Kafka详解五、Kafka Consumer的底层API- SimpleConsumer

1.Kafka提供了两套API给Consumer

1. The high-level Consumer API
2. The SimpleConsumer API     

第一种高度抽象的Consumer API，它使用起来简单、方便，但是对于某些特殊的需求我们可能要用到第二种更底层的API，那么先介绍下第二种API能够帮助我们做哪些事情

• 一个消息读取多次
• 在一个处理过程中只消费Partition其中的一部分消息
• 添加事务管理机制以保证消息被处理且仅被处理一次

2.使用SimpleConsumer有哪些弊端呢？

• 必须在程序中跟踪offset值
• 必须找出指定Topic Partition中的lead broker
• 必须处理broker的变动

3.使用SimpleConsumer的步骤

1. 从所有活跃的broker中找出哪个是指定Topic Partition中的leader broker
2. 找出指定Topic Partition中的所有备份broker
3. 构造请求
4. 发送请求查询数据
5. 处理leader broker变更

4.代码实例

package bonree.consumer;import java.nio.ByteBuffer;import java.util.ArrayList;import java.util.Collections;import java.util.HashMap;import java.util.List;import java.util.Map;import kafka.api.FetchRequest;import kafka.api.FetchRequestBuilder;import kafka.api.PartitionOffsetRequestInfo;import kafka.common.ErrorMapping;import kafka.common.TopicAndPartition;import kafka.javaapi.FetchResponse;import kafka.javaapi.OffsetResponse;import kafka.javaapi.PartitionMetadata;import kafka.javaapi.TopicMetadata;import kafka.javaapi.TopicMetadataRequest;import kafka.javaapi.consumer.SimpleConsumer;import kafka.message.MessageAndOffset;public class SimpleExample {private List<String> m_replicaBrokers = new ArrayList<String>();public SimpleExample() {m_replicaBrokers = new ArrayList<String>();}public static void main(String args[]) {SimpleExample example = new SimpleExample();// 最大读取消息数量long maxReads = Long.parseLong("3");// 要订阅的topicString topic = "mytopic";// 要查找的分区int partition = Integer.parseInt("0");// broker节点的ipList<String> seeds = new ArrayList<String>();seeds.add("192.168.4.30");seeds.add("192.168.4.31");seeds.add("192.168.4.32");// 端口int port = Integer.parseInt("9092");try {example.run(maxReads, topic, partition, seeds, port);} catch (Exception e) {System.out.println("Oops:" + e);e.printStackTrace();}}public void run(long a_maxReads, String a_topic, int a_partition, List<String> a_seedBrokers, int a_port) throws Exception {// 获取指定Topic partition的元数据PartitionMetadata metadata = findLeader(a_seedBrokers, a_port, a_topic, a_partition);if (metadata == null) {System.out.println("Can't find metadata for Topic and Partition. Exiting");return;}if (metadata.leader() == null) {System.out.println("Can't find Leader for Topic and Partition. Exiting");return;}String leadBroker = metadata.leader().host();String clientName = "Client_" + a_topic + "_" + a_partition;SimpleConsumer consumer = new SimpleConsumer(leadBroker, a_port, 100000, 64 * 1024, clientName);long readOffset = getLastOffset(consumer, a_topic, a_partition, kafka.api.OffsetRequest.EarliestTime(), clientName);int numErrors = 0;while (a_maxReads > 0) {if (consumer == null) {consumer = new SimpleConsumer(leadBroker, a_port, 100000, 64 * 1024, clientName);}FetchRequest req = new FetchRequestBuilder().clientId(clientName).addFetch(a_topic, a_partition, readOffset, 100000).build();FetchResponse fetchResponse = consumer.fetch(req);if (fetchResponse.hasError()) {numErrors++;// Something went wrong!short code = fetchResponse.errorCode(a_topic, a_partition);System.out.println("Error fetching data from the Broker:" + leadBroker + " Reason: " + code);if (numErrors > 5)break;if (code == ErrorMapping.OffsetOutOfRangeCode()) {// We asked for an invalid offset. For simple case ask for// the last element to resetreadOffset = getLastOffset(consumer, a_topic, a_partition, kafka.api.OffsetRequest.LatestTime(), clientName);continue;}consumer.close();consumer = null;leadBroker = findNewLeader(leadBroker, a_topic, a_partition, a_port);continue;}numErrors = 0;long numRead = 0;for (MessageAndOffset