内容简介：kafka0.9 Consumer poll()方法阻塞
最近项目中用到了Kafka0.9，在使用0.9的Consumer API的时候遇到了poll()方法阻塞的问题。程序没有报任何错误，只是持续在poll()方法处阻塞。深入poll()方法可以看到是在AbstractCoordinator.ensureCoordinatorKnown()方法中出现了死循环。在循环中不停地输出如下DEBUG日志：
DEBUG main org.apache.kafka.clients.consumer.internals.AbstractCoordinator - Issuing group metadata request to broker 2
DEBUG main org.apache.kafka.clients.consumer.internals.AbstractCoordinator - Group metadata response ClientResponse(receivedTimeMs=1495335769027, disconnected=false, request=ClientRequest(expectResponse=true, callback=org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient$RequestFutureCompletionHandler@5c3bd550, request=RequestSend(header={api_key=10,api_version=0,correlation_id=90,client_id=consumer-1}, body={group_id=stats_consume_group}), createdTimeMs=1495335768924, sendTimeMs=1495335768924), responseBody={error_code=15,coordinator={node_id=-1,host=,port=-1}})
DEBUG main org.apache.kafka.clients.NetworkClient - Sending metadata request ClientRequest(expectResponse=true, callback=null, request=RequestSend(header={api_key=3,api_version=0,correlation_id=91,client_id=consumer-1}, body={topics=[statistic_test]}), isInitiatedByNetworkClient, createdTimeMs=1495335769027, sendTimeMs=0) to node 2
DEBUG main org.apache.kafka.clients.Metadata - Updated cluster metadata version 47 to Cluster(nodes = [Node(0, kafka-node, 9092), Node(2, sz-pg-adtracking-bigdisk-003, 9092), Node(1, sz-pg-adtracking-bigdisk-002, 9092)], partitions = [Partition(topic = statistic_test, partition = 1, leader = 2, replicas = [2,1,], isr = [1,2,], Partition(topic = statistic_test, partition = 2, leader = 0, replicas = [0,2,], isr = [0,2,], Partition(topic = statistic_test, partition = 0, leader = 1, replicas = [1,0,], isr = [0,1,], Partition(topic = statistic_test, partition = 5, leader = 0, replicas = [0,1,], isr = [0,1,], Partition(topic = statistic_test, partition = 6, leader = 1, replicas = [1,0,], isr = [0,1,], Partition(topic = statistic_test, partition = 3, leader = 1, replicas = [1,2,], isr = [1,2,], Partition(topic = statistic_test, partition = 4, leader = 2, replicas = [2,0,], isr = [0,2,], Partition(topic = statistic_test, partition = 7, leader = 2, replicas = [2,1,], isr = [1,2,], Partition(topic = statistic_test, partition = 8, leader = 0, replicas = [0,2,], isr = [0,2,]])
需要关注的是这处信息：
error_code=15,coordinator={node_id=-1,host=,port=-1}
看样子是kafka的连接出了问题。不过我的Producer向kafka写数据是没问题的，使用kafka提供的消费工具kafka-console-consumer.sh执行消费也是没问题的。
在网上找到了一些关于这个现象的解释：在客户端进行消费之前会为ConsumerGroup向Kafka集群申请coordinater节点。kafka集群在配置或分配coordinater节点的时候可能会短暂的报这个错误。
我这里不是短暂的报错，而是陷入了死循环。目前可以想到的就是我的kafka集群配置出现问题了。在简单粗暴地将zookeeper上kafka的配置完全删掉再重启Kafka后，消费可以正常执行了。至于问题具体出在哪儿还没有找到。目前只能是持续关注，等问题再次出现了。
就这样。
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