ZookeeperNet太难用，写了个RetryHelper来进行配套使用

浏览数：21 / 时间：2015年06月09日

普通的zk用法，如下写法：

zk.Exists("/aaa", true);
zk.Create(...);

但是由于这些API会抛Zookeeper的Exception，比如ConnectionLossException, NoNodeException等，所以必须配合一堆try/catch的机制来catch错误，catch后再处理...

写起来很麻烦

因此写了个RetryHelper来封装上面这个try/catch行为，用起来也比较方便，如下：

RetryHelper helper=RetryHelper.Make();

helper.CreateNodeStructure = () => { Console.WriteLine("CreateNodeStructure"); };
helper.FixConnectionLossAction = () => { Console.WriteLine("FixConnectionLossAction");};
helper.IfErrorThen = () => { Console.WriteLine("IfErrorThen"); };
helper.Execute(() =>
{
     this.zk.GetChildren(...);
});

上面的意思是如果在Execute中，如果报错了，则会看报错的是哪种类型，如果是ConnectionLoss则执行FixConnectionLossAction委托，如果是NoNode则执行建立节点的委托

也就是将最常见的2个zookeeper动作给结构化了：建立节点目录结构以及连接丢失时的重新连接动作

RetryHelper代码：

public class RetryHelper
    {
        private int retryDelay = 500;
        private long signal = 0;
        public Action IfErrorThen;
        public Action CreateNodeStructure;
        public Action FixConnectionLossAction;

        public static RetryHelper Make()
        {
            return new RetryHelper();
        }

        public void Execute(Action action)
        {
            while (true)
            {
                try
                {
                    action();
                    break;
                }
                catch (ZooKeeperNet.KeeperException.NoNodeException ex)
                {
                    //create node structure

                    Console.WriteLine("retry helper NoNodeException: " + ex.Message);

                    if (CreateNodeStructure != null)
                        RetryHelper.Make().Execute(CreateNodeStructure);
                    continue;
                }
                catch (ZooKeeperNet.KeeperException.ConnectionLossException ex)
                {
                    Console.WriteLine("retry helper ConnectionLossException: " + ex.Message);

                    long attempSignal = Interlocked.Read(ref signal);

                    while (Interlocked.Read(ref signal) > 0)
                        Thread.Sleep(retryDelay);

                    if (attempSignal == 0)
                    {
                        Interlocked.Increment(ref signal);

                        if (FixConnectionLossAction != null)
                            RetryHelper.Make().Execute(FixConnectionLossAction);

                        Interlocked.Decrement(ref signal);
                    }

                    continue;
                }
                catch (Exception ex)
                {
                    Console.WriteLine("retry helper catch: " + ex.Message);
                    Thread.Sleep(retryDelay);

                    if (IfErrorThen != null)
                        IfErrorThen();
                    continue;
                }
            }
        }
    }

仔细看上面代码的朋友肯定也注意到里面catch connectionloss exception的代码块中使用了Interlocked，这是因为：在多线程系统侠，如果zk连接丢失了，由于多个地方都在尝试zk操作，所以会导致并发性的进入catch loss connection exception代码处理块，如果此时不加判断的处理所有并发请求，则会出现连接多次到zk，严重影响性能；因此，这里的代码实际上意图是将多次连接请求合并为一次连接。此处特别感谢我同事的code review，哈哈。

下面是个测试并发消除的demo，为了让结果清晰，我把RetryHelper的catch中的Console.WriteLine注释了

static void Main(string[] args)
        {
            RetryHelper helper=RetryHelper.Make();

            helper.CreateNodeStructure = () => { Console.WriteLine("CreateNodeStructure"); };
            helper.FixConnectionLossAction = () => 
            { 
                Console.WriteLine(Thread.CurrentThread.ManagedThreadId+" FixConnectionLossAction BEGIN "+DateTime.Now.ToString());
                Thread.Sleep(2000);
                Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " FixConnectionLossAction END " + DateTime.Now.ToString());
            };
            helper.IfErrorThen = () => { Console.WriteLine("IfErrorThen"); };

            var tasks=new List<Task>();
            for (int i = 0; i < 10; i++)
            {
                var task = new Task(() => {
                    helper.Execute(() =>
                    {
                        throw new ZooKeeperNet.KeeperException.ConnectionLossException();
                    });
                });
                tasks.Add(task);
            }

            tasks.ForEach(t=>t.Start());

            Task.WaitAll(tasks.ToArray());

            Console.ReadKey();
        }