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8. SOFAJRaft源码分析— JRaft是如何实现日志复制的？

luozhiyun 人气:1

前言

前几天和腾讯的大佬一起吃饭聊天，说起我对SOFAJRaft的理解，我自然以为我是很懂了的，但是大佬问起了我那SOFAJRaft集群之间的日志是怎么复制的？
我当时哑口无言，说不出是怎么实现的，所以这次来分析一下SOFAJRaft中日志复制是怎么做的。

Leader发送探针获取Follower的LastLogIndex

Leader 节点在通过 Replicator 和 Follower 建立连接之后，要发送一个 Probe 类型的探针请求，目的是知道 Follower 已经拥有的的日志位置，以便于向 Follower 发送后续的日志。

大致的流程如下：

NodeImpl#becomeLeader->replicatorGroup#addReplicator->Replicator#start->Replicator#sendEmptyEntries

最后会通过调用Replicator的sendEmptyEntries方法来发送探针来获取Follower的LastLogIndex

Replicator#sendEmptyEntries

private void sendEmptyEntries(final boolean isHeartbeat,
                              final RpcResponseClosure<AppendEntriesResponse> heartBeatClosure) {
    final AppendEntriesRequest.Builder rb = AppendEntriesRequest.newBuilder();
    //将集群配置设置到rb中，例如Term，GroupId，ServerId等
    if (!fillCommonFields(rb, this.nextIndex - 1, isHeartbeat)) {
        // id is unlock in installSnapshot
        installSnapshot();
        if (isHeartbeat && heartBeatClosure != null) {
            Utils.runClosureInThread(heartBeatClosure, new Status(RaftError.EAGAIN,
                "Fail to send heartbeat to peer %s", this.options.getPeerId()));
        }
        return;
    }
    try {
        final long monotonicSendTimeMs = Utils.monotonicMs();
        final AppendEntriesRequest request = rb.build();

        if (isHeartbeat) {
            ....//省略心跳代码
        } else {
            //statInfo这个类没看到哪里有用到，
            // Sending a probe request.
            //leader发送探针获取Follower的LastLogIndex
            this.statInfo.runningState = RunningState.APPENDING_ENTRIES;
            //将lastLogIndex设置为比firstLogIndex小1
            this.statInfo.firstLogIndex = this.nextIndex;
            this.statInfo.lastLogIndex = this.nextIndex - 1;
            this.appendEntriesCounter++;
            //设置当前Replicator为发送探针
            this.state = State.Probe;
            final int stateVersion = this.version;
            //返回reqSeq，并将reqSeq加一
            final int seq = getAndIncrementReqSeq();
            final Future<Message> rpcFuture = this.rpcService.appendEntries(this.options.getPeerId().getEndpoint(),
                request, -1, new RpcResponseClosureAdapter<AppendEntriesResponse>() {

                    @Override
                    public void run(final Status status) {
                        onRpcReturned(Replicator.this.id, RequestType.AppendEntries, status, request,
                            getResponse(), seq, stateVersion, monotonicSendTimeMs);
                    }

                });
            //Inflight 是对批量发送出去的 logEntry 的一种抽象，他表示哪些 logEntry 已经被封装成日志复制 request 发送出去了
            //这里是将logEntry封装到Inflight中
            addInflight(RequestType.AppendEntries, this.nextIndex, 0, 0, seq, rpcFuture);
        }
        LOG.debug("Node {} send HeartbeatRequest to {} term {} lastCommittedIndex {}", this.options.getNode()
            .getNodeId(), this.options.getPeerId(), this.options.getTerm(), request.getCommittedIndex());
    } finally {
        this.id.unlock();
    }
}

在调用sendEmptyEntries方法的时候，会传入isHeartbeat为false和heartBeatClosure为null，因为我们这个方法主要是发送探针获取Follower的位移。
首先调用fillCommonFields方法，将任期，groupId，ServerId，PeerIdLogIndex等设置到rb中，如：

private boolean fillCommonFields(final AppendEntriesRequest.Builder rb, long prevLogIndex, final boolean isHeartbeat) {
    final long prevLogTerm = this.options.getLogManager().getTerm(prevLogIndex);
    ....
    rb.setTerm(this.options.getTerm());
    rb.setGroupId(this.options.getGroupId());
    rb.setServerId(this.options.getServerId().toString());
    rb.setPeerId(this.options.getPeerId().toString());
    rb.setPrevLogIndex(prevLogIndex);
    rb.setPrevLogTerm(prevLogTerm);
    rb.setCommittedIndex(this.options.getBallotBox().getLastCommittedIndex());
    return true;
}

注意prevLogIndex是nextIndex-1，表示当前的index
继续往下走，会设置statInfo实例里面的属性，但是statInfo这个对象我没看到哪里有用到过。
然后向该Follower发送一个AppendEntriesRequest请求，onRpcReturned负责响应请求。
发送完请求后调用addInflight初始化一个Inflight实例，加入到inflights集合中，如下：

private void addInflight(final RequestType reqType, final long startIndex, final int count, final int size,
                         final int seq, final Future<Message> rpcInfly) {
    this.rpcInFly = new Inflight(reqType, startIndex, count, size, seq, rpcInfly);
    this.inflights.add(this.rpcInFly);
    this.nodeMetrics.recordSize("replicate-inflights-count", this.inflights.size());
}

