序
本文主要研究一下elasticsearch的TransportProxyClient
TransportProxyClient
elasticsearch-6.4.3-sources.jar!/org/elasticsearch/client/transport/TransportProxyClient.java
final class TransportProxyClient {
private final TransportClientNodesService nodesService;
private final Map proxies;
TransportProxyClient(Settings settings, TransportService transportService,
TransportClientNodesService nodesService, List actions) {
this.nodesService = nodesService;
Map proxies = new HashMap<>();
for (GenericAction action : actions) {
if (action instanceof Action) {
proxies.put((Action) action, new TransportActionNodeProxy(settings, action, transportService));
}
}
this.proxies = unmodifiableMap(proxies);
}
public > void execute(final Action action,
final Request request, ActionListener listener) {
final TransportActionNodeProxy proxy = proxies.get(action);
assert proxy != null : "no proxy found for action: " + action;
nodesService.execute((n, l) -> proxy.execute(n, request, l), listener);
}
}
- TransportProxyClient的构造器接收Settings、TransportService、TransportClientNodesService、List
四个参数 - TransportProxyClient的构造器会根据actions来给每个action创建TransportActionNodeProxy，并放入到名为proxies的map中
- TransportProxyClient主要是提供了execute方法，该方法从proxies取出对应的TransportActionNodeProxy，然后通过TransportClientNodesService的execute方法来执行proxy.execute方法
TransportActionNodeProxy
elasticsearch-6.4.3-sources.jar!/org/elasticsearch/action/TransportActionNodeProxy.java
public class TransportActionNodeProxy extends AbstractComponent {
private final TransportService transportService;
private final GenericAction action;
private final TransportRequestOptions transportOptions;
public TransportActionNodeProxy(Settings settings, GenericAction action, TransportService transportService) {
super(settings);
this.action = action;
this.transportService = transportService;
this.transportOptions = action.transportOptions(settings);
}
public void execute(final DiscoveryNode node, final Request request, final ActionListener listener) {
ActionRequestValidationException validationException = request.validate();
if (validationException != null) {
listener.onFailure(validationException);
return;
}
transportService.sendRequest(node, action.name(), request, transportOptions,
new ActionListenerResponseHandler<>(listener, action::newResponse));
}
}
- TransportActionNodeProxy的构造器要求输入Settings、GenericAction、TransportService三个参数；TransportActionNodeProxy提供了execute方法，它的方法参数要求输入DiscoveryNode、Request、ActionListener，该方法主要是对ActionListener包装为ActionListenerResponseHandler，然后调用transportService.sendRequest
Nodes
TransportClientNodesService Nodes
elasticsearch-6.4.3-sources.jar!/org/elasticsearch/client/transport/TransportClientNodesService.java
final class TransportClientNodesService extends AbstractComponent implements Closeable {
private final TimeValue nodesSamplerInterval;
private final long pingTimeout;
private final ClusterName clusterName;
private final TransportService transportService;
private final ThreadPool threadPool;
private final Version minCompatibilityVersion;
// nodes that are added to be discovered
private volatile List listedNodes = Collections.emptyList();
private final Object mutex = new Object();
private volatile List nodes = Collections.emptyList();
// Filtered nodes are nodes whose cluster name does not match the configured cluster name
private volatile List filteredNodes = Collections.emptyList();
private final AtomicInteger tempNodeIdGenerator = new AtomicInteger();
private final NodeSampler nodesSampler;
private volatile ScheduledFuture nodesSamplerFuture;
private final AtomicInteger randomNodeGenerator = new AtomicInteger(Randomness.get().nextInt());
private final boolean ignoreClusterName;
private volatile boolean closed;
private final TransportClient.HostFailureListener hostFailureListener;
//......
