RocketMQ-消息拉取Pull

一、問題思考

消息拉取在實踐過程中，有以下幾個問題需要考慮：

1、如何全量拉取消息？

2、如何指定MessageQueue從指定offset處拉取消息？

3、如何更新MessageQueue的Offset標誌位？

4、Pull模式下如何實現負載均衡？

二、Pull模式下常用Demo

1、更新MessageQueue的Offset標誌位

consumer.updateConsumeOffset(mq, pullResult.getNextBeginOffset());

2、Pull模式簡單Demo

public static void main(String[] args) throws MQClientException {
		DefaultMQPullConsumer consumer = new DefaultMQPullConsumer("PullConsumer");
		consumer.setNamesrvAddr("10.72.86.241:9876;10.72.86.229:9876");

		consumer.start();
		
		// 從指定topic中拉取所有消息隊列
		Set<MessageQueue> mqs = consumer.fetchSubscribeMessageQueues("user-info");
		
		for (MessageQueue mq : mqs) {
			// 獲取消息的offset，指定從store中獲取
			long offset = consumer.fetchConsumeOffset(mq, true);
			System.out.println("consumer from the queue:" + mq + ":" + offset);
			
			try {
				while (true) {
					PullResult pullResult = consumer.pullBlockIfNotFound(mq,
							null, consumer.fetchConsumeOffset(mq, false), 1);
					
					consumer.updateConsumeOffset(mq, pullResult.getNextBeginOffset());

					switch (pullResult.getPullStatus()) {
					case FOUND:
						List<MessageExt> messageExtList = pullResult.getMsgFoundList();
						for (MessageExt m : messageExtList) {
							System.out.println(m.toString());
						}
						break;
					case NO_MATCHED_MSG:
						break;
					case NO_NEW_MSG:
						break;
					case OFFSET_ILLEGAL:
						break;
					default:
						break;
					}
				}
				
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
		consumer.shutdown();
	}

3、指定MessageQueue拉取消息

public class PullMQConsumerMain {
	public static void main(String[] args) throws MQClientException {
		DefaultMQPullConsumer consumer = new DefaultMQPullConsumer("PullConsumer");
		consumer.setNamesrvAddr("10.72.86.241:9876;10.72.86.229:9876");

		consumer.start();
		
		try {
			//從指定topic的MessageQueue中拉取消息
			MessageQueue mq = new MessageQueue();
			mq.setQueueId(0);
			mq.setTopic("user-info");
			mq.setBrokerName("broker-a");
			//指定獲取消息的offset
			long offset = 26;
			
			PullResult pullResult = consumer.pullBlockIfNotFound(mq, null, offset, 32);
			//更新offset
			consumer.updateConsumeOffset(mq, pullResult.getNextBeginOffset());
			
			System.out.println("最新 offset :" + consumer.fetchConsumeOffset(mq, false));

			switch (pullResult.getPullStatus()) {
			case FOUND:
				List<MessageExt> messageExtList = pullResult
						.getMsgFoundList();
				for (MessageExt m : messageExtList) {
					System.out.println(m.toString());
				}
				break;
			case NO_MATCHED_MSG:
				break;
			case NO_NEW_MSG:
				break;
			case OFFSET_ILLEGAL:
				break;
			default:
				break;
			}
        } catch (Exception e) {
            e.printStackTrace();
        }

        consumer.shutdown();
	}
}

4、Pull模式下負載均衡（MQPullConsumerScheduleService）

public class PullScheduleMain {
	public static void main(String[] args) throws MQClientException {
		final MQPullConsumerScheduleService scheduleService = new MQPullConsumerScheduleService("PullSchedule");
		scheduleService.getDefaultMQPullConsumer().setNamesrvAddr("10.72.86.241:9876;10.72.86.229:9876");
		scheduleService.setMessageModel(MessageModel.CLUSTERING);
		
		scheduleService.registerPullTaskCallback("user-info", new PullTaskCallback() {
			
			@Override
			public void doPullTask(MessageQueue mq, PullTaskContext context) {
				// TODO Auto-generated method stub
				MQPullConsumer consumer = context.getPullConsumer();
				
				try {
					long offset = consumer.fetchConsumeOffset(mq, false);
					if(offset < 0)
						offset = 0;
					
					PullResult pullResult = consumer.pullBlockIfNotFound(mq, null, offset, 4);

					switch (pullResult.getPullStatus()) {
					case FOUND:
						List<MessageExt> messageExtList = pullResult
								.getMsgFoundList();
						for (MessageExt m : messageExtList) {
							System.out.println(m.toString());
						}
						break;
					case NO_MATCHED_MSG:
						break;
					case NO_NEW_MSG:
						break;
					case OFFSET_ILLEGAL:
						break;
					default:
						break;
					}
					
					consumer.updateConsumeOffset(mq, pullResult.getNextBeginOffset());
					context.setPullNextDelayTimeMillis(10000);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
		
		scheduleService.start();
	}
}

三、總結

      分別用四個例子回答了上面的4個問題，後續會通過源碼解析的方式描述Pull模式的實現機制。

      遺留問題：

      調用fetchMessageQueuesInBalance函數時，返回MessageQueue爲空