Inflight 是对批量发送出去的 logEntry 的一种抽象，他表示哪些 logEntry 已经被封装成日志复制 request 发送出去了，这里是将logEntry封装到Inflight中。

Leader批量的发送日志给Follower

Replicator#sendEntries

private boolean sendEntries(final long nextSendingIndex) {
    final AppendEntriesRequest.Builder rb = AppendEntriesRequest.newBuilder();
    //填写当前Replicator的配置信息到rb中
    if (!fillCommonFields(rb, nextSendingIndex - 1, false)) {
        // unlock id in installSnapshot
        installSnapshot();
        return false;
    }

    ByteBufferCollector dataBuf = null;
    //获取最大的size为1024
    final int maxEntriesSize = this.raftOptions.getMaxEntriesSize();

    //这里使用了类似对象池的技术，避免重复创建对象
    final RecyclableByteBufferList byteBufList = RecyclableByteBufferList.newInstance();
    try {
        //循环遍历出所有的logEntry封装到byteBufList和emb中
        for (int i = 0; i < maxEntriesSize; i++) {
            final RaftOutter.EntryMeta.Builder emb = RaftOutter.EntryMeta.newBuilder();
            //nextSendingIndex代表下一个要发送的index，i代表偏移量
            if (!prepareEntry(nextSendingIndex, i, emb, byteBufList)) {
                break;
            }
            rb.addEntries(emb.build());
        }
        //如果EntriesCount为0的话，说明LogManager里暂时没有新数据
        if (rb.getEntriesCount() == 0) {
            if (nextSendingIndex < this.options.getLogManager().getFirstLogIndex()) {
                installSnapshot();
                return false;
            }
            // _id is unlock in _wait_more
            waitMoreEntries(nextSendingIndex);
            return false;
        }
        //将byteBufList里面的数据放入到rb中
        if (byteBufList.getCapacity() > 0) {
            dataBuf = ByteBufferCollector.allocateByRecyclers(byteBufList.getCapacity());
            for (final ByteBuffer b : byteBufList) {
                dataBuf.put(b);
            }
            final ByteBuffer buf = dataBuf.getBuffer();
            buf.flip();
            rb.setData(ZeroByteStringHelper.wrap(buf));
        }
    } finally {
        //回收一下byteBufList
        RecycleUtil.recycle(byteBufList);
    }

    final AppendEntriesRequest request = rb.build();
    if (LOG.isDebugEnabled()) {
        LOG.debug(
            "Node {} send AppendEntriesRequest to {} term {} lastCommittedIndex {} prevLogIndex {} prevLogTerm {} logIndex {} count {}",
            this.options.getNode().getNodeId(), this.options.getPeerId(), this.options.getTerm(),
            request.getCommittedIndex(), request.getPrevLogIndex(), request.getPrevLogTerm(), nextSendingIndex,
            request.getEntriesCount());
    }
    //statInfo没找到哪里有用到过
    this.statInfo.runningState = RunningState.APPENDING_ENTRIES;
    this.statInfo.firstLogIndex = rb.getPrevLogIndex() + 1;
    this.statInfo.lastLogIndex = rb.getPrevLogIndex() + rb.getEntriesCount();

    final Recyclable recyclable = dataBuf;
    final int v = this.version;
    final long monotonicSendTimeMs = Utils.monotonicMs();
    final int seq = getAndIncrementReqSeq();
    final Future<Message> rpcFuture = this.rpcService.appendEntries(this.options.getPeerId().getEndpoint(),
        request, -1, new RpcResponseClosureAdapter<AppendEntriesResponse>() {

            @Override
            public void run(final Status status) {
                //回收资源
                RecycleUtil.recycle(recyclable);
                onRpcReturned(Replicator.this.id, RequestType.AppendEntries, status, request, getResponse(), seq,
                    v, monotonicSendTimeMs);
            }

        });
    //添加Inflight
    addInflight(RequestType.AppendEntries, nextSendingIndex, request.getEntriesCount(), request.getData().size(),
        seq, rpcFuture);
    return true;

}
  1. 首先会调用fillCommonFields方法，填写当前Replicator的配置信息到rb中；
  2. 调用prepareEntry，根据当前的I和nextSendingIndex计算出当前的偏移量，然后去LogManager找到对应的LogEntry，再把LogEntry里面的属性设置到emb中，并把LogEntry里面的数据加入到RecyclableByteBufferList中；
  3. 如果LogEntry里面没有新的数据，那么EntriesCount会为0，那么就返回；
  4. 遍历byteBufList里面的数据，将数据添加到rb中，这样rb里面的数据就是前面是任期、类型、数据长度等信息，rb后面就是真正的数据；
  5. 新建AppendEntriesRequest实例发送请求；
  6. 添加 Inflight 到队列中。Leader 维护一个 queue，每发出一批 logEntry 就向 queue 中 添加一个代表这一批 logEntry 的 Inflight，这样当它知道某一批 logEntry 复制失败之后，就可以依赖 queue 中的 Inflight 把该批次 logEntry 以及后续的所有日志重新复制给 follower。既保证日志复制能够完成，又保证了复制日志的顺序不变