public TransportClientNodesService addTransportAddresses(TransportAddress... transportAddresses) {
synchronized (mutex) {
if (closed) {
throw new IllegalStateException("transport client is closed, can't add an address");
}
List filtered = new ArrayList<>(transportAddresses.length);
for (TransportAddress transportAddress : transportAddresses) {
boolean found = false;
for (DiscoveryNode otherNode : listedNodes) {
if (otherNode.getAddress().equals(transportAddress)) {
found = true;
logger.debug("address [{}] already exists with [{}], ignoring...", transportAddress, otherNode);
break;
}
}
if (!found) {
filtered.add(transportAddress);
}
}
if (filtered.isEmpty()) {
return this;
}
List builder = new ArrayList<>(listedNodes);
for (TransportAddress transportAddress : filtered) {
DiscoveryNode node = new DiscoveryNode("#transport#-" + tempNodeIdGenerator.incrementAndGet(),
transportAddress, Collections.emptyMap(), Collections.emptySet(), minCompatibilityVersion);
logger.debug("adding address [{}]", node);
builder.add(node);
}
listedNodes = Collections.unmodifiableList(builder);
nodesSampler.sample();
}
return this;
}
public TransportClientNodesService removeTransportAddress(TransportAddress transportAddress) {
synchronized (mutex) {
if (closed) {
throw new IllegalStateException("transport client is closed, can't remove an address");
}
List listNodesBuilder = new ArrayList<>();
for (DiscoveryNode otherNode : listedNodes) {
if (!otherNode.getAddress().equals(transportAddress)) {
listNodesBuilder.add(otherNode);
} else {
logger.debug("removing address [{}] from listed nodes", otherNode);
}
}
listedNodes = Collections.unmodifiableList(listNodesBuilder);
List nodesBuilder = new ArrayList<>();
for (DiscoveryNode otherNode : nodes) {
if (!otherNode.getAddress().equals(transportAddress)) {
nodesBuilder.add(otherNode);
} else {
logger.debug("disconnecting from node with address [{}]", otherNode);
transportService.disconnectFromNode(otherNode);
}
}
nodes = Collections.unmodifiableList(nodesBuilder);
nodesSampler.sample();
}
return this;
}
//......
}
- TransportClientNodesService定义了三个关于DiscoveryNode的List属性，分别是listedNodes、nodes、filteredNodes
- addTransportAddresses方法会更新listedNodes，然后调用nodesSampler.sample()更新nodes及filteredNodes；removeTransportAddress方法会更新listedNodes，nodes，然后调用nodesSampler.sample()更新nodes及filteredNodes
- listedNodes即为通过addTransportAddresses方法添加的node(
一般是通过配置文件指定的clusterNodes)；nodesSampler.sample()方法会对listedNodes进行进一步检测，比如将clusterName不是当前配置的clusterName的放到filteredNodes，剩下的再进行连接的建立，成功的放到nodes里头
TransportClient Nodes
elasticsearch-6.4.3-sources.jar!/org/elasticsearch/client/transport/TransportClient.java
public abstract class TransportClient extends AbstractClient {
private final TransportClientNodesService nodesService;
private final TransportProxyClient proxy;
//......
/**
* Returns the current connected transport nodes that this client will use.
*
* The nodes include all the nodes that are currently alive based on the transport
* addresses provided.
*/
public List connectedNodes() {
return nodesService.connectedNodes();
}
/**
* The list of filtered nodes that were not connected to, for example, due to
* mismatch in cluster name.
*/
public List filteredNodes() {
return nodesService.filteredNodes();
}
/**
* Returns the listed nodes in the transport client (ones added to it).
*/
public List listedNodes() {
return nodesService.listedNodes();
}
/**
* Adds a transport address that will be used to connect to.
*
* The Node this transport address represents will be used if its possible to connect to it.
* If it is unavailable, it will be automatically connected to once it is up.
*
* In order to get the list of all the current connected nodes, please see {@link #connectedNodes()}.
*/
public TransportClient addTransportAddress(TransportAddress transportAddress) {
nodesService.addTransportAddresses(transportAddress);
return this;
}
/**
* Adds a list of transport addresses that will be used to connect to.
*
* The Node this transport address represents will be used if its possible to connect to it.
* If it is unavailable, it will be automatically connected to once it is up.
