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本文主要研究一下 elasticsearch 的 ZenPing

elasticsearch-7.0.0/server/src/main/java/org/elasticsearch/discovery/zen/ZenPing.java

public interface ZenPing extends Releasable {void start();

    void ping(Consumer<PingCollection> resultsConsumer, TimeValue timeout);

    class PingResponse implements Writeable {//......}

    class PingCollection {//......}
}

ZenPing 接口继承了 Releasable 接口，另外它还定义了 start、ping 方法；除此之外还定义了 PingResponse、PingCollection 这两个类

elasticsearch-7.0.0/server/src/main/java/org/elasticsearch/discovery/zen/ZenPing.java

    class PingResponse implements Writeable {

        /**
         * An ID of a ping response that was generated on behalf of another node. Needs to be less than all other ping IDs so that fake ping
         * responses don't override real ones.
         */
        public static long FAKE_PING_ID = -1;

        private static final AtomicLong idGenerator = new AtomicLong();

        // an always increasing unique identifier for this ping response.
        // lower values means older pings.
        private final long id;

        private final ClusterName clusterName;

        private final DiscoveryNode node;

        private final DiscoveryNode master;

        private final long clusterStateVersion;

        /**
         * @param node                the node which this ping describes
         * @param master              the current master of the node
         * @param clusterName         the cluster name of the node
         * @param clusterStateVersion the current cluster state version of that node
         *                            ({@link ElectMasterService.MasterCandidate#UNRECOVERED_CLUSTER_VERSION} for not recovered)
         */
        public PingResponse(DiscoveryNode node, DiscoveryNode master, ClusterName clusterName, long clusterStateVersion) {this(idGenerator.incrementAndGet(), node, master, clusterName, clusterStateVersion);
        }

        /**
         * @param id                  the ping's ID
         * @param node                the node which this ping describes
         * @param master              the current master of the node
         * @param clusterName         the cluster name of the node
         * @param clusterStateVersion the current cluster state version of that node
*                            ({@link ElectMasterService.MasterCandidate#UNRECOVERED_CLUSTER_VERSION} for not recovered)
         */
        public PingResponse(long id, DiscoveryNode node, DiscoveryNode master, ClusterName clusterName, long clusterStateVersion) {
            this.id = id;
            this.node = node;
            this.master = master;
            this.clusterName = clusterName;
            this.clusterStateVersion = clusterStateVersion;
        }

        public PingResponse(DiscoveryNode node, DiscoveryNode master, ClusterState state) {this(node, master, state.getClusterName(),
                state.blocks().hasGlobalBlock(STATE_NOT_RECOVERED_BLOCK) ?
                    ElectMasterService.MasterCandidate.UNRECOVERED_CLUSTER_VERSION : state.version());
        }

        PingResponse(StreamInput in) throws IOException {this.clusterName = new ClusterName(in);
            this.node = new DiscoveryNode(in);
            this.master = in.readOptionalWriteable(DiscoveryNode::new);
            this.clusterStateVersion = in.readLong();
            this.id = in.readLong();}

        @Override
        public void writeTo(StreamOutput out) throws IOException {clusterName.writeTo(out);
            node.writeTo(out);
            out.writeOptionalWriteable(master);
            out.writeLong(clusterStateVersion);
            out.writeLong(id);
        }

        /**
         * an always increasing unique identifier for this ping response.
         * lower values means older pings.
         */
        public long id() {return this.id;}

        /**
         * the name of the cluster this node belongs to
         */
        public ClusterName clusterName() {return this.clusterName;}

        /** the node which this ping describes */
        public DiscoveryNode node() {return node;}

        /** the current master of the node */
        public DiscoveryNode master() {return master;}

        /**
         * the current cluster state version of that node ({@link ElectMasterService.MasterCandidate#UNRECOVERED_CLUSTER_VERSION}
         * for not recovered) */
        public long getClusterStateVersion() {return clusterStateVersion;}

        @Override
        public String toString() {return "ping_response{node [" + node + "], id[" + id + "], master [" + master + "]," +
                   "cluster_state_version [" + clusterStateVersion + "], cluster_name[" + clusterName.value() + "]}";
        }
    }

PingResponse 实现了 Writeable 接口，其 writeTo 方法会依次写入 clusterName、node、master、clusterStateVersion、id

elasticsearch-7.0.0/server/src/main/java/org/elasticsearch/discovery/zen/ZenPing.java

    class PingCollection {

        Map<DiscoveryNode, PingResponse> pings;

        public PingCollection() {pings = new HashMap<>();
        }

        /**
         * adds a ping if newer than previous pings from the same node
         *
         * @return true if added, false o.w.
         */
        public synchronized boolean addPing(PingResponse ping) {PingResponse existingResponse = pings.get(ping.node());
            // in case both existing and new ping have the same id (probably because they come
            // from nodes from version <1.4.0) we prefer to use the last added one.
            if (existingResponse == null || existingResponse.id() <= ping.id()) {pings.put(ping.node(), ping);
                return true;
            }
            return false;
        }

        /** serialize current pings to a list. It is guaranteed that the list contains one ping response per node */
        public synchronized List<PingResponse> toList() {return new ArrayList<>(pings.values());
        }

        /** the number of nodes for which there are known pings */
        public synchronized int size() {return pings.size();
        }
    }

PingCollection 定义了 DiscoveryNode 与 PingResponse 的 map，并提供了 addPing 方法来添加 pingResponse