其中RecyclableByteBufferList采用对象池进行实例化，对象池的相关信息可以看我这篇：7. SOFAJRaft源码分析—如何实现一个轻量级的对象池？

下面我们详解一下sendEntries里面的具体方法。

prepareEntry填充emb属性

Replicator#prepareEntry

boolean prepareEntry(final long nextSendingIndex, final int offset, final RaftOutter.EntryMeta.Builder emb,
                     final RecyclableByteBufferList dateBuffer) {
    if (dateBuffer.getCapacity() >= this.raftOptions.getMaxBodySize()) {
        return false;
    }
    //设置当前要发送的index
    final long logIndex = nextSendingIndex + offset;
    //如果这个index已经在LogManager中找不到了，那么直接返回
    final LogEntry entry = this.options.getLogManager().getEntry(logIndex);
    if (entry == null) {
        return false;
    }
    //下面就是把LogEntry里面的属性设置到emb中
    emb.setTerm(entry.getId().getTerm());
    if (entry.hasChecksum()) {
        emb.setChecksum(entry.getChecksum()); //since 1.2.6
    }
    emb.setType(entry.getType());
    if (entry.getPeers() != null) {
        Requires.requireTrue(!entry.getPeers().isEmpty(), "Empty peers at logIndex=%d", logIndex);
        for (final PeerId peer : entry.getPeers()) {
            emb.addPeers(peer.toString());
        }
        if (entry.getOldPeers() != null) {
            for (final PeerId peer : entry.getOldPeers()) {
                emb.addOldPeers(peer.toString());
            }
        }
    } else {
        Requires.requireTrue(entry.getType() != EnumOutter.EntryType.ENTRY_TYPE_CONFIGURATION,
            "Empty peers but is ENTRY_TYPE_CONFIGURATION type at logIndex=%d", logIndex);
    }
    final int remaining = entry.getData() != null ? entry.getData().remaining() : 0;
    emb.setDataLen(remaining);
    //把LogEntry里面的数据放入到dateBuffer中
    if (entry.getData() != null) {
        // should slice entry data
        dateBuffer.add(entry.getData().slice());
    }
    return true;
}
  1. 对比一下传入的dateBuffer的容量是否已经超过了系统设置的容量（512 * 1024），如果超过了则返回false
  2. 根据给定的起始的index和偏移量offset计算logIndex，然后去LogManager里面根据index获取LogEntry，如果返回的为则说明找不到了，那么就直接返回false，外层的if判断会执行break跳出循环
  3. 然后将LogEntry里面的属性设置到emb对象中，最后将LogEntry里面的数据添加到dateBuffer，这里要做到数据和属性分离

Follower处理Leader发送的日志复制请求

在leader发送完AppendEntriesRequest请求之后，请求的数据会在Follower中被AppendEntriesRequestProcessor所处理

具体的处理方法是processRequest0

public Message processRequest0(final RaftServerService service, final AppendEntriesRequest request,
                               final RpcRequestClosure done) {

    final Node node = (Node) service;

    //默认使用pipeline
    if (node.getRaftOptions().isReplicatorPipeline()) {
        final String groupId = request.getGroupId();
        final String peerId = request.getPeerId();
        //获取请求的次数，以groupId+peerId为一个维度
        final int reqSequence = getAndIncrementSequence(groupId, peerId, done.getBizContext().getConnection());
        //Follower处理leader发过来的日志请求
        final Message response = service.handleAppendEntriesRequest(request, new SequenceRpcRequestClosure(done,
            reqSequence, groupId, peerId));
        //正常的数据只返回null，异常的数据会返回response
        if (response != null) {
            sendSequenceResponse(groupId, peerId, reqSequence, done.getAsyncContext(), done.getBizContext(),
                response);
        }
        return null;
    } else {
        return service.handleAppendEntriesRequest(request, done);
    }
}

调用service的handleAppendEntriesRequest会调用到NodeIml的handleAppendEntriesRequest方法中，handleAppendEntriesRequest方法只是异常情况和leader没有发送数据时才会返回，正常情况是返回null

