ES选主策略

ES版本5.6.3

1、整个流程的开始，实在node启动后触发的，Node.java中start()方法，通过调用ZenDiscovery.java中的doStart()方法，之后会调用startInitialJoin方法开始进行加入现有的cluster或者选主。

public void startInitialJoin() {
        // start the join thread from a cluster state update. See {@link JoinThreadControl} for details.
        clusterService.submitStateUpdateTask("initial_join", new LocalClusterUpdateTask() {

            @Override
            public ClusterTasksResult<LocalClusterUpdateTask> execute(ClusterState currentState) throws Exception {
                // do the join on a different thread, the DiscoveryService waits for 30s anyhow till it is discovered
                joinThreadControl.startNewThreadIfNotRunning();
                return unchanged();
            }

            @Override
            public void onFailure(String source, @org.elasticsearch.common.Nullable Exception e) {
                logger.warn("failed to start initial join process", e);
            }
        });
 }

2、ZenDiscovery类中startNewThreadIfNotRunning方法中innerJoinCluster()为实质性进行选主操作，其中findMaster()选择master节点。 

private void innerJoinCluster() {
        DiscoveryNode masterNode = null;
        final Thread currentThread = Thread.currentThread();
        nodeJoinController.startElectionContext();
        while (masterNode == null && joinThreadControl.joinThreadActive(currentThread)) {
            masterNode = findMaster();
        }

       ......
    }

3、在findMaster()中。通过pingAndWait()方法获取当前可以ping通的节点，并获取PingResponse，此信息中包含节点信息以及该节点当前的master节点信息。之后，根据获取的节点开始进行选主。

　　此处有一参数需要注意：discovery.zen.master_election.ignore_non_master_pings，默认值为false，表明数据节点（node.master: false    node.data: true）是否参与选主，一般我们集群节点数较少时，不用修改此配置，如果集群规模很大，可以考虑只允许主节点参与选主操作。

　　然后，根据获取的pingResponses来判断当前是否有master节点存在，存储在activeMasters中，对于master的候选节点存储在masterCandidates中。

　　如果activeMasters为空，表明当前并未有master节点存在，则进行选主操作，即步骤4。这里需要注意的是discovery.zen.minimum_master_nodes，候选节点数必选大于等于该参数，选主才能继续，否则是无法选主的。该参数一般配置为(N/2)+1，防止集群出现脑裂。

　　如果activeMasters不为空，则当前的master节点，即为步骤5中的找到的master节点。

 private DiscoveryNode findMaster() {
        logger.trace("starting to ping");
        List<ZenPing.PingResponse> fullPingResponses = pingAndWait(pingTimeout).toList();
        if (fullPingResponses == null) {
            logger.trace("No full ping responses");
            return null;
        }
        if (logger.isTraceEnabled()) {
            StringBuilder sb = new StringBuilder();
            if (fullPingResponses.size() == 0) {
                sb.append(" {none}");
            } else {
                for (ZenPing.PingResponse pingResponse : fullPingResponses) {
                    sb.append("\n\t--> ").append(pingResponse);
                }
            }
            logger.trace("full ping responses:{}", sb);
        }

        final DiscoveryNode localNode = clusterService.localNode();

        // add our selves
        assert fullPingResponses.stream().map(ZenPing.PingResponse::node)
            .filter(n -> n.equals(localNode)).findAny().isPresent() == false;

        fullPingResponses.add(new ZenPing.PingResponse(localNode, null, clusterService.state()));

        // filter responses
        final List<ZenPing.PingResponse> pingResponses = filterPingResponses(fullPingResponses, masterElectionIgnoreNonMasters, logger);

        List<DiscoveryNode> activeMasters = new ArrayList<>();
        for (ZenPing.PingResponse pingResponse : pingResponses) {
            // We can't include the local node in pingMasters list, otherwise we may up electing ourselves without
            // any check / verifications from other nodes in ZenDiscover#innerJoinCluster()
            if (pingResponse.master() != null && !localNode.equals(pingResponse.master())) {
                activeMasters.add(pingResponse.master());
            }
        }

