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1 简介

1.1 概述

为避免单点故障，通常会部署多台服务器。作为服务消费者，在调用时需要考虑：

• ① 选择哪个服务提供者进行调用


• ② 若服务提供者调用失败，将如何操作

ClusterInvoker选择服务提供者的大概过程如下：

• 首先，调用Directory#list()获取Invoker列表（该列表经过路由过滤）：


• 最后，通过LoadBalance#select()获取最终的Invoker

1.2 Cluster和ClusterInvoker

Dubbo定义了两个集群对象：

• Cluster接口：用于创建ClusterInvoker


• ClusterInvoker抽象类：实现Invoker接口

示例代码如下：

public class FailoverCluster implements Cluster {
public final static String NAME = "failover";
@Override
public Invoker join(Directory directory) throws RpcException {
return new FailoverClusterInvoker(directory);
}
}

2 ClusterInvoker分析

2.1 抽象基类

2.1.1 invoke

该方法会执行如下逻辑：

• 绑定attachments到invocation中.


• 从服务目录中获取Invoker列表


• 执行Invoker

@Override
public Result invoke(final Invocation invocation) throws RpcException {
checkWhetherDestroyed();
LoadBalance loadbalance = null;
// 绑定attachments到invocation中.
Map cOntextAttachments= RpcContext.getContext().getAttachments();
if (contextAttachments != null && contextAttachments.size() != 0) {
((RpcInvocation) invocation).addAttachments(contextAttachments);
}
// 从服务目录中获取Invoker列表
List> invokers = list(invocation);
if (invokers != null && !invokers.isEmpty()) {
loadbalance = ExtensionLoader.getExtensionLoader(LoadBalance.class).getExtension(invokers.get(0).getUrl()
.getMethodParameter(RpcUtils.getMethodName(invocation), Constants.LOADBALANCE_KEY, Constants.DEFAULT_LOADBALANCE));
}
RpcUtils.attachInvocationIdIfAsync(getUrl(), invocation);
// 执行Invoker
return doInvoke(invocation, invokers, loadbalance);
}

2.1.2 list

该方法用于从服务目录中获取Invoker列表，代码十分简单，如下：

protected List> list(Invocation invocation) throws RpcException {
List> invokers = directory.list(invocation);
return invokers;
}

2.1.3 select

select()方法主要逻辑：

• 对粘带属性的支持：让服务消费者尽可能的调用同一个服务提供者，除非该提供者挂了再进行切换。


• doSelect()方法：根据负载均衡算法获取一个服务提供者Invoker

// invokers：服务目录中获取的所有Invoker列表
// invoked：在本次调用中，已经执行过的invokers列表
protected Invoker select(LoadBalance loadbalance, Invocation invocation, List> invokers, List> selected) throws RpcException {
if (invokers == null || invokers.isEmpty())
return null;
// 获取调用方法
String methodName = invocation == null ? "" : invocation.getMethodName();
// 获取sticky属性（粘带属性）
boolean sticky = invokers.get(0).getUrl().getMethodParameter(methodName, Constants.CLUSTER_STICKY_KEY, Constants.DEFAULT_CLUSTER_STICKY);
{
// 如果stickyInvoker!=null且invokers列表也不包含stickyInvoker，说明stickyInvoker代表的服务提供者挂了
if (stickyInvoker != null && !invokers.contains(stickyInvoker)) {
stickyInvoker = null;
}
//如果sticky==true且stickyInvoker!=null，此时如果selected包含stickyInvoker
//说明stickyInvoker对应服务提供者可能因为网络问题未能成功提供服务，但该提供者并没有挂（invokers仍包含stickyInvoker）
if (sticky && stickyInvoker != null && (selected == null || !selected.contains(stickyInvoker))) {
// availablecheck是否开启可用性检查，若开启，检测stickyInvoker是否可用
if (availablecheck && stickyInvoker.isAvailable()) {
return stickyInvoker;
}
}
}
// 若代码走到此处，说明stickyInvoker==null，或不可用
Invoker invoker = doSelect(loadbalance, invocation, invokers, selected);
if (sticky) {
stickyInvoker = invoker;
}
return invoker;
}