处理响应日志复制请求

NodeIml#handleAppendEntriesRequest

public Message handleAppendEntriesRequest(final AppendEntriesRequest request, final RpcRequestClosure done) {
    boolean doUnlock = true;
    final long startMs = Utils.monotonicMs();
    this.writeLock.lock();
    //获取entryLog个数
    final int entriesCount = request.getEntriesCount();
    try {
        //校验当前节点是否活跃
        if (!this.state.isActive()) {
            LOG.warn("Node {} is not in active state, currTerm={}.", getNodeId(), this.currTerm);
            return RpcResponseFactory.newResponse(RaftError.EINVAL, "Node %s is not in active state, state %s.",
                getNodeId(), this.state.name());
        }
        //校验传入的serverId是否能被正常解析
        final PeerId serverId = new PeerId();
        if (!serverId.parse(request.getServerId())) {
            LOG.warn("Node {} received AppendEntriesRequest from {} serverId bad format.", getNodeId(),
                request.getServerId());
            return RpcResponseFactory.newResponse(RaftError.EINVAL, "Parse serverId failed: %s.",
                request.getServerId());
        }
        //校验任期
        // Check stale term
        if (request.getTerm() < this.currTerm) {
            LOG.warn("Node {} ignore stale AppendEntriesRequest from {}, term={}, currTerm={}.", getNodeId(),
                request.getServerId(), request.getTerm(), this.currTerm);
            return AppendEntriesResponse.newBuilder() //
                .setSuccess(false) //
                .setTerm(this.currTerm) //
                .build();
        }

        // Check term and state to step down
        //当前节点如果不是Follower节点的话要执行StepDown操作
        checkStepDown(request.getTerm(), serverId);
        //这说明请求的节点不是当前节点的leader
        if (!serverId.equals(this.leaderId)) {
            LOG.error("Another peer {} declares that it is the leader at term {} which was occupied by leader {}.",
                serverId, this.currTerm, this.leaderId);
            // Increase the term by 1 and make both leaders step down to minimize the
            // loss of split brain
            stepDown(request.getTerm() + 1, false, new Status(RaftError.ELEADERCONFLICT,
                "More than one leader in the same term."));
            return AppendEntriesResponse.newBuilder() //
                .setSuccess(false) //
                .setTerm(request.getTerm() + 1) //
                .build();
        }

        updateLastLeaderTimestamp(Utils.monotonicMs());

        //校验是否正在生成快照
        if (entriesCount > 0 && this.snapshotExecutor != null && this.snapshotExecutor.isInstallingSnapshot()) {
            LOG.warn("Node {} received AppendEntriesRequest while installing snapshot.", getNodeId());
            return RpcResponseFactory.newResponse(RaftError.EBUSY, "Node %s:%s is installing snapshot.",
                this.groupId, this.serverId);
        }
        //传入的是发起请求节点的nextIndex-1
        final long prevLogIndex = request.getPrevLogIndex();
        final long prevLogTerm = request.getPrevLogTerm();
        final long localPrevLogTerm = this.logManager.getTerm(prevLogIndex);
        //发起请求的节点prevLogIndex对应的任期和当前节点的index所对应的任期不匹配
        if (localPrevLogTerm != prevLogTerm) {
            final long lastLogIndex = this.logManager.getLastLogIndex();

            LOG.warn(
                "Node {} reject term_unmatched AppendEntriesRequest from {}, term={}, prevLogIndex={}, prevLogTerm={}, localPrevLogTerm={}, lastLogIndex={}, entriesSize={}.",
                getNodeId(), request.getServerId(), request.getTerm(), prevLogIndex, prevLogTerm, localPrevLogTerm,
                lastLogIndex, entriesCount);

            return AppendEntriesResponse.newBuilder() //
                .setSuccess(false) //
                .setTerm(this.currTerm) //
                .setLastLogIndex(lastLogIndex) //
                .build();
        }
        //响应心跳或者发送的是sendEmptyEntry
        if (entriesCount == 0) {
            // heartbeat
            final AppendEntriesResponse.Builder respBuilder = AppendEntriesResponse.newBuilder() //
                .setSuccess(true) //
                .setTerm(this.currTerm)
                //  返回当前节点的最新的index
                .setLastLogIndex(this.logManager.getLastLogIndex());
            doUnlock = false;
            this.writeLock.unlock();
            // see the comments at FollowerStableClosure#run()
            this.ballotBox.setLastCommittedIndex(Math.min(request.getCommittedIndex(), prevLogIndex));
            return respBuilder.build();
        }

        // Parse request
        long index = prevLogIndex;
        final List<LogEntry> entries = new ArrayList<>(entriesCount);
        ByteBuffer allData = null;
        if (request.hasData()) {
            allData = request.getData().asReadOnlyByteBuffer();
        }
        //获取所有数据
        final List<RaftOutter.EntryMeta> entriesList = request.getEntriesList();
        for (int i = 0; i < entriesCount; i++) {
            final RaftOutter.EntryMeta entry = entriesList.get(i);
            index++;
            if (entry.getType() != EnumOutter.EntryType.ENTRY_TYPE_UNKNOWN) {
                //给logEntry属性设值
                final LogEntry logEntry = new LogEntry();
                logEntry.setId(new LogId(index, entry.getTerm()));
                logEntry.setType(entry.getType());
                if (entry.hasChecksum()) {
                    logEntry.setChecksum(entry.getChecksum()); // since 1.2.6
                }
                //将数据填充到logEntry
                final long dataLen = entry.getDataLen();
                if (dataLen > 0) {
                    final byte[] bs = new byte[(int) dataLen];
                    assert allData != null;
                    allData.get(bs, 0, bs.length);
                    logEntry.setData(ByteBuffer.wrap(bs));
                }

