RockeMQ-存储机制-可用性策略

broker的可用性策略-快速失败机制

具体实现在BrokerFastFailure，会执行一个定时任务扫描写消息任务的队列，当发现ospagecache繁忙的时候，就取出一个请求任务快速返回，直到OSPageCache不在繁忙。

然后会遍历任务队列，如果发现某一个请求任务已经超时，那么也会立即返回避免producer堵塞。

具体实现逻辑：

    private void cleanExpiredRequest() {
        while (this.brokerController.getMessageStore().isOSPageCacheBusy()) {
            try {
                if (!this.brokerController.getSendThreadPoolQueue().isEmpty()) {
                    final Runnable runnable = this.brokerController.getSendThreadPoolQueue().poll(0, TimeUnit.SECONDS);
                    if (null == runnable) {
                        break;
                    }

                    final RequestTask rt = castRunnable(runnable);
                    rt.returnResponse(RemotingSysResponseCode.SYSTEM_BUSY, String.format("[PCBUSY_CLEAN_QUEUE]broker busy, start flow control for a while, period in queue: %sms, size of queue: %d", System.currentTimeMillis() - rt.getCreateTimestamp(), this.brokerController.getSendThreadPoolQueue().size()));
                } else {
                    break;
                }
            } catch (Throwable ignored) {
            }
        }

        while (true) {
            try {
                if (!this.brokerController.getSendThreadPoolQueue().isEmpty()) {
                    // peek不删除元素
                    final Runnable runnable = this.brokerController.getSendThreadPoolQueue().peek();
                    if (null == runnable) {
                        break;
                    }
                    final RequestTask rt = castRunnable(runnable);
                    if (rt == null || rt.isStopRun()) {
                        break;
                    }

                    final long behind = System.currentTimeMillis() - rt.getCreateTimestamp();
                    if (behind >= this.brokerController.getBrokerConfig().getWaitTimeMillsInSendQueue()) {
                        if (this.brokerController.getSendThreadPoolQueue().remove(runnable)) {
                            rt.setStopRun(true);
                            rt.returnResponse(RemotingSysResponseCode.SYSTEM_BUSY, String.format("[TIMEOUT_CLEAN_QUEUE]broker busy, start flow control for a while, period in queue: %sms, size of queue: %d", behind, this.brokerController.getSendThreadPoolQueue().size()));
                        }
                    } else {
                        break;
                    }
                } else {
                    break;
                }
            } catch (Throwable ignored) {
            }
        }
    }

在remote的NettyRemotingAbstract类中的processRequestCommand，会把远程请求处理包装成一个RequestTask 然后放到sendMessageExecutor中

 final RequestTask requestTask = new RequestTask(run, ctx.channel(), cmd);
                pair.getObject2().submit(requestTask);

实际上会把任务放到队列中 this.sendThreadPoolQueue = new LinkedBlockingQueue<Runnable>(this.brokerConfig.getSendThreadPoolQueueCapacity());

取出来的时候，进行强转。final RequestTask rt = castRunnable(runnable);