Committing " + Long.toHexString(zxid)
                    + " without seeing txn");
            return;
        }
        // 隊首元素的zxid
        long firstElementZxid = pendingTxns.element().zxid;
        // 如果隊首元素的zxid不等於需要提交的zxid，則退出程序
        if (firstElementZxid != zxid) {
            LOG.error("Committing zxid 0x" + Long.toHexString(zxid)
                    + " but next pending txn 0x"
                    + Long.toHexString(firstElementZxid));
            System.exit(12);
        }
        // 從待處理事務請求隊列中移除隊首請求
        Request request = pendingTxns.remove();
        // 提交該請求
        commitProcessor.commit(request);
    }

2、ObserverZooKeeperServer

2.1、類屬性

// 同步處理器是否可用，系統參數控制
private boolean syncRequestProcessorEnabled = this.self.getSyncEnabled();
// 待同步請求隊列
ConcurrentLinkedQueue<Request> pendingSyncs = 
        new ConcurrentLinkedQueue<Request>();

2.2、核心方法

2.2.1、setupRequestProcessors

構建請求處理鏈，ObserverZooKeeperServer的請求處理鏈是：ObserverRequestProcessor->CommitProcessor->FinalRequestProcessor，可能會存在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
         */
             //是否使用同步處理器，看系統參數配置，會影響性能
        if (syncRequestProcessorEnabled) {
            syncProcessor = new SyncRequestProcessor(this, null);
            syncProcessor.start();
        }
    }

2.2.2、commitRequest

同步處理器可用，則使用同步處理器進行處理（放入同步處理器的queuedRequests隊列中），然後提交請求（放入提交請求處理器的committedRequests隊列中）

public void commitRequest(Request request) {
        if (syncRequestProcessorEnabled) {
            // Write to txnlog and take periodic snapshot
                        //寫事務日誌，並定期快照
            syncProcessor.proce***equest(request);
        }
        commitProcessor.commit(request);
    }