                if (entry.getPeersCount() > 0) {
                    //只有配置类型的entry才有多个Peer
                    if (entry.getType() != EnumOutter.EntryType.ENTRY_TYPE_CONFIGURATION) {
                        throw new IllegalStateException(
                                "Invalid log entry that contains peers but is not ENTRY_TYPE_CONFIGURATION type: "
                                        + entry.getType());
                    }

                    final List<PeerId> peers = new ArrayList<>(entry.getPeersCount());
                    for (final String peerStr : entry.getPeersList()) {
                        final PeerId peer = new PeerId();
                        peer.parse(peerStr);
                        peers.add(peer);
                    }
                    logEntry.setPeers(peers);

                    if (entry.getOldPeersCount() > 0) {
                        final List<PeerId> oldPeers = new ArrayList<>(entry.getOldPeersCount());
                        for (final String peerStr : entry.getOldPeersList()) {
                            final PeerId peer = new PeerId();
                            peer.parse(peerStr);
                            oldPeers.add(peer);
                        }
                        logEntry.setOldPeers(oldPeers);
                    }
                } else if (entry.getType() == EnumOutter.EntryType.ENTRY_TYPE_CONFIGURATION) {
                    throw new IllegalStateException(
                            "Invalid log entry that contains zero peers but is ENTRY_TYPE_CONFIGURATION type");
                }

                // Validate checksum
                if (this.raftOptions.isEnableLogEntryChecksum() && logEntry.isCorrupted()) {
                    long realChecksum = logEntry.checksum();
                    LOG.error(
                            "Corrupted log entry received from leader, index={}, term={}, expectedChecksum={}, " +
                             "realChecksum={}",
                            logEntry.getId().getIndex(), logEntry.getId().getTerm(), logEntry.getChecksum(),
                            realChecksum);
                    return RpcResponseFactory.newResponse(RaftError.EINVAL,
                            "The log entry is corrupted, index=%d, term=%d, expectedChecksum=%d, realChecksum=%d",
                            logEntry.getId().getIndex(), logEntry.getId().getTerm(), logEntry.getChecksum(),
                            realChecksum);
                }

                entries.add(logEntry);
            }
        }
        //存储日志，并回调返回response
        final FollowerStableClosure closure = new FollowerStableClosure(request, AppendEntriesResponse.newBuilder()
            .setTerm(this.currTerm), this, done, this.currTerm);
        this.logManager.appendEntries(entries, closure);
        // update configuration after _log_manager updated its memory status
        this.conf = this.logManager.checkAndSetConfiguration(this.conf);
        return null;
    } finally {
        if (doUnlock) {
            this.writeLock.unlock();
        }
        this.metrics.recordLatency("handle-append-entries", Utils.monotonicMs() - startMs);
        this.metrics.recordSize("handle-append-entries-count", entriesCount);
    }
}

handleAppendEntriesRequest方法写的很长，但是实际上做了很多校验的事情，具体的处理逻辑不多

  1. 校验当前的Node节点是否还处于活跃状态，如果不是的话，那么直接返回一个error的response
  2. 校验请求的serverId的格式是否正确，不正确则返回一个error的response
  3. 校验请求的任期是否小于当前的任期，如果是那么返回一个AppendEntriesResponse类型的response
  4. 调用checkStepDown方法检测当前节点的任期，以及状态，是否有leader等
  5. 如果请求的serverId和当前节点的leaderId是不是同一个，用来校验是不是leader发起的请求，如果不是返回一个AppendEntriesResponse
  6. 校验是否正在生成快照
  7. 获取请求的Index在当前节点中对应的LogEntry的任期是不是和请求传入的任期相同，不同的话则返回AppendEntriesResponse
  8. 如果传入的entriesCount为零，那么leader发送的可能是心跳或者发送的是sendEmptyEntry，返回AppendEntriesResponse，并将当前任期和最新index封装返回
  9. 请求的数据不为空，那么遍历所有的数据
  10. 实例化一个logEntry，并且将数据和属性设置到logEntry实例中，最后将logEntry放入到entries集合中
  11. 调用logManager将数据批量提交日志写入 RocksDB