        // nodes discovered during pinging
        List<ElectMasterService.MasterCandidate> masterCandidates = new ArrayList<>();
        for (ZenPing.PingResponse pingResponse : pingResponses) {
            if (pingResponse.node().isMasterNode()) {
                masterCandidates.add(new ElectMasterService.MasterCandidate(pingResponse.node(), pingResponse.getClusterStateVersion()));
            }
        }

        if (activeMasters.isEmpty()) {
            if (electMaster.hasEnoughCandidates(masterCandidates)) {
                final ElectMasterService.MasterCandidate winner = electMaster.electMaster(masterCandidates);
                logger.trace("candidate {} won election", winner);
                return winner.getNode();
            } else {
                // if we don't have enough master nodes, we bail, because there are not enough master to elect from
                logger.warn("not enough master nodes discovered during pinging (found [{}], but needed [{}]), pinging again",
                            masterCandidates, electMaster.minimumMasterNodes());
                return null;
            }
        } else {
            assert !activeMasters.contains(localNode) : "local node should never be elected as master when other nodes indicate an active master";
            // lets tie break between discovered nodes
            return electMaster.tieBreakActiveMasters(activeMasters);
        }
    }

4、ElectMasterService.java中的electMaster()方法为选主的具体实现，逻辑十分简单根据当前的候选节点进行排序，排在第一个的即为master节点。

public MasterCandidate electMaster(Collection<MasterCandidate> candidates) {
        assert hasEnoughCandidates(candidates);
        List<MasterCandidate> sortedCandidates = new ArrayList<>(candidates);
        sortedCandidates.sort(MasterCandidate::compare);
        return sortedCandidates.get(0);
    }

 5、当存在master节点时，则加入现有的集群中，如果是多个master节点，则会选择排在第一个的master节点作为需要加入的集群。

public DiscoveryNode tieBreakActiveMasters(Collection<DiscoveryNode> activeMasters) {
        return activeMasters.stream().min(ElectMasterService::compareNodes).get();
    }

6、在选择完master节点后，需要进行集群组建了。如果当前选择出的master节点为本节点，则本节点需要等待其他节点来加入。这个逻辑不太确定，是异步实现的。大体如下：在每个node启动时，均会注册internal:discovery/zen/join请求，待其为maser后，其他节点通过该请求与之通信加入到master所在集群。在MembershipListener.java中注册监听，当有节点加入时，通过异步函数将信息存储在NodeJoinController下的内部类ElectionContext中，具体判断是否获取了足够的节点，判断当次选举是否成功。--------这块逻辑比较绕，不确定理解是否正确，如哪位大神比较了解，望不吝赐教啊！！！！

7、如果选择的master节点不是本节点，则选择加入该集群ZenDiscovery.java中joinElectedMaster()，尝试次数为discovery.zen.join_retry_attempts由控制，默认为3次，每次的超时时间：discovery.zen.join_timeout控制，默认值为discovery.zen.ping_timeout*20也就是60ms。所以这个参数不宜配置过长，否则在选举失败的超时时间就会比较长。如果加入master失败或者超时，则会进行新的一轮选主，直到选则出满足条件的master节点。

private boolean joinElectedMaster(DiscoveryNode masterNode) {
        try {
            // first, make sure we can connect to the master
            transportService.connectToNode(masterNode);
        } catch (Exception e) {
            logger.warn((Supplier<?>) () -> new ParameterizedMessage("failed to connect to master [{}], retrying...", masterNode), e);
            return false;
        }
        int joinAttempt = 0; // we retry on illegal state if the master is not yet ready
        while (true) {
            try {
                logger.trace("joining master {}", masterNode);
                membership.sendJoinRequestBlocking(masterNode, clusterService.localNode(), joinTimeout);
                return true;
            } catch (Exception e) {
                final Throwable unwrap = ExceptionsHelper.unwrapCause(e);
                if (unwrap instanceof NotMasterException) {
                    if (++joinAttempt == this.joinRetryAttempts) {
                        logger.info("failed to send join request to master [{}], reason [{}], tried [{}] times", masterNode, ExceptionsHelper.detailedMessage(e), joinAttempt);
                        return false;
                    } else {
                        logger.trace("master {} failed with [{}]. retrying... (attempts done: [{}])", masterNode, ExceptionsHelper.detailedMessage(e), joinAttempt);
                    }
                } else {
                    if (logger.isTraceEnabled()) {
                        logger.trace((Supplier<?>) () -> new ParameterizedMessage("failed to send join request to master [{}]", masterNode), e);
                    } else {
                        logger.info("failed to send join request to master [{}], reason [{}]", masterNode, ExceptionsHelper.detailedMessage(e));
                    }
                    return false;
                }
            }

            try {
                Thread.sleep(this.joinRetryDelay.millis());
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }
    }

至此，master就已经选择完成了。大概逻辑就是这样，可能中间一些细节有待进一步深究。