doSelect方法用于真正去筛选Invoker：

• 利用负载均衡算法，选出一个Invoker


• 判断Invoker是否可用，若不可用则进行重选（只做一次）

private Invoker doSelect(LoadBalance loadbalance, Invocation invocation, List> invokers, List> selected) throws RpcException {
if (invokers == null || invokers.isEmpty())
return null;
if (invokers.size() == 1)
return invokers.get(0);
if (loadbalance == null) {
loadbalance = ExtensionLoader.getExtensionLoader(LoadBalance.class).getExtension(Constants.DEFAULT_LOADBALANCE);
}
Invoker invoker = loadbalance.select(invokers, getUrl(), invocation);
//如果selected包含负载均衡选择出的Invoker，或者该Invoker无法经过可用性检查，此时进行重选
if ((selected != null && selected.contains(invoker))
|| (!invoker.isAvailable() && getUrl() != null && availablecheck)) {
try {
Invoker rinvoker = reselect(loadbalance, invocation, invokers, selected, availablecheck);
if (rinvoker != null) {
invoker = rinvoker;
} else {
//Check the index of current selected invoker, if it&＃39;s not the last one, choose the one at index+1.
int index = invokers.indexOf(invoker);
try {
//Avoid collision
invoker = index } catch (Exception e) { /* 日志 */ }
}
} catch (Throwable t) { /* 日志 */ }
}
return invoker;
}

2.2 FailoverClusterInvoker

十分简单：失败则循环调用。循环次数可配置，默认3次。

public class FailoverClusterInvoker extends AbstractClusterInvoker {
private static final Logger logger = LoggerFactory.getLogger(FailoverClusterInvoker.class);
public FailoverClusterInvoker(Directory directory) {
super(directory);
}
@Override
@SuppressWarnings({"unchecked", "rawtypes"})
public Result doInvoke(Invocation invocation, final List> invokers, LoadBalance loadbalance) throws RpcException {
List> copyinvokers = invokers;
checkInvokers(copyinvokers, invocation);
int len = getUrl().getMethodParameter(invocation.getMethodName(), Constants.RETRIES_KEY, Constants.DEFAULT_RETRIES) + 1;
if (len <= 0) {
len = 1;
}
// retry loop.
RpcException le = null; // last exception.
List> invoked = new ArrayList>(copyinvokers.size()); // invoked invokers.
Set providers = new HashSet(len);
for (int i = 0; i //Reselect before retry to avoid a change of candidate `invokers`.
//NOTE: if `invokers` changed, then `invoked` also lose accuracy.
if (i > 0) {
checkWhetherDestroyed();
copyinvokers = list(invocation);
// check again
checkInvokers(copyinvokers, invocation);
}
Invoker invoker = select(loadbalance, invocation, copyinvokers, invoked);
invoked.add(invoker);
RpcContext.getContext().setInvokers((List) invoked);
try {
Result result = invoker.invoke(invocation);
if (le != null && logger.isWarnEnabled()) {
//
}
return result;
} catch (RpcException e) {
if (e.isBiz()) { // biz exception.
throw e;
}
le = e;
} catch (Throwable e) {
le = new RpcException(e.getMessage(), e);
} finally {
providers.add(invoker.getUrl().getAddress());
}
}
throw new RpcException(...);
}
}

