1.node概述

Any time that you start an instance of Elasticsearch, you are starting a node. A collection of connected nodes is called a cluster. If you are running a single node of Elasticsearch, then you have a cluster of one node.

Every node in the cluster can handle HTTP and Transport traffic by default. The transport layer is used exclusively for communication between nodes and the Java TransportClient
 
; the HTTP layer is used only by external REST clients.

All nodes know about all the other nodes in the cluster and can forward client requests to the appropriate node. Besides that, each node serves one or more purpose:

Master-eligible node
A node that has node.master set to true (default), which makes it eligible to be elected as the master node, which controls the cluster.
Data node
 

A node that has node.data set to true (default). Data nodes hold data and perform data related operations such as CRUD, search, and aggregations.
Ingest node
A node that has node.ingest set to true (default). Ingest nodes are able to apply an ingest pipeline to a document in order to transform and enrich the document before indexing. With a heavy ingest load
 
, it makes sense to use dedicated ingest nodes and to mark the master and data nodes as node.ingest: false.
Tribe node
A tribe node, configured via the tribe.* settings, is a special type of coordinating only node that can connect to multiple clusters and perform search and other operations across all connected clusters.
By default a node is a master-eligible node and a data node, plus it can pre-process documents through ingest pipelines. This is very convenient for small clusters but, as the cluster grows, it becomes important to consider separating dedicated master-eligible nodes from dedicated data nodes.

2.配置Node類

/**
 * A node represent a node within a cluster ({@code cluster.name}). The {@link #client()} can be used
 * in order to use a {@link Client} to perform actions/operations against the cluster.
 */

    public static final Setting<Boolean> WRITE_PORTS_FILE_SETTING =
        Setting.boolSetting("node.portsfile", false, Property.NodeScope);
    public static final Setting<Boolean> NODE_DATA_SETTING = Setting.boolSetting("node.data", true, Property.NodeScope);
    public static final Setting<Boolean> NODE_MASTER_SETTING =
        Setting.boolSetting("node.master", true, Property.NodeScope);
    public static final Setting<Boolean> NODE_INGEST_SETTING =
        Setting.boolSetting("node.ingest", true, Property.NodeScope);

3.node通訊 :NodeClient.java

    private <    Request extends ActionRequest,
                Response extends ActionResponse
            > TransportAction<Request, Response> transportAction(Action<Response> action) {
        if (actions == null) {
            throw new IllegalStateException("NodeClient has not been initialized");
        }
        TransportAction<Request, Response> transportAction = actions.get(action);
        if (transportAction == null) {
            throw new IllegalStateException("failed to find action [" + action + "] to execute");
        }
        return transportAction;
    }

4.TransportAction.java（node之間通訊，走tcp）

    /**
     * Execute the transport action on the local node, returning the {@link Task} used to track its execution and accepting a
     * {@link TaskListener} which listens for the completion of the action.
     */
    public final Task execute(Request request, TaskListener<Response> listener) {
        Task task = taskManager.register("transport", actionName, request);
        execute(task, request, new ActionListener<Response>() {
            @Override
            public void onResponse(Response response) {
                if (task != null) {
                    taskManager.unregister(task);
                }
                listener.onResponse(task, response);
            }

            @Override
            public void onFailure(Exception e) {
                if (task != null) {
                    taskManager.unregister(task);
                }
                listener.onFailure(task, e);
            }
        });
        return task;
    }