发送响应给leader

最终发送给leader的响应是通过AppendEntriesRequestProcessor的sendSequenceResponse来发送的

void sendSequenceResponse(final String groupId, final String peerId, final int seq,
                          final AsyncContext asyncContext, final BizContext bizContext, final Message msg) {
    final Connection connection = bizContext.getConnection();
    //获取context，维度是groupId和peerId
    final PeerRequestContext ctx = getPeerRequestContext(groupId, peerId, connection);
    final PriorityQueue<SequenceMessage> respQueue = ctx.responseQueue;
    assert (respQueue != null);

    synchronized (Utils.withLockObject(respQueue)) {
        //将要响应的数据放入到优先队列中
        respQueue.add(new SequenceMessage(asyncContext, msg, seq));
        //校验队列里面的数据是否超过了256
        if (!ctx.hasTooManyPendingResponses()) {
            while (!respQueue.isEmpty()) {
                final SequenceMessage queuedPipelinedResponse = respQueue.peek();
                //如果序列对应不上，那么就不发送响应
                if (queuedPipelinedResponse.sequence != getNextRequiredSequence(groupId, peerId, connection)) {
                    // sequence mismatch, waiting for next response.
                    break;
                }
                respQueue.remove();
                try {
                    //发送响应
                    queuedPipelinedResponse.sendResponse();
                } finally {
                    //序列加一
                    getAndIncrementNextRequiredSequence(groupId, peerId, connection);
                }
            }
        } else {
            LOG.warn("Closed connection to peer {}/{}, because of too many pending responses, queued={}, max={}",
                ctx.groupId, peerId, respQueue.size(), ctx.maxPendingResponses);
            connection.close();
            // Close the connection if there are too many pending responses in queue.
            removePeerRequestContext(groupId, peerId);
        }
    }
}

这个方法会将要发送的数据依次压入到PriorityQueue优先队列中进行排序，然后获取序列号最小的元素和nextRequiredSequence比较，如果不相等，那么则是出现了乱序的情况，那么就不发送请求

Leader处理日志复制的Response

Leader收到Follower发过来的Response响应之后会调用Replicator的onRpcReturned方法

static void onRpcReturned(final ThreadId id, final RequestType reqType, final Status status, final Message request,
                          final Message response, final int seq, final int stateVersion, final long rpcSendTime) {
    if (id == null) {
        return;
    }
    final long startTimeMs = Utils.nowMs();
    Replicator r;
    if ((r = (Replicator) id.lock()) == null) {
        return;
    }
    //检查版本号，因为每次resetInflights都会让version加一，所以检查一下
    if (stateVersion != r.version) {
        LOG.debug(
            "Replicator {} ignored old version response {}, current version is {}, request is {}\n, and response is {}\n, status is {}.",
            r, stateVersion, r.version, request, response, status);
        id.unlock();
        return;
    }
    //使用优先队列按seq排序,最小的会在第一个
    final PriorityQueue<RpcResponse> holdingQueue = r.pendingResponses;
    //这里用一个优先队列是因为响应是异步的，seq小的可能响应比seq大慢
    holdingQueue.add(new RpcResponse(reqType, seq, status, request, response, rpcSendTime));
    //默认holdingQueue队列里面的数量不能超过256
    if (holdingQueue.size() > r.raftOptions.getMaxReplicatorInflightMsgs()) {
        LOG.warn("Too many pending responses {} for replicator {}, maxReplicatorInflightMsgs={}",
            holdingQueue.size(), r.options.getPeerId(), r.raftOptions.getMaxReplicatorInflightMsgs());
        //重新发送探针
        //清空数据
        r.resetInflights();
        r.state = State.Probe;
        r.sendEmptyEntries(false);
        return;
    }

    boolean continueSendEntries = false;

    final boolean isLogDebugEnabled = LOG.isDebugEnabled();
    StringBuilder sb = null;
    if (isLogDebugEnabled) {
        sb = new StringBuilder("Replicator ").append(r).append(" is processing RPC responses,");
    }
    try {
        int processed = 0;
        while (!holdingQueue.isEmpty()) {
            //取出holdingQueue里seq最小的数据
            final RpcResponse queuedPipelinedResponse = holdingQueue.peek();