messageAndOffset : fetchResponse.messageSet(a_topic, a_partition)) {long currentOffset = messageAndOffset.offset();if (currentOffset < readOffset) {System.out.println("Found an old offset: " + currentOffset + " Expecting: " + readOffset);continue;}readOffset = messageAndOffset.nextOffset();ByteBuffer payload = messageAndOffset.message().payload();byte[] bytes = new byte[payload.limit()];payload.get(bytes);System.out.println(String.valueOf(messageAndOffset.offset()) + ": " + new String(bytes, "UTF-8"));numRead++;a_maxReads--;}if (numRead == 0) {try {Thread.sleep(1000);} catch (InterruptedException ie) {}}}if (consumer != null)consumer.close();}public static long getLastOffset(SimpleConsumer consumer, String topic, int partition, long whichTime, String clientName) {TopicAndPartition topicAndPartition = new TopicAndPartition(topic, partition);Map<TopicAndPartition, PartitionOffsetRequestInfo> requestInfo = new HashMap<TopicAndPartition, PartitionOffsetRequestInfo>();requestInfo.put(topicAndPartition, new PartitionOffsetRequestInfo(whichTime, 1));kafka.javaapi.OffsetRequest request = new kafka.javaapi.OffsetRequest(requestInfo, kafka.api.OffsetRequest.CurrentVersion(), clientName);OffsetResponse response = consumer.getOffsetsBefore(request);if (response.hasError()) {System.out.println("Error fetching data Offset Data the Broker. Reason: " + response.errorCode(topic, partition));return 0;}long[] offsets = response.offsets(topic, partition);return offsets[0];}/** * @param a_oldLeader * @param a_topic * @param a_partition * @param a_port * @return String * @throws Exception *             找一个leader broker */private String findNewLeader(String a_oldLeader, String a_topic, int a_partition, int a_port) throws Exception {for (int i = 0; i < 3; i++) {boolean goToSleep = false;PartitionMetadata metadata = findLeader(m_replicaBrokers, a_port, a_topic, a_partition);if (metadata == null) {goToSleep = true;} else if (metadata.leader() == null) {goToSleep = true;} else if (a_oldLeader.equalsIgnoreCase(metadata.leader().host()) && i == 0) {// first time through if the leader hasn't changed give// ZooKeeper a second to recover// second time, assume the broker did recover before failover,// or it was a non-Broker issue//goToSleep = true;} else {return metadata.leader().host();}if (goToSleep) {try {Thread.sleep(1000);} catch (InterruptedException ie) {}}}System.out.println("Unable to find new leader after Broker failure. Exiting");throw new Exception("Unable to find new leader after Broker failure. Exiting");}private PartitionMetadata findLeader(List<String> a_seedBrokers, int a_port, String a_topic, int a_partition) {PartitionMetadata returnMetaData = null;loop: for (String seed : a_seedBrokers) {SimpleConsumer consumer = null;try {consumer = new SimpleConsumer(seed, a_port, 100000, 64 * 1024, "leaderLookup");List<String> topics = Collections.singletonList(a_topic);TopicMetadataRequest req = new TopicMetadataRequest(topics);kafka.javaapi.TopicMetadataResponse resp = consumer.send(req);List<TopicMetadata> metaData = resp.topicsMetadata();for (TopicMetadata item : metaData) {for (PartitionMetadata part : item.partitionsMetadata()) {if (part.partitionId() == a_partition) {returnMetaData = part;break loop;}}}} catch (Exception e) {System.out.println("Error communicating with Broker [" + seed + "] to find Leader for [" + a_topic + ", " + a_partition + "] Reason: " + e);} finally {if (consumer != null)consumer.close();}}if (returnMetaData != null) {m_replicaBrokers.clear();for (kafka.cluster.Broker replica : returnMetaData.replicas()) {m_replicaBrokers.add(replica.host());}}return returnMetaData;}}