【Motan源码学习】8-高可用策略

Motan 实现了 FailFast、FailOver、FailBack 这几种HA策略。
1.FailFast
Fail-fast is a property of a system or module with respect to its response to failures. A fail-fast system is designed to immediately report at its interface anyfailure or condition that is likely to lead to failure. Fail-fast systems are usually designed to stop normal operation rather than attempt to continue a possibly flawed process. Such designs often check the system’s state at several points in an operation, so any failures can be detected early. A fail-fast module passes the responsibility for handling errors, but not detecting them, to the next-higher system design level.

这是维基百科关于FailFast的描述，就是错误要尽早发现，失败立即报错结束。 Motan 的 FailFast 实现的比较简单，和简单的调用一样。这让我想起了Hystrix这个熔断框架，里面也有FailFast的机制，可以去了解一下。
2.FailOver
In computing, failover is switching to a redundant or standby computer server, system, hardware component or network upon the failure or abnormal termination of the previously active application,[1] server, system, hardware component, or network. Failover and switchover are essentially the same operation, except that failover is automatic and usually operates without warning, while switchover requires human intervention.

这是维基百科关于FailOver的描述，即失效转移，一个节点失败了，立刻尝试别的节点。具体到 Motan 的服务调用，是这样实现的, 即一个Refer失败了，就尝试别的Refer.

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public Response call(Request request, LoadBalance<T> loadBalance) {
    List<Referer<T>> referers = selectReferers(request, loadBalance);
    URL refUrl = referers.get(0).getUrl();
    // 先使用method的配置
    int tryCount = refUrl.getMethodParameter(request.getMethodName(), request.getParamtersDesc(), URLParamType.retries.getName(),URLParamType.retries.getIntValue());
    for (int i = 0; i <= tryCount; i++) {
        Referer<T> refer = referers.get(i % referers.size());
        try {
            request.setRetries(i);
            return refer.call(request);
        } catch (RuntimeException e) {
            ....
        }
    }
    throw new MotanFrameworkException("FailoverHaStrategy.call should not come here!");
}

3.FailBack
Failback is the process of restoring a system, component, or service previously in a state of failure back to its original, working state, and having the standby system go from functioning back to standby.
这是维基百科关于FailBack的描述, 即失败自动恢复，在实现上一般是后台记录失败请求，定时重发。Motan 在服务调用的时候并没有做FailBack. 而是在服务注册的时候使用了FailBack的机制。

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public FailbackRegistry(URL url) {
    super(url);
    long retryPeriod = url.getIntParameter(URLParamType.registryRetryPeriod.getName(), URLParamType.registryRetryPeriod.getIntValue());
    retryExecutor.scheduleAtFixedRate(new Runnable() {
        @Override
        public void run() {
            try {
                retry();
            } catch (Exception e) {
                LoggerUtil.warn(String.format("[%s] False when retry in failback registry", registryClassName), e);
            }
        }
    }, retryPeriod, retryPeriod, TimeUnit.MILLISECONDS);
}

FailbackRegistry 中会专门有个现成定时的去重试，注册失败的服务会被记录到failedRegistered这个Set中，重试的时候就去重新注册这个Set里的服务，成功了就移除，失败了下次继续。

Motan 还实现了一种异常隔离机制，调用Server连续失败超过指定次数后置为不可用，然后定期进行心跳探测，OK后再将Server置为可用。这个功能是在 transport 层实现的，在讲心跳机制的时候有说过。

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以上皆是阅读源码 https://github.com/weibocom/motan （tag 0.3.1）所得，文中贴的代码或配置有所删减