public class FailoverClusterInvoker<T> extends AbstractClusterInvoker<T> {
    public FailoverClusterInvoker(Directory<T> directory) {
        super(directory);
    }
    @Override
    public Result doInvoke(Invocation invocation, final List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
        // 所有生產(chǎn)者Invokers
        List<Invoker<T>> copyInvokers = invokers;
        checkInvokers(copyInvokers, invocation);
        String methodName = RpcUtils.getMethodName(invocation);
        // 獲取重試次數(shù)
        int len = getUrl().getMethodParameter(methodName, Constants.RETRIES_KEY, Constants.DEFAULT_RETRIES) + 1;
        if (len <= 0) {
            len = 1;
        }
        RpcException le = null;
        // 已經(jīng)調(diào)用過(guò)的生產(chǎn)者
        List<Invoker<T>> invoked = new ArrayList<Invoker<T>>(copyInvokers.size());
        Set<String> providers = new HashSet<String>(len);
        // 重試直到達(dá)到最大次數(shù)
        for (int i = 0; i < len; i++) {
            if (i > 0) {
                // 如果當(dāng)前實(shí)例被銷毀則拋出異常
                checkWhetherDestroyed();
                // 根據(jù)路由策略選出可用生產(chǎn)者Invokers
                copyInvokers = list(invocation);
                // 重新檢查
                checkInvokers(copyInvokers, invocation);
            }
            // 負(fù)載均衡選擇一個(gè)生產(chǎn)者Invoker
            Invoker<T> invoker = select(loadbalance, invocation, copyInvokers, invoked);
            invoked.add(invoker);
            RpcContext.getContext().setInvokers((List) invoked);
            try {
                // 服務(wù)消費(fèi)發(fā)起遠(yuǎn)程調(diào)用
                Result result = invoker.invoke(invocation);
                if (le != null && logger.isWarnEnabled()) {
                    logger.warn("Although retry the method " + methodName + " in the service " + getInterface().getName() + " was successful by the provider " + invoker.getUrl().getAddress() + ", but there have been failed providers " + providers + " (" + providers.size() + "/" + copyInvokers.size() + ") from the registry " + directory.getUrl().getAddress() + " on the consumer " + NetUtils.getLocalHost() + " using the dubbo version " + Version.getVersion() + ". Last error is: " + le.getMessage(), le);
                }
                // 有結(jié)果則返回
                return result;
            } catch (RpcException e) {
                // 業(yè)務(wù)異常直接拋出
                if (e.isBiz()) {
                    throw e;
                }
                le = e;
            } catch (Throwable e) {
                // RpcException不拋出繼續(xù)重試
                le = new RpcException(e.getMessage(), e);
            } finally {
                // 保存已經(jīng)訪問(wèn)過(guò)的生產(chǎn)者
                providers.add(invoker.getUrl().getAddress());
            }
        }
        throw new RpcException(le.getCode(), "Failed to invoke the method " + methodName + " in the service " + getInterface().getName() + ". Tried " + len + " times of the providers " + providers + " (" + providers.size() + "/" + copyInvokers.size() + ") from the registry " + directory.getUrl().getAddress() + " on the consumer " + NetUtils.getLocalHost() + " using the dubbo version " + Version.getVersion() + ". Last error is: " + le.getMessage(), le.getCause() != null ? le.getCause() : le);
    }
}

public class FailfastClusterInvoker<T> extends AbstractClusterInvoker<T> {
    public FailfastClusterInvoker(Directory<T> directory) {
        super(directory);
    }
    @Override
    public Result doInvoke(Invocation invocation, List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
        // 檢查生產(chǎn)者Invokers是否合法
        checkInvokers(invokers, invocation);
        // 負(fù)載均衡選擇一個(gè)生產(chǎn)者Invoker
        Invoker<T> invoker = select(loadbalance, invocation, invokers, null);
        try {
            // 服務(wù)消費(fèi)發(fā)起遠(yuǎn)程調(diào)用
            return invoker.invoke(invocation);
        } catch (Throwable e) {
            // 服務(wù)消費(fèi)失敗不重試直接拋出異常
            if (e instanceof RpcException && ((RpcException) e).isBiz()) {
                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);
        }
    }
}

public class FailsafeClusterInvoker<T> extends AbstractClusterInvoker<T> {
    private static final Logger logger = LoggerFactory.getLogger(FailsafeClusterInvoker.class);
    public FailsafeClusterInvoker(Directory<T> directory) {
        super(directory);
    }
    @Override
    public Result doInvoke(Invocation invocation, List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
        try {
            // 檢查生產(chǎn)者Invokers是否合法
            checkInvokers(invokers, invocation);
            // 負(fù)載均衡選擇一個(gè)生產(chǎn)者Invoker
            Invoker<T> invoker = select(loadbalance, invocation, invokers, null);
            // 服務(wù)消費(fèi)發(fā)起遠(yuǎn)程調(diào)用
            return invoker.invoke(invocation);
        } catch (Throwable e) {
            // 消費(fèi)失敗包裝為一個(gè)空結(jié)果對(duì)象
            logger.error("Failsafe ignore exception: " + e.getMessage(), e);
            return new RpcResult();
        }
    }
}