*
* In order to get the list of all the current connected nodes, please see {@link #connectedNodes()}.
*/
public TransportClient addTransportAddresses(TransportAddress... transportAddress) {
nodesService.addTransportAddresses(transportAddress);
return this;
}
/**
* Removes a transport address from the list of transport addresses that are used to connect to.
*/
public TransportClient removeTransportAddress(TransportAddress transportAddress) {
nodesService.removeTransportAddress(transportAddress);
return this;
}
//......
}
- TransportClient提供了connectedNodes、filteredNodes、listedNodes方法，可以看到它们内部都是调用的TransportClientNodesService对应的方法；从注释上可以看到，connectedNodes返回的是当前已经建立连接的nodes，供client端使用；filteredNodes返回的是因为clusterName不匹配导致被过滤掉的nodes，这些nodes不会被client使用；listedNodes返回的是通过addTransportAddresses添加的nodes
NodeSampler
ScheduledNodeSampler
elasticsearch-6.4.3-sources.jar!/org/elasticsearch/client/transport/TransportClientNodesService.java
TransportClientNodesService(Settings settings, TransportService transportService,
ThreadPool threadPool, TransportClient.HostFailureListener hostFailureListener) {
super(settings);
this.clusterName = ClusterName.CLUSTER_NAME_SETTING.get(settings);
this.transportService = transportService;
this.threadPool = threadPool;
this.minCompatibilityVersion = Version.CURRENT.minimumCompatibilityVersion();
this.nodesSamplerInterval = TransportClient.CLIENT_TRANSPORT_NODES_SAMPLER_INTERVAL.get(this.settings);
this.pingTimeout = TransportClient.CLIENT_TRANSPORT_PING_TIMEOUT.get(this.settings).millis();
this.ignoreClusterName = TransportClient.CLIENT_TRANSPORT_IGNORE_CLUSTER_NAME.get(this.settings);
if (logger.isDebugEnabled()) {
logger.debug("node_sampler_interval[{}]", nodesSamplerInterval);
}
if (TransportClient.CLIENT_TRANSPORT_SNIFF.get(this.settings)) {
this.nodesSampler = new SniffNodesSampler();
} else {
this.nodesSampler = new SimpleNodeSampler();
}
this.hostFailureListener = hostFailureListener;
this.nodesSamplerFuture = threadPool.schedule(nodesSamplerInterval, ThreadPool.Names.GENERIC, new ScheduledNodeSampler());
}
//......
class ScheduledNodeSampler implements Runnable {
@Override
public void run() {
try {
nodesSampler.sample();
if (!closed) {
nodesSamplerFuture = threadPool.schedule(nodesSamplerInterval, ThreadPool.Names.GENERIC, this);
}
} catch (Exception e) {
logger.warn("failed to sample", e);
}
}
}
//......
- TransportClientNodesService的构造器里头会根据settings的client.transport.sniff配置(
默认是false)来判断是创建SniffNodesSampler还是SimpleNodeSampler，通过threadPool注册一个调度任务，每隔nodesSamplerInterval执行ScheduledNodeSampler；ScheduledNodeSampler实现了Runnable接口，其fun方法主要是调用nodesSampler.sample()，之后只要TransportClientNodesService没有close，则会继续注册调度任务，并更新nodesSamplerFuture
NodeSampler
elasticsearch-6.4.3-sources.jar!/org/elasticsearch/client/transport/TransportClientNodesService.java
abstract class NodeSampler {
public void sample() {
synchronized (mutex) {
if (closed) {
return;
}
doSample();
}
}
protected abstract void doSample();
/**
* Establishes the node connections. If validateInHandshake is set to true, the connection will fail if
* node returned in the handshake response is different than the discovery node.
