Flink 消费RabbitMQ 和 Kafka

在消息RabbitMQ时,我们关心的一个问题是手动ack还是自动ack,如果是自动ack就怕出现丢消息的情况
Flink以RabbitMQ作为Source,是怎么保证消息唯一性的呢,是怎么保证ack的.

首先引入依赖包
<dependency>
     <groupId>org.apache.flink</groupId>
     <artifactId>flink-connector-kafka-0.10_${scala.version}</artifactId>
     <version>${flink.version}</version>
</dependency>

RMQSource类,可以看到如果设置了checkpointing,则默认autoAck是false,是手动控制提交的
那什么时候提交呢,flink checkpointing有个时间间隔,每次checkpointing触发时,才能ack,也就是说,不是一条消息ack一下,而是定时ack
这个跟kafka,update offset一样,都是在checkpoint的时候处理

@Override
	public void open(Configuration config) throws Exception {
		super.open(config);
		ConnectionFactory factory = setupConnectionFactory();
		try {
			connection = factory.newConnection();
			channel = connection.createChannel();
			if (channel == null) {
				throw new RuntimeException("None of RabbitMQ channels are available");
			}
			setupQueue();
			consumer = new QueueingConsumer(channel);

			RuntimeContext runtimeContext = getRuntimeContext();
			if (runtimeContext instanceof StreamingRuntimeContext
					&& ((StreamingRuntimeContext) runtimeContext).isCheckpointingEnabled()) {
				autoAck = false;
				// enables transaction mode
				channel.txSelect();
			} else {
				autoAck = true;
			}

			LOG.debug("Starting RabbitMQ source with autoAck status: " + autoAck);
			channel.basicConsume(queueName, autoAck, consumer);

		} catch (IOException e) {
			throw new RuntimeException("Cannot create RMQ connection with " + queueName + " at "
					+ rmqConnectionConfig.getHost(), e);
		}
		running = true;
	}

RMQSource

@Override
	public void run(SourceContext<OUT> ctx) throws Exception {
		while (running) {
			QueueingConsumer.Delivery delivery = consumer.nextDelivery();

			synchronized (ctx.getCheckpointLock()) {

				OUT result = schema.deserialize(delivery.getBody());

				if (schema.isEndOfStream(result)) {
					break;
				}

				if (!autoAck) {
					final long deliveryTag = delivery.getEnvelope().getDeliveryTag();
					if (usesCorrelationId) {
						final String correlationId = delivery.getProperties().getCorrelationId();
						Preconditions.checkNotNull(correlationId, "RabbitMQ source was instantiated " +
							"with usesCorrelationId set to true but a message was received with " +
							"correlation id set to null!");
						if (!addId(correlationId)) {
							// we have already processed this message
							continue;
						}
					}
					sessionIds.add(deliveryTag);
				}

				ctx.collect(result);
			}
		}
	}

@Override
	protected void acknowledgeSessionIDs(List<Long> sessionIds) {
		try {
			for (long id : sessionIds) {
				channel.basicAck(id, false);
			}
			channel.txCommit();
		} catch (IOException e) {
			throw new RuntimeException("Messages could not be acknowledged during checkpoint creation.", e);
		}
	}