Dubbo源码学习--FailoverCluster集群容错

Failover Cluster

失败自动切换，当出现失败，重试其它服务器。通常用于读操作，但重试会带来更长延迟。可通过?retries="2"?来设置重试次数(不含第一次)。

重试次数配置如下：

或

或

FailoverCluster中实现的失败重试的机制是通过FailoverClusterInvoker来实现。

public class FailoverCluster implements Cluster {

    public final static String NAME = "failover";

    public  Invoker join(Directory directory) throws RpcException {
        return new FailoverClusterInvoker(directory);
    }

}

FailoverClusterInvoker中的实现还是比较简单，通过for循环依次调用服务，如果成功则直接返回，如果失败则尝试重新调用其他服务。

public class FailoverClusterInvoker extends AbstractClusterInvoker {

    private static final Logger logger = LoggerFactory.getLogger(FailoverClusterInvoker.class);

    public FailoverClusterInvoker(Directory directory) {
        super(directory);
    }

    @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);
		//如果失败会重试调用所有服务
        for (int i = 0; i < len; 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);
            }
			//负载均衡获取服务调用者
            Invoker invoker = select(loadbalance, invocation, copyinvokers, invoked);
            invoked.add(invoker);
            RpcContext.getContext().setInvokers((List) invoked);
            try {
				//调用成功则直接返回
                Result result = invoker.invoke(invocation);
                if (le != null && logger.isWarnEnabled()) {
                    logger.warn("Although retry the method " + invocation.getMethodName()
                            + " 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);
                }
                return result;
			//失败的话再尝试重新调用
            } 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(le != null ? le.getCode() : 0, "Failed to invoke the method "
                + invocation.getMethodName() + " 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 != null ? le.getMessage() : ""), le != null && le.getCause() != null ? le.getCause() : le);
    }

}