*/
List establishNodeConnections(Set nodes) {
for (Iterator it = nodes.iterator(); it.hasNext(); ) {
DiscoveryNode node = it.next();
if (!transportService.nodeConnected(node)) {
try {
logger.trace("connecting to node [{}]", node);
transportService.connectToNode(node);
} catch (Exception e) {
it.remove();
logger.debug(() -> new ParameterizedMessage("failed to connect to discovered node [{}]", node), e);
}
}
}
return Collections.unmodifiableList(new ArrayList<>(nodes));
}
}
- NodeSampler是个抽象类，它定义了sample方法，其内部是调用定义的抽象方法doSample；NodeSampler还提供了establishNodeConnections方法，它通过transportService.nodeConnected(node)来判断node是否是connected的，如果不是则会通过transportService.connectToNode(node)再尝试连接一次，如果抛异常则将该节点移除掉，最后返回这次检测是connected的nodes；它有两个子类，分别是SimpleNodeSampler、SniffNodesSampler
SimpleNodeSampler
elasticsearch-6.4.3-sources.jar!/org/elasticsearch/client/transport/TransportClientNodesService.java
class SimpleNodeSampler extends NodeSampler {
@Override
protected void doSample() {
HashSet newNodes = new HashSet<>();
ArrayList newFilteredNodes = new ArrayList<>();
for (DiscoveryNode listedNode : listedNodes) {
try (Transport.Connection connection = transportService.openConnection(listedNode, LISTED_NODES_PROFILE)){
final PlainTransportFuture handler = new PlainTransportFuture<>(
new FutureTransportResponseHandler() {
@Override
public LivenessResponse read(StreamInput in) throws IOException {
LivenessResponse response = new LivenessResponse();
response.readFrom(in);
return response;
}
});
transportService.sendRequest(connection, TransportLivenessAction.NAME, new LivenessRequest(),
TransportRequestOptions.builder().withType(TransportRequestOptions.Type.STATE).withTimeout(pingTimeout).build(),
handler);
final LivenessResponse livenessResponse = handler.txGet();
if (!ignoreClusterName && !clusterName.equals(livenessResponse.getClusterName())) {
logger.warn("node {} not part of the cluster {}, ignoring...", listedNode, clusterName);
newFilteredNodes.add(listedNode);
} else {
// use discovered information but do keep the original transport address,
// so people can control which address is exactly used.
DiscoveryNode nodeWithInfo = livenessResponse.getDiscoveryNode();
newNodes.add(new DiscoveryNode(nodeWithInfo.getName(), nodeWithInfo.getId(), nodeWithInfo.getEphemeralId(),
nodeWithInfo.getHostName(), nodeWithInfo.getHostAddress(), listedNode.getAddress(),
nodeWithInfo.getAttributes(), nodeWithInfo.getRoles(), nodeWithInfo.getVersion()));
}
} catch (ConnectTransportException e) {
logger.debug(() -> new ParameterizedMessage("failed to connect to node [{}], ignoring...", listedNode), e);
hostFailureListener.onNodeDisconnected(listedNode, e);
} catch (Exception e) {
logger.info(() -> new ParameterizedMessage("failed to get node info for {}, disconnecting...", listedNode), e);
}
}
nodes = establishNodeConnections(newNodes);
filteredNodes = Collections.unmodifiableList(newFilteredNodes);
}
}
- SimpleNodeSampler的doSample方法会对nodes进行更进一步的存活检测，主要是发送LivenessRequest，如果能成功返回LivenessResponse，则判断clusterName是否一致，不一致的添加到newFilteredNodes，最后赋值给filteredNodes；一致的添加到newNodes中，最后通过establishNodeConnections方法建立连接并移除连接失败的node(
重试一次)最后赋值给nodes
SniffNodesSampler
elasticsearch-6.4.3-sources.jar!/org/elasticsearch/client/transport/TransportClientNodesService.java
class SniffNodesSampler extends NodeSampler {
@Override
protected void doSample() {
// the nodes we are going to ping include the core listed nodes that were added
// and the last round of discovered nodes
Set nodesToPing = new HashSet<>();
for (DiscoveryNode node : listedNodes) {
nodesToPing.add(node);
}
for (DiscoveryNode node : nodes) {
nodesToPing.add(node);
}
final CountDownLatch latch = new CountDownLatch(nodesToPing.size());
final ConcurrentMap clusterStateResponses = ConcurrentCollections.newConcurrentMap();
try {
for (final DiscoveryNode nodeToPing : nodesToPing) {
threadPool.executor(ThreadPool.Names.MANAGEMENT).execute(new AbstractRunnable() {
/**
* we try to reuse existing connections but if needed we will open a temporary connection
* that will be closed at the end of the execution.
