Dubbo源码学习--ForkingCluster集群容错（八）

Forking Cluster

并行调用多个服务器，只要一个成功即返回。通常用于实时性要求较高的读操作，但需要浪费更多服务资源。可通过 forks="2" 来设置最大并行数。

在ForkingCluster中是由ForkingClusterInvoker来帮助完成实现的。

public class ForkingCluster implements Cluster {

    public final static String NAME = "forking";

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

}

在ForkingClusterInvoker中选择多个服务提供方，并发创建多个线程添加到线程池中调用服务，根据服务调用的结果来判断，如果存在调用成功的服务则返回结果，否则返回异常信息。

public class ForkingClusterInvoker extends AbstractClusterInvoker {

    private final ExecutorService executor = Executors.newCachedThreadPool(new NamedThreadFactory("forking-cluster-timer", true));

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

    @SuppressWarnings({"unchecked", "rawtypes"})
    public Result doInvoke(final Invocation invocation, List> invokers, LoadBalance loadbalance) throws RpcException {
        checkInvokers(invokers, invocation);
        final List> selected;
        final int forks = getUrl().getParameter(Constants.FORKS_KEY, Constants.DEFAULT_FORKS);
        final int timeout = getUrl().getParameter(Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);
		//选择多个服务提供方
        if (forks <= 0 || forks >= invokers.size()) {
            selected = invokers;
        } else {
            selected = new ArrayList>();
            for (int i = 0; i < forks; i++) {
                // TODO. Add some comment here, refer chinese version for more details.
                Invoker invoker = select(loadbalance, invocation, invokers, selected);
                if (!selected.contains(invoker)) {//Avoid add the same invoker several times.
                    selected.add(invoker);
                }
            }
        }
        RpcContext.getContext().setInvokers((List) selected);
        final AtomicInteger count = new AtomicInteger();
        final BlockingQueue