public class FailbackClusterInvoker<T> extends AbstractClusterInvoker<T> {
    private static final Logger logger = LoggerFactory.getLogger(FailbackClusterInvoker.class);
    private static final long RETRY_FAILED_PERIOD = 5;
    private final int retries;
    private final int failbackTasks;
    private volatile Timer failTimer;
    public FailbackClusterInvoker(Directory<T> directory) {
        super(directory);
        int retriesConfig = getUrl().getParameter(Constants.RETRIES_KEY, Constants.DEFAULT_FAILBACK_TIMES);
        if (retriesConfig <= 0) {
            retriesConfig = Constants.DEFAULT_FAILBACK_TIMES;
        }
        int failbackTasksConfig = getUrl().getParameter(Constants.FAIL_BACK_TASKS_KEY, Constants.DEFAULT_FAILBACK_TASKS);
        if (failbackTasksConfig <= 0) {
            failbackTasksConfig = Constants.DEFAULT_FAILBACK_TASKS;
        }
        retries = retriesConfig;
        failbackTasks = failbackTasksConfig;
    }
    private void addFailed(LoadBalance loadbalance, Invocation invocation, List<Invoker<T>> invokers, Invoker<T> lastInvoker) {
        if (failTimer == null) {
            synchronized (this) {
                if (failTimer == null) {
                    // 創(chuàng)建定時(shí)器
                    failTimer = new HashedWheelTimer(new NamedThreadFactory("failback-cluster-timer", true), 1, TimeUnit.SECONDS, 32, failbackTasks);
                }
            }
        }
        // 構(gòu)造定時(shí)任務(wù)
        RetryTimerTask retryTimerTask = new RetryTimerTask(loadbalance, invocation, invokers, lastInvoker, retries, RETRY_FAILED_PERIOD);
        try {
            // 定時(shí)任務(wù)放入定時(shí)器等待執(zhí)行
            failTimer.newTimeout(retryTimerTask, RETRY_FAILED_PERIOD, TimeUnit.SECONDS);
        } catch (Throwable e) {
            logger.error("Failback background works error,invocation->" + invocation + ", exception: " + e.getMessage());
        }
    }
    @Override
    protected Result doInvoke(Invocation invocation, List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
        Invoker<T> invoker = null;
        try {
            // 檢查生產(chǎn)者Invokers是否合法
            checkInvokers(invokers, invocation);
            // 負(fù)責(zé)均衡選擇一個(gè)生產(chǎn)者Invoker
            invoker = select(loadbalance, invocation, invokers, null);
            // 消費(fèi)服務(wù)發(fā)起遠(yuǎn)程調(diào)用
            return invoker.invoke(invocation);
        } catch (Throwable e) {
            logger.error("Failback to invoke method " + invocation.getMethodName() + ", wait for retry in background. Ignored exception: " + e.getMessage() + ", ", e);
            // 如果服務(wù)消費(fèi)失敗則記錄失敗請(qǐng)求
            addFailed(loadbalance, invocation, invokers, invoker);
            // 返回空結(jié)果
            return new RpcResult();
        }
    }
    @Override
    public void destroy() {
        super.destroy();
        if (failTimer != null) {
            failTimer.stop();
        }
    }
    /**
     * RetryTimerTask
     */
    private class RetryTimerTask implements TimerTask {
        private final Invocation invocation;
        private final LoadBalance loadbalance;
        private final List<Invoker<T>> invokers;
        private final int retries;
        private final long tick;
        private Invoker<T> lastInvoker;
        private int retryTimes = 0;
        RetryTimerTask(LoadBalance loadbalance, Invocation invocation, List<Invoker<T>> invokers, Invoker<T> lastInvoker, int retries, long tick) {
            this.loadbalance = loadbalance;
            this.invocation = invocation;
            this.invokers = invokers;
            this.retries = retries;
            this.tick = tick;
            this.lastInvoker = lastInvoker;
        }
        @Override
        public void run(Timeout timeout) {
            try {
                // 負(fù)載均衡選擇一個(gè)生產(chǎn)者Invoker
                Invoker<T> retryInvoker = select(loadbalance, invocation, invokers, Collections.singletonList(lastInvoker));
                lastInvoker = retryInvoker;
                // 服務(wù)消費(fèi)發(fā)起遠(yuǎn)程調(diào)用
                retryInvoker.invoke(invocation);
            } catch (Throwable e) {
                logger.error("Failed retry to invoke method " + invocation.getMethodName() + ", waiting again.", e);
                // 超出最大重試次數(shù)記錄日志不拋出異常
                if ((++retryTimes) >= retries) {
                    logger.error("Failed retry times exceed threshold (" + retries + "), We have to abandon, invocation->" + invocation);
                } else {
                    // 未超出最大重試次數(shù)重新放入定時(shí)器
                    rePut(timeout);
                }
            }
        }
        private void rePut(Timeout timeout) {
            if (timeout == null) {
                return;
            }
            Timer timer = timeout.timer();
            if (timer.isStop() || timeout.isCancelled()) {
                return;
            }
            timer.newTimeout(timeout.task(), tick, TimeUnit.SECONDS);
        }
    }
}

