负载均衡策略之RoundRobinRule和RetryRule源码解读

一 RoundRobinRule和RetryRule类图位置

二 RoundRobinRule源码解读

//该策略实现安装线性轮询的方式依次选择每个服务实例
public class RoundRobinRule extends AbstractLoadBalancerRule {

    private AtomicInteger nextServerCyclicCounter;
    private static final boolean AVAILABLE_ONLY_SERVERS = true;
    private static final boolean ALL_SERVERS = false;

    private static Logger log = LoggerFactory.getLogger(RoundRobinRule.class);

    public RoundRobinRule() {
        nextServerCyclicCounter = new AtomicInteger(0);
    }

    public RoundRobinRule(ILoadBalancer lb) {
        this();
        setLoadBalancer(lb);
    }

    public Server choose(ILoadBalancer lb, Object key) {
        if (lb == null) {
            log.warn("no load balancer");
            return null;
        }

        Server server = null;
        int count = 0;
        //最多选择十次，就会结束尝试
        while (server == null && count++ < 10) {
            List<Server> reachableServers = lb.getReachableServers();
            List<Server> allServers = lb.getAllServers();
            int upCount = reachableServers.size();
            int serverCount = allServers.size();

            if ((upCount == 0) || (serverCount == 0)) {
                log.warn("No up servers available from load balancer: " + lb);
                return null;
            }

            int nextServerIndex = incrementAndGetModulo(serverCount);
            server = allServers.get(nextServerIndex);

            if (server == null) {
                /* Transient. */
                Thread.yield();
                continue;
            }

            if (server.isAlive() && (server.isReadyToServe())) {
                return (server);
            }

            // Next.
            server = null;
        }

        if (count >= 10) {
            log.warn("No available alive servers after 10 tries from load balancer: "
                    + lb);
        }
        return server;
    }

    //通过递增实现轮询
    private int incrementAndGetModulo(int modulo) {
        for (;;) {
            int current = nextServerCyclicCounter.get();
            int next = (current + 1) % modulo;
            if (nextServerCyclicCounter.compareAndSet(current, next))
                return next;
        }
    }

    @Override
    public Server choose(Object key) {
        return choose(getLoadBalancer(), key);
    }

    @Override
    public void initWithNiwsConfig(IClientConfig clientConfig) {
    }
}

三 RetryRule源码解读

//具备重试机制的实例选择功能
public class RetryRule extends AbstractLoadBalancerRule {
    //默认使用RoundRobinRule实例
    IRule subRule = new RoundRobinRule();
    //阈值为500ms
    long maxRetryMillis = 500;

    public RetryRule() {
    }

    public RetryRule(IRule subRule) {
        this.subRule = (subRule != null) ? subRule : new RoundRobinRule();
    }

    public RetryRule(IRule subRule, long maxRetryMillis) {
        this.subRule = (subRule != null) ? subRule : new RoundRobinRule();
        this.maxRetryMillis = (maxRetryMillis > 0) ? maxRetryMillis : 500;
    }

    public void setRule(IRule subRule) {
        this.subRule = (subRule != null) ? subRule : new RoundRobinRule();
    }

    public IRule getRule() {
        return subRule;
    }

    public void setMaxRetryMillis(long maxRetryMillis) {
        if (maxRetryMillis > 0) {
            this.maxRetryMillis = maxRetryMillis;
        } else {
            this.maxRetryMillis = 500;
        }
    }

    public long getMaxRetryMillis() {
        return maxRetryMillis;
    }

    
    
    @Override
    public void setLoadBalancer(ILoadBalancer lb) {        
        super.setLoadBalancer(lb);
        subRule.setLoadBalancer(lb);
    }


    public Server choose(ILoadBalancer lb, Object key) {
        long requestTime = System.currentTimeMillis();
        long deadline = requestTime + maxRetryMillis;

        Server answer = null;

        answer = subRule.choose(key);

        if (((answer == null) || (!answer.isAlive()))
                && (System.currentTimeMillis() < deadline)) {

            InterruptTask task = new InterruptTask(deadline
                    - System.currentTimeMillis());
            //反复重试
            while (!Thread.interrupted()) {
                //选择实例
                answer = subRule.choose(key);
                //500ms内没选择到就返回null
                if (((answer == null) || (!answer.isAlive()))
                        && (System.currentTimeMillis() < deadline)) {
                    /* pause and retry hoping it's transient */
                    Thread.yield();
                } 
                else //若能选择到实例，就返回
                {
                    break;
                }
            }

            task.cancel();
        }

        if ((answer == null) || (!answer.isAlive())) {
            return null;
        } else {
            return answer;
        }
    }

    @Override
    public Server choose(Object key) {
        return choose(getLoadBalancer(), key);
    }

    @Override
    public void initWithNiwsConfig(IClientConfig clientConfig) {
    }
}