            //如果Follower没有响应的话就会出现次序对不上的情况，那么就不往下走了
            //sequence mismatch, waiting for next response.
            if (queuedPipelinedResponse.seq != r.requiredNextSeq) {
                // 如果之前存在处理，则到此直接break循环
                if (processed > 0) {
                    if (isLogDebugEnabled) {
                        sb.append("has processed ").append(processed).append(" responses,");
                    }
                    break;
                } else {
                    //Do not processed any responses, UNLOCK id and return.
                    continueSendEntries = false;
                    id.unlock();
                    return;
                }
            }
            //走到这里说明seq对的上，那么就移除优先队列里面seq最小的数据
            holdingQueue.remove();
            processed++;
            //获取inflights队列里的第一个元素
            final Inflight inflight = r.pollInflight();
            //发起一个请求的时候会将inflight放入到队列中
            //如果为空，那么就忽略
            if (inflight == null) {
                // The previous in-flight requests were cleared.
                if (isLogDebugEnabled) {
                    sb.append("ignore response because request not found:").append(queuedPipelinedResponse)
                        .append(",\n");
                }
                continue;
            }
            //seq没有对上，说明顺序乱了，重置状态
            if (inflight.seq != queuedPipelinedResponse.seq) {
                // reset state
                LOG.warn(
                    "Replicator {} response sequence out of order, expect {}, but it is {}, reset state to try again.",
                    r, inflight.seq, queuedPipelinedResponse.seq);
                r.resetInflights();
                r.state = State.Probe;
                continueSendEntries = false;
                // 锁住节点，根据错误类别等待一段时间
                r.block(Utils.nowMs(), RaftError.EREQUEST.getNumber());
                return;
            }
            try {
                switch (queuedPipelinedResponse.requestType) {
                    case AppendEntries:
                        //处理日志复制的response
                        continueSendEntries = onAppendEntriesReturned(id, inflight, queuedPipelinedResponse.status,
                            (AppendEntriesRequest) queuedPipelinedResponse.request,
                            (AppendEntriesResponse) queuedPipelinedResponse.response, rpcSendTime, startTimeMs, r);
                        break;
                    case Snapshot:
                        //处理快照的response
                        continueSendEntries = onInstallSnapshotReturned(id, r, queuedPipelinedResponse.status,
                            (InstallSnapshotRequest) queuedPipelinedResponse.request,
                            (InstallSnapshotResponse) queuedPipelinedResponse.response);
                        break;
                }
            } finally {
                if (continueSendEntries) {
                    // Success, increase the response sequence.
                    r.getAndIncrementRequiredNextSeq();
                } else {
                    // The id is already unlocked in onAppendEntriesReturned/onInstallSnapshotReturned, we SHOULD break out.
                    break;
                }
            }
        }
    } finally {
        if (isLogDebugEnabled) {
            sb.append(", after processed, continue to send entries: ").append(continueSendEntries);
            LOG.debug(sb.toString());
        }
        if (continueSendEntries) {
            // unlock in sendEntries.
            r.sendEntries();
        }
    }
}
  1. 检查版本号，因为每次resetInflights都会让version加一，所以检查一下是不是同一批的数据
  2. 获取Replicator的pendingResponses队列，然后将当前响应的数据封装成RpcResponse实例加入到队列中
  3. 校验队列里面的元素是否大于256，大于256则清空数据重新同步
  4. 校验holdingQueue队列里面的seq最小的序列数据序列和当前的requiredNextSeq是否相同，不同的话如果是刚进入循环那么直接break退出循环
  5. 获取inflights队列中第一个元素，如果seq没有对上，说明顺序乱了，重置状态
  6. 调用onAppendEntriesReturned方法处理日志复制的response
  7. 如果处理成功，那么则调用sendEntries继续发送复制日志到Follower

Replicator#onAppendEntriesReturned

private static boolean onAppendEntriesReturned(final ThreadId id, final Inflight inflight, final Status status,
                                               final AppendEntriesRequest request,
                                               final AppendEntriesResponse response, final long rpcSendTime,
                                               final long startTimeMs, final Replicator r) {
    //校验数据序列有没有错
    if (inflight.startIndex != request.getPrevLogIndex() + 1) {
        LOG.warn(
            "Replicator {} received invalid AppendEntriesResponse, in-flight startIndex={}, request prevLogIndex={}, reset the replicator state and probe again.",
            r, inflight.startIndex, request.getPrevLogIndex());
        r.resetInflights();
        r.state = State.Probe;
        // unlock id in sendEmptyEntries
        r.sendEmptyEntries(false);
        return false;
    }
    //度量
    // record metrics
    if (request.getEntriesCount() > 0) {
        r.nodeMetrics.recordLatency("replicate-entries", Utils.monotonicMs() - rpcSendTime);
        r.nodeMetrics.recordSize("replicate-entries-count", request.getEntriesCount());
        r.nodeMetrics.recordSize("replicate-entries-bytes", request.getData() != null ? request.getData().size()
            : 0);
    }