*/
Transport.Connection connectionToClose = null;
void onDone() {
try {
IOUtils.closeWhileHandlingException(connectionToClose);
} finally {
latch.countDown();
}
}
@Override
public void onFailure(Exception e) {
onDone();
if (e instanceof ConnectTransportException) {
logger.debug(() -> new ParameterizedMessage("failed to connect to node [{}], ignoring...", nodeToPing), e);
hostFailureListener.onNodeDisconnected(nodeToPing, e);
} else {
logger.info(() -> new ParameterizedMessage(
"failed to get local cluster state info for {}, disconnecting...", nodeToPing), e);
}
}
@Override
protected void doRun() throws Exception {
Transport.Connection pingConnection = null;
if (nodes.contains(nodeToPing)) {
try {
pingConnection = transportService.getConnection(nodeToPing);
} catch (NodeNotConnectedException e) {
// will use a temp connection
}
}
if (pingConnection == null) {
logger.trace("connecting to cluster node [{}]", nodeToPing);
connectionToClose = transportService.openConnection(nodeToPing, LISTED_NODES_PROFILE);
pingConnection = connectionToClose;
}
transportService.sendRequest(pingConnection, ClusterStateAction.NAME,
Requests.clusterStateRequest().clear().nodes(true).local(true),
TransportRequestOptions.builder().withType(TransportRequestOptions.Type.STATE)
.withTimeout(pingTimeout).build(),
new TransportResponseHandler() {
@Override
public ClusterStateResponse newInstance() {
return new ClusterStateResponse();
}
@Override
public String executor() {
return ThreadPool.Names.SAME;
}
@Override
public void handleResponse(ClusterStateResponse response) {
clusterStateResponses.put(nodeToPing, response);
onDone();
}
@Override
public void handleException(TransportException e) {
logger.info(() -> new ParameterizedMessage(
"failed to get local cluster state for {}, disconnecting...", nodeToPing), e);
try {
hostFailureListener.onNodeDisconnected(nodeToPing, e);
} finally {
onDone();
}
}
});
}
});
}
latch.await();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
return;
}
HashSet newNodes = new HashSet<>();
HashSet newFilteredNodes = new HashSet<>();
for (Map.Entry entry : clusterStateResponses.entrySet()) {
if (!ignoreClusterName && !clusterName.equals(entry.getValue().getClusterName())) {
logger.warn("node {} not part of the cluster {}, ignoring...",
entry.getValue().getState().nodes().getLocalNode(), clusterName);
newFilteredNodes.add(entry.getKey());
continue;
}
for (ObjectCursor cursor : entry.getValue().getState().nodes().getDataNodes().values()) {
newNodes.add(cursor.value);
}
}
nodes = establishNodeConnections(newNodes);
filteredNodes = Collections.unmodifiableList(new ArrayList<>(newFilteredNodes));
}
}
- SniffNodesSampler的doSample方法首先将listedNodes及nodes合并为名为nodesToPing的Set，之后就挨个将nodesToPing的node放入到线程池异步执行检测，这里通过CountDownLatch来等待所有节点异步执行完毕
- 异步线程池检测的逻辑是对node发送Requests.clusterStateRequest().clear().nodes(true).local(true)请求，如果成功则返回ClusterStateResponse，并添加到clusterStateResponses这个ConcurrentMap中
- 之后遍历clusterStateResponses这个ConcurrentMap，clusterName不一致的node添加到newFilteredNodes，最后赋值给filteredNodes；clusterName一致的则遍历ClusterStateResponse.getState().nodes().getDataNodes().values()，将这些node添加到newNodes，最后通过establishNodeConnections方法建立连接并移除连接失败的node(
重试一次)最后赋值给nodes
TransportClientNodesService.execute
elasticsearch-6.4.3-sources.jar!/org/elasticsearch/client/transport/TransportClientNodesService.java
final class TransportClientNodesService extends AbstractComponent implements Closeable {
private final AtomicInteger randomNodeGenerator = new AtomicInteger(Randomness.get().nextInt());
//......
