上一篇主要介绍了基于zookeeper的服务注册实现：
http://idcbgp.cn/article/274027
对于我们的系统还需要一些策略,比如worker机器的选择策略，请求失败的策略，让我们的系统更加的可靠。

客户端负载均衡

对于loadbalance，主要是实现几个方法，让我们的manager客户端进行worker机器进行合适的选择。比如轮训，随机，或者权重。

具体实现

0-工具方法
 private int getNextNonNegative() {
        return MathUtil.getNonNegative(idx.getAndIncrement());
 }
public static int getNonNegative(int originValue){
        //通过二进制位操作将originValue转化为非负数
        return 0x7fffffff & originValue;
}
1-随机
  @Override
    public Endpoint doSelect(RequestPacket request) {

        List<Endpoint> endpoints = getEndpoints();

        int idx = (int) (ThreadLocalRandom.current().nextDouble() * endpoints.size());
        for (int i = 0; i < endpoints.size(); i++) {
            Endpoint ref = endpoints.get((i + idx) % endpoints.size());
            //TODO 判断是否存活
            return ref;
        }
        return null;
    }

2-轮训
  @Override
    public Endpoint doSelect(RequestPacket request) {
        List<Endpoint> endpoints = getEndpoints();
        int index = getNextNonNegative();
        for (int i = 0; i < endpoints.size(); i++) {
            Endpoint ref = endpoints.get((i + index) % endpoints.size());
            return ref;
        }
        return null;
    }

3-低并发度优先： referer的某时刻的call数越小优先级越高
 低并发referer获取策略：
 由于Referer List可能很多，比如上百台，如果每次都要从这上百个Referer或者最低并  发的几个，性能有些损耗，因此 random.nextInt(list.size()) 获取一个起始index，然后获取最多不超过MAX_REFERER_COUNT的状态是isAvailable的referer进行判断activeCount.
 protected Endpoint doSelect(RequestPacket request) {
        List<Endpoint> endpoints = getEndpoints();

        int refererSize = endpoints.size();
        int startIndex = ThreadLocalRandom.current().nextInt(refererSize);
        int currentCursor = 0;
        int currentAvailableCursor = 0;
        Endpoint endpoint = null;
        while (currentAvailableCursor < MAX_REFERER_COUNT && currentCursor < refererSize) {
            Endpoint temp = endpoints.get((startIndex + currentCursor) % refererSize);
            currentCursor++;
            currentAvailableCursor++;
            if (endpoint == null) {
                endpoint = temp;
            } else {
                if (compare(endpoint, temp) > 0) {
                    endpoint = temp;
                }
            }
        }

        return endpoint;
    }

4 ......更多

Ha策略

这里就简单实现两种:
1- FailfastHaStrategy  快速失败
我们通过loadBalance选择一个worker机器地址，然后发起请求，如果出现错误就立刻报错失败。
2- FailoverHaStrategy  会带有重试功能
我们通过loadBalance会选择一组worker机器，如果第一个失败，会轮训调用后面的机器，直到成功，或者全部失败

实现

1-FailfastHaStrategy  

    //快速失败策略
    public ResponsePacket call(RequestPacket request, LoadBalance loadBalance) {
        //获取执行的服务
        Endpoint endpoint = loadBalance.select(request);
        log.info("{}FailfastHaStrategy start to call {},request:{}", Constants.LOG_PREFIX, endpoint.getHost(), request.toString());

        //获取nettyClient  发送RPC请求

        return request(endpoint, request);
    }


2-FailoverHaStrategy  
public class FailoverHaStrategy extends AbstractHaStrategy {

    protected ThreadLocal<List<Endpoint>> endpointHolder = new ThreadLocal<List<Endpoint>>() {
        @Override
        protected java.util.List<Endpoint> initialValue() {
            return new ArrayList<Endpoint>();
        }
    };

    public ResponsePacket call(RequestPacket request, LoadBalance loadBalance) {

        //根据规则获取一组endpoint
        List<Endpoint> endpointList = selectReferers(request, loadBalance);
        if (endpointList.isEmpty()) {
            throw new CommonException(999999, String.format("FailoverHaStrategy No Endpoint　loadbalance:%s", loadBalance));
        }
        int tryCount = request.getRetries();//获取用户配置的重试次数
        // 如果有问题，则设置为不重试
        if (tryCount < 0) {
            tryCount = 0;
        }
        for (int i = 0; i <= tryCount; i++) {
            Endpoint endpoint = endpointList.get(i % endpointList.size());
            log.info("{}FailoverHaStrategy start to call ......{},tryCount:{},request:{}", Constants.LOG_PREFIX, endpoint.getHost(), (i + 1), request.toString());
            try {
                //获取nettyClient  发送RPC请求
                return request(endpoint, request);

            } catch (RuntimeException e) {
                // 对于业务异常，直接抛出，不进行重试
                if (e instanceof CommonException) {
                    throw e;
                } else if (i >= tryCount) {
                    log.info("{}tryCount is over......throw e", Constants.LOG_PREFIX);
                    throw e;
                }
                log.info("{}try run ,tryCount:{}", Constants.LOG_PREFIX, (i + 1));
            }
        }
        throw new CommonException(999999, "FailoverHaStrategy.call should not come here!");
    }

    protected List<Endpoint> selectReferers(RequestPacket request, LoadBalance loadBalance) {
        List<Endpoint> endpoints = endpointHolder.get();
        endpoints.clear();
        loadBalance.selectToHolder(request, endpoints);
        return endpoints;
    }

}

总结

后续我们会继续介绍一些核心功能，比如代理的实现和transport层的实现
完整代码见github： https://github.com/lizu18xz/faya-job