public class ForkingClusterInvoker<T> extends AbstractClusterInvoker<T> {
    private final ExecutorService executor = Executors.newCachedThreadPool(new NamedInternalThreadFactory("forking-cluster-timer", true));
    public ForkingClusterInvoker(Directory<T> directory) {
        super(directory);
    }
    @Override
    public Result doInvoke(final Invocation invocation, List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
        try {
            checkInvokers(invokers, invocation);
            final List<Invoker<T>> selected;
            // 獲取配置參數(shù)
            final int forks = getUrl().getParameter(Constants.FORKS_KEY, Constants.DEFAULT_FORKS);
            final int timeout = getUrl().getParameter(Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);
            // 獲取并行執(zhí)行的Invoker列表
            if (forks <= 0 || forks >= invokers.size()) {
                selected = invokers;
            } else {
                selected = new ArrayList<>();
                for (int i = 0; i < forks; i++) {
                    // 選擇生產(chǎn)者
                    Invoker<T> invoker = select(loadbalance, invocation, invokers, selected);
                    // 防止重復(fù)增加Invoker
                    if (!selected.contains(invoker)) {
                        selected.add(invoker);
                    }
                }
            }
            RpcContext.getContext().setInvokers((List) selected);
            final AtomicInteger count = new AtomicInteger();
            final BlockingQueue<Object> ref = new LinkedBlockingQueue<>();
            for (final Invoker<T> invoker : selected) {
                // 在線程池中并發(fā)執(zhí)行
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            // 執(zhí)行消費(fèi)邏輯
                            Result result = invoker.invoke(invocation);
                            // 存儲(chǔ)消費(fèi)結(jié)果
                            ref.offer(result);
                        } catch (Throwable e) {
                            // 如果異常次數(shù)大于等于forks參數(shù)值說(shuō)明全部調(diào)用失敗，則把異常放入隊(duì)列
                            int value = count.incrementAndGet();
                            if (value >= selected.size()) {
                                ref.offer(e);
                            }
                        }
                    }
                });
            }
            try {
                // 從隊(duì)列獲取結(jié)果
                Object ret = ref.poll(timeout, TimeUnit.MILLISECONDS);
                // 如果異常類型表示全部調(diào)用失敗則拋出異常
                if (ret instanceof Throwable) {
                    Throwable e = (Throwable) ret;
                    throw new RpcException(e instanceof RpcException ? ((RpcException) e).getCode() : 0, "Failed to forking invoke provider " + selected + ", but no luck to perform the invocation. Last error is: " + e.getMessage(), e.getCause() != null ? e.getCause() : e);
                }
                return (Result) ret;
            } catch (InterruptedException e) {
                throw new RpcException("Failed to forking invoke provider " + selected + ", but no luck to perform the invocation. Last error is: " + e.getMessage(), e);
            }
        } finally {
            RpcContext.getContext().clearAttachments();
        }
    }
}

public class BroadcastClusterInvoker<T> extends AbstractClusterInvoker<T> {
    private static final Logger logger = LoggerFactory.getLogger(BroadcastClusterInvoker.class);
    public BroadcastClusterInvoker(Directory<T> directory) {
        super(directory);
    }
    @Override
    public Result doInvoke(final Invocation invocation, List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
        checkInvokers(invokers, invocation);
        RpcContext.getContext().setInvokers((List) invokers);
        RpcException exception = null;
        Result result = null;
        // 遍歷調(diào)用所有生產(chǎn)者節(jié)點(diǎn)
        for (Invoker<T> invoker : invokers) {
            try {
                // 執(zhí)行消費(fèi)邏輯
                result = invoker.invoke(invocation);
            } catch (RpcException e) {
                exception = e;
                logger.warn(e.getMessage(), e);
            } catch (Throwable e) {
                exception = new RpcException(e.getMessage(), e);
                logger.warn(e.getMessage(), e);
            }
        }
        // 任何一個(gè)出現(xiàn)異常則拋出異常
        if (exception != null) {
            throw exception;
        }
        return result;
    }
}

<!-- 設(shè)置重試次數(shù)為零 -->
<dubbo:reference id="helloService" interface="com.java.front.dubbo.demo.provider.HelloService" retries="0" />
<!-- 選擇集群容錯(cuò)方案 -->
<dubbo:reference id="helloService" interface="com.java.front.dubbo.demo.provider.HelloService" cluster="failfast" />

select * from table where col='xxx' for update 

update table set xxx,
version = #{version} + 1 
where id = #{id} 
and version = #{version}