2.3 FailbackClusterInvoker

调用失败，返回空结果给服务提供者，定时任务对失败的调用进行补偿，适合执行消息通知等操作，如下：

public class FailbackClusterInvoker extends AbstractClusterInvoker {
private static final Logger logger = LoggerFactory.getLogger(FailbackClusterInvoker.class);
private static final long RETRY_FAILED_PERIOD = 5 * 1000;
private final ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(2,
new NamedInternalThreadFactory("failback-cluster-timer", true));
private final ConcurrentMap> failed = new ConcurrentHashMap>();
private volatile ScheduledFuture retryFuture;
public FailbackClusterInvoker(Directory directory) {
super(directory);
}
// 放入失败队列Map结构中
private void addFailed(Invocation invocation, AbstractClusterInvoker router) {
if (retryFuture == null) {
synchronized (this) {
if (retryFuture == null) {
retryFuture = scheduledExecutorService.scheduleWithFixedDelay(new Runnable() {
@Override
public void run() {
// collect retry statistics
try {
retryFailed();
} catch (Throwable t) { // Defensive fault tolerance
logger.error("Unexpected error occur at collect statistic", t);
}
}
}, RETRY_FAILED_PERIOD, RETRY_FAILED_PERIOD, TimeUnit.MILLISECONDS);
}
}
}
failed.put(invocation, router);
}
// 失败重试
void retryFailed() {
if (failed.size() == 0) {
return;
}
for (Map.Entry> entry : new HashMap>(
failed).entrySet()) {
Invocation invocation = entry.getKey();
Invoker invoker = entry.getValue();
try {
invoker.invoke(invocation);
failed.remove(invocation);
} catch (Throwable e) {
logger.error("Failed retry to invoke method " + invocation.getMethodName() + ", waiting again.", e);
}
}
}
@Override
protected Result doInvoke(Invocation invocation, List> invokers, LoadBalance loadbalance) throws RpcException {
try {
checkInvokers(invokers, invocation);
Invoker invoker = select(loadbalance, invocation, invokers, null);
return invoker.invoke(invocation);
} catch (Throwable e) {
/* 日志 */
addFailed(invocation, this);
return new RpcResult(); // ignore
}
}
}

2.4 FailfastClusterInvoker

调用一次，快速失败，失败后立即抛出异常：

public class FailfastClusterInvoker extends AbstractClusterInvoker {
public FailfastClusterInvoker(Directory directory) {
super(directory);
}
@Override
public Result doInvoke(Invocation invocation, List> invokers, LoadBalance loadbalance) throws RpcException {
checkInvokers(invokers, invocation);
Invoker invoker = select(loadbalance, invocation, invokers, null);
try {
return invoker.invoke(invocation);
} catch (Throwable e) {
if (e instanceof RpcException && ((RpcException) e).isBiz()) { // biz exception.
throw (RpcException) e;
}
throw new RpcException(e instanceof RpcException ? ((RpcException) e).getCode() : 0, "Failfast invoke providers " + invoker.getUrl() + " " + loadbalance.getClass().getSimpleName() + " select from all providers " + invokers + " for service " + getInterface().getName() + " method " + invocation.getMethodName() + " on consumer " + NetUtils.getLocalHost() + " use dubbo version " + Version.getVersion() + ", but no luck to perform the invocation. Last error is: " + e.getMessage(), e.getCause() != null ? e.getCause() : e);
}
}
}


2.5 FailsafeClusterInvoker

调用一次，快速失败，失败仅打印日志，不抛出异常

public class FailsafeClusterInvoker extends AbstractClusterInvoker {
private static final Logger logger = LoggerFactory.getLogger(FailsafeClusterInvoker.class);
public FailsafeClusterInvoker(Directory directory) {
super(directory);
}
@Override
public Result doInvoke(Invocation invocation, List> invokers, LoadBalance loadbalance) throws RpcException {
try {
checkInvokers(invokers, invocation);
Invoker invoker = select(loadbalance, invocation, invokers, null);
return invoker.invoke(invocation);
} catch (Throwable e) {
logger.error("Failsafe ignore exception: " + e.getMessage(), e);
return new RpcResult(); // ignore
}
}
}