public void execute(NodeListenerCallback callback, ActionListener listener) {
// we first read nodes before checking the closed state; this
// is because otherwise we could be subject to a race where we
// read the state as not being closed, and then the client is
// closed and the nodes list is cleared, and then a
// NoNodeAvailableException is thrown
// it is important that the order of first setting the state of
// closed and then clearing the list of nodes is maintained in
// the close method
final List nodes = this.nodes;
if (closed) {
throw new IllegalStateException("transport client is closed");
}
ensureNodesAreAvailable(nodes);
int index = getNodeNumber();
RetryListener retryListener = new RetryListener<>(callback, listener, nodes, index, hostFailureListener);
DiscoveryNode node = retryListener.getNode(0);
try {
callback.doWithNode(node, retryListener);
} catch (Exception e) {
try {
//this exception can't come from the TransportService as it doesn't throw exception at all
listener.onFailure(e);
} finally {
retryListener.maybeNodeFailed(node, e);
}
}
}
private void ensureNodesAreAvailable(List nodes) {
if (nodes.isEmpty()) {
String message = String.format(Locale.ROOT, "None of the configured nodes are available: %s", this.listedNodes);
throw new NoNodeAvailableException(message);
}
}
private int getNodeNumber() {
int index = randomNodeGenerator.incrementAndGet();
if (index < 0) {
index = 0;
randomNodeGenerator.set(0);
}
return index;
}
public static class RetryListener implements ActionListener {
private final NodeListenerCallback callback;
private final ActionListener listener;
private final List nodes;
private final int index;
private final TransportClient.HostFailureListener hostFailureListener;
private volatile int i;
RetryListener(NodeListenerCallback callback, ActionListener listener,
List nodes, int index, TransportClient.HostFailureListener hostFailureListener) {
this.callback = callback;
this.listener = listener;
this.nodes = nodes;
this.index = index;
this.hostFailureListener = hostFailureListener;
}
@Override
public void onResponse(Response response) {
listener.onResponse(response);
}
@Override
public void onFailure(Exception e) {
Throwable throwable = ExceptionsHelper.unwrapCause(e);
if (throwable instanceof ConnectTransportException) {
maybeNodeFailed(getNode(this.i), (ConnectTransportException) throwable);
int i = ++this.i;
if (i >= nodes.size()) {
listener.onFailure(new NoNodeAvailableException("None of the configured nodes were available: " + nodes, e));
} else {
try {
callback.doWithNode(getNode(i), this);
} catch(final Exception inner) {
inner.addSuppressed(e);
// this exception can't come from the TransportService as it doesn't throw exceptions at all
listener.onFailure(inner);
}
}
} else {
listener.onFailure(e);
}
}
final DiscoveryNode getNode(int i) {
return nodes.get((index + i) % nodes.size());
}
final void maybeNodeFailed(DiscoveryNode node, Exception ex) {
if (ex instanceof NodeDisconnectedException || ex instanceof NodeNotConnectedException) {
hostFailureListener.onNodeDisconnected(node, ex);
}
}
}
//......