    final boolean isLogDebugEnabled = LOG.isDebugEnabled();
    StringBuilder sb = null;
    if (isLogDebugEnabled) {
        sb = new StringBuilder("Node "). //
            append(r.options.getGroupId()).append(":").append(r.options.getServerId()). //
            append(" received AppendEntriesResponse from "). //
            append(r.options.getPeerId()). //
            append(" prevLogIndex=").append(request.getPrevLogIndex()). //
            append(" prevLogTerm=").append(request.getPrevLogTerm()). //
            append(" count=").append(request.getEntriesCount());
    }
    //如果follower因为崩溃，RPC调用失败等原因没有收到成功响应
    //那么需要阻塞一段时间再进行调用
    if (!status.isOk()) {
        // If the follower crashes, any RPC to the follower fails immediately,
        // so we need to block the follower for a while instead of looping until
        // it comes back or be removed
        // dummy_id is unlock in block
        if (isLogDebugEnabled) {
            sb.append(" fail, sleep.");
            LOG.debug(sb.toString());
        }
        //如果注册了Replicator状态监听器，那么通知所有监听器
        notifyReplicatorStatusListener(r, ReplicatorEvent.ERROR, status);
        if (++r.consecutiveErrorTimes % 10 == 0) {
            LOG.warn("Fail to issue RPC to {}, consecutiveErrorTimes={}, error={}", r.options.getPeerId(),
                r.consecutiveErrorTimes, status);
        }
        r.resetInflights();
        r.state = State.Probe;
        // unlock in in block
        r.block(startTimeMs, status.getCode());
        return false;
    }
    r.consecutiveErrorTimes = 0;
    //响应失败
    if (!response.getSuccess()) {
        // Leader 的切换，表明可能出现过一次网络分区，从新跟随新的 Leader
        if (response.getTerm() > r.options.getTerm()) {
            if (isLogDebugEnabled) {
                sb.append(" fail, greater term ").append(response.getTerm()).append(" expect term ")
                    .append(r.options.getTerm());
                LOG.debug(sb.toString());
            }
            // 获取当前本节点的表示对象——NodeImpl
            final NodeImpl node = r.options.getNode();
            r.notifyOnCaughtUp(RaftError.EPERM.getNumber(), true);
            r.destroy();
            // 调整自己的 term 任期值
            node.increaseTermTo(response.getTerm(), new Status(RaftError.EHIGHERTERMRESPONSE,
                "Leader receives higher term heartbeat_response from peer:%s", r.options.getPeerId()));
            return false;
        }
        if (isLogDebugEnabled) {
            sb.append(" fail, find nextIndex remote lastLogIndex ").append(response.getLastLogIndex())
                .append(" local nextIndex ").append(r.nextIndex);
            LOG.debug(sb.toString());
        }
        if (rpcSendTime > r.lastRpcSendTimestamp) {
            r.lastRpcSendTimestamp = rpcSendTime;
        }
        // Fail, reset the state to try again from nextIndex.
        r.resetInflights();
        //如果Follower最新的index小于下次要发送的index，那么设置为Follower响应的index
        // prev_log_index and prev_log_term doesn't match
        if (response.getLastLogIndex() + 1 < r.nextIndex) {
            LOG.debug("LastLogIndex at peer={} is {}", r.options.getPeerId(), response.getLastLogIndex());
            // The peer contains less logs than leader
            r.nextIndex = response.getLastLogIndex() + 1;
        } else {
            // The peer contains logs from old term which should be truncated,
            // decrease _last_log_at_peer by one to test the right index to keep
            if (r.nextIndex > 1) {
                LOG.debug("logIndex={} dismatch", r.nextIndex);
                r.nextIndex--;
            } else {
                LOG.error("Peer={} declares that log at index=0 doesn't match, which is not supposed to happen",
                    r.options.getPeerId());
            }
        }
        //响应失败需要重新获取Follower的日志信息，用来重新同步
        // dummy_id is unlock in _send_heartbeat
        r.sendEmptyEntries(false);
        return false;
    }
    if (isLogDebugEnabled) {
        sb.append(", success");
        LOG.debug(sb.toString());
    }
    // success
    //响应成功检查任期
    if (response.getTerm() != r.options.getTerm()) {
        r.resetInflights();
        r.state = State.Probe;
        LOG.error("Fail, response term {} dismatch, expect term {}", response.getTerm(), r.options.getTerm());
        id.unlock();
        return false;
    }
    if (rpcSendTime > r.lastRpcSendTimestamp) {
        r.lastRpcSendTimestamp = rpcSendTime;
    }
    // 本次提交的日志数量
    final int entriesSize = request.getEntriesCount();
    if (entriesSize > 0) {
        // 节点确认提交
        r.options.getBallotBox().commitAt(r.nextIndex, r.nextIndex + entriesSize - 1, r.options.getPeerId());
        if (LOG.isDebugEnabled()) {
            LOG.debug("Replicated logs in [{}, {}] to peer {}", r.nextIndex, r.nextIndex + entriesSize - 1,
                r.options.getPeerId());
        }
    } else {
        // The request is probe request, change the state into Replicate.
        r.state = State.Replicate;
    }
    r.nextIndex += entriesSize;
    r.hasSucceeded = true;
    r.notifyOnCaughtUp(RaftError.SUCCESS.getNumber(), false);
    // dummy_id is unlock in _send_entries
    if (r.timeoutNowIndex > 0 && r.timeoutNowIndex < r.nextIndex) {
        r.sendTimeoutNow(false, false);
    }
    return true;
}

onAppendEntriesReturned方法也非常的长，但是我们要有点耐心往下看

  1. 校验数据序列有没有错
  2. 进行度量和拼接日志操作
  3. 判断一下返回的状态如果不是正常的，那么就通知监听器，进行重置操作并阻塞一定时间后再发送
  4. 如果返回Success状态为false，那么校验一下任期，因为Leader 的切换，表明可能出现过一次网络分区，需要重新跟随新的 Leader；如果任期没有问题那么就进行重置操作,并根据Follower返回的最新的index来重新设值nextIndex
  5. 如果各种校验都没有问题的话，那么进行日志提交确认，更新最新的日志提交位置索引

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