}
- TransportClientNodesService提供的execute方法主要是做了两个事情，一个是对nodes节点进行客户端的负载均衡，一个是通过RetryListener对请求增加重试机制
- ensureNodesAreAvailable方法首先确保nodes这个列表不为空，如果为空则抛出NoNodeAvailableException；之后通过getNodeNumber方法来确定index值，该方法使用randomNodeGenerator递增得到index，如果index大于等于0则返回，如果index小于0则重置randomNodeGenerator的值为0并返回0；这里randomNodeGenerator是AtomicInteger类型，其初始值为Randomness.get().nextInt()
- RetryListener的构造器接收上一步计算出来的index值，它有一个i变量，初始为0，在onFailure的时候，如果是ConnectTransportException异常，则会进行重试，重试的时候首先将i递增，之后判断如果i>=nodes大小则停止重试，抛出NoNodeAvailableException，否则继续调用callback.doWithNode进行重试，重试时是通过getNode方法获取node，同时传入当前的listener；getNode方法采取的是(index + i) % nodes.size()来获取node的index，形成Round Robin的效果；对于RetryListener来说，内部重试时i会递增，对于execute方法来说，index值也是递增的，因而无论请求成功还是失败，对nodes的方法都形成Round Robin的效果
小结
- TransportProxyClient主要是提供了execute方法，该方法从proxies取出对应的TransportActionNodeProxy，然后通过TransportClientNodesService的execute方法来执行proxy.execute方法；TransportActionNodeProxy提供了execute方法，它的方法参数要求输入DiscoveryNode、Request、ActionListener，该方法主要是对ActionListener包装为ActionListenerResponseHandler，然后调用transportService.sendRequest
- TransportClientNodesService定义了三个关于DiscoveryNode的List属性，分别是listedNodes、nodes、filteredNodes；其中listedNodes是通过addTransportAddresses添加的nodes；nodes是当前已经建立连接的node列表，供client端使用；filteredNodes是因为clusterName不匹配导致被过滤掉的nodes，这些nodes不会被client使用
- TransportClientNodesService的构造器里头会根据settings的client.transport.sniff配置(
默认是false)来判断是创建SniffNodesSampler还是SimpleNodeSampler，通过threadPool注册一个调度任务，每隔nodesSamplerInterval执行ScheduledNodeSampler；ScheduledNodeSampler实现了Runnable接口，其fun方法主要是调用nodesSampler.sample()，之后只要TransportClientNodesService没有close，则会继续注册调度任务，并更新nodesSamplerFuture - NodeSampler是个抽象类，它定义了sample方法，其内部是调用定义的抽象方法doSample；NodeSampler还提供了establishNodeConnections方法，它通过transportService.nodeConnected(node)来判断node是否是connected的，如果不是则会通过transportService.connectToNode(node)再尝试连接一次，如果抛异常则将该节点移除掉，最后返回这次检测是connected的nodes；它有两个子类，分别是SimpleNodeSampler、SniffNodesSampler
- SimpleNodeSampler的doSample方法会对nodes进行更进一步的存活检测，主要是发送LivenessRequest，如果能成功返回LivenessResponse，则判断clusterName是否一致，不一致的添加到newFilteredNodes，最后赋值给filteredNodes；一致的添加到newNodes中，最后通过establishNodeConnections方法建立连接并移除连接失败的node(
重试一次)最后赋值给nodes - SniffNodesSampler的doSample方法首先将listedNodes及nodes合并为名为nodesToPing的Set，之后就挨个将nodesToPing的node放入到线程池异步执行检测，这里通过CountDownLatch来等待所有节点异步执行完毕；异步线程池检测的逻辑是对node发送Requests.clusterStateRequest().clear().nodes(true).local(true)请求，如果成功则返回ClusterStateResponse，并添加到clusterStateResponses这个ConcurrentMap中；之后遍历clusterStateResponses这个ConcurrentMap，clusterName不一致的node添加到newFilteredNodes，最后赋值给filteredNodes；clusterName一致的则遍历ClusterStateResponse.getState().nodes().getDataNodes().values()，将这些node添加到newNodes，最后通过establishNodeConnections方法建立连接并移除连接失败的node(
重试一次)最后赋值给nodes - TransportClientNodesService提供的execute方法主要是做了两个事情，一个是对nodes节点进行客户端的负载均衡，一个是通过RetryListener对请求增加重试机制；其对nodes的负载均衡策略为Round Robin，而RetryListener只对ConnectTransportException异常进行重试，最大重试次数为nodes.size()-1
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