Kafka如何获取topic最近n条消息

刺骨的言语ヽ痛彻心扉 2022-03-25 10:40 860阅读 0赞

# 问题来源 #

项目运行中我们经常需要诊断个个环节是否正确，其中到kafka就需要查看最新的消息到达kafka没有，达到的内容是什么，这就需要查看kafka指定topic的最近的n条消息（将kakfa消息全部打印出来非常耗时而且不必要）。当然我们可以使用第三方提供的kafka工具查看，可是使用这些工具耗时费力而且不能很好集成到项目中。

备注：第三方工具包括kakfa命令行工具一起其他第三方的工具。  
**注意事项**：本博客所有代码是为了介绍相关内容而编写或者引用的，示例代码并非可直接用于生产的代码。仅供参考而已。

# 大体思路 #

我们知道如果使用KafkaConsumer类，要么从topic最老的消息开始，要么从最新的消息开始。下面是官方文档中关于auto.offset.reset 的解释。

The default is “latest,” which means that lacking a valid offset,  
the consumer will start reading from the newest records (records that were written after the consumer started running).  
The alternative is “earliest,” which means that lacking a valid offset, the consumer will read all the data in the partition,  
starting from the very beginning.

因此直接使用KafkaConsumer就行不通了。 但是我们可以先通过给指定coisumerGroup设置offset，然后在让KafkaConsumer以该coisumerGroup读取数据。这样就能从指定offset开始读取消息了。

# 具体代码 #

完整的代码在[这里][Link 1]，欢迎加星和fork。 谢谢！

package com.yq.consumer;
    
    import lombok.extern.slf4j.Slf4j;
    import org.apache.kafka.clients.admin.AdminClient;
    import org.apache.kafka.clients.admin.DescribeTopicsResult;
    import org.apache.kafka.clients.admin.TopicDescription;
    import org.apache.kafka.clients.consumer.ConsumerConfig;
    import org.apache.kafka.clients.consumer.ConsumerRecord;
    import org.apache.kafka.clients.consumer.ConsumerRecords;
    import org.apache.kafka.clients.consumer.KafkaConsumer;
    import org.apache.kafka.clients.consumer.OffsetAndMetadata;
    import org.apache.kafka.common.KafkaFuture;
    import org.apache.kafka.common.Node;
    import org.apache.kafka.common.TopicPartition;
    import org.apache.kafka.common.TopicPartitionInfo;
    
    import java.util.Arrays;
    import java.util.Collections;
    import java.util.Iterator;
    import java.util.List;
    import java.util.Map;
    import java.util.Properties;
    
    /** * 本例子试着读取当前topic每个分区内最新的30条消息(如果topic额分区内有没有30条，就获取实际消息) * className: ReceiveLatestMessageMain * * @author EricYang * @version 2019/01/10 11:30 */
    @Slf4j
    public class ReceiveLatestMessageMain { 
        private static final int COUNT = 30;
        public static void main(String... args) throws Exception { 
            Properties props = new Properties();
    
            props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "ubuntu:9092");
            props.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
            props.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
    
            props.put(ConsumerConfig.GROUP_ID_CONFIG, "yq-consumer12");
            props.put(ConsumerConfig.MAX_POLL_RECORDS_CONFIG, 20);
            props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, false);
            props.put(ConsumerConfig.AUTO_COMMIT_INTERVAL_MS_CONFIG, "1000");
            props.put(ConsumerConfig.SESSION_TIMEOUT_MS_CONFIG, "30000");
            props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "latest");
    
    
            System.out.println("create KafkaConsumer");
            final KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
            AdminClient adminClient = AdminClient.create(props);
            String topic = "topic01";
            adminClient.describeTopics(Arrays.asList(topic));
            try { 
                DescribeTopicsResult topicResult = adminClient.describeTopics(Arrays.asList(topic));
                Map<String, KafkaFuture<TopicDescription>> descMap = topicResult.values();
                Iterator<Map.Entry<String, KafkaFuture<TopicDescription>>> itr = descMap.entrySet().iterator();
                while(itr.hasNext()) { 
                    Map.Entry<String, KafkaFuture<TopicDescription>> entry = itr.next();
                    System.out.println("key: " + entry.getKey());
                    List<TopicPartitionInfo> topicPartitionInfoList = entry.getValue().get().partitions();
                    topicPartitionInfoList.forEach((e) -> { 
                        int partitionId = e.partition();
                        Node node  = e.leader();
                        TopicPartition topicPartition = new TopicPartition(topic, partitionId);
                        Map<TopicPartition, Long> mapBeginning = consumer.beginningOffsets(Arrays.asList(topicPartition));
                        Iterator<Map.Entry<TopicPartition, Long>> itr2 = mapBeginning.entrySet().iterator();
                        long beginOffset = 0;
                        //mapBeginning只有一个元素，因为Arrays.asList(topicPartition)只有一个topicPartition
                        while(itr2.hasNext()) { 
                            Map.Entry<TopicPartition, Long> tmpEntry = itr2.next();
                            beginOffset =  tmpEntry.getValue();
                        }
                        Map<TopicPartition, Long> mapEnd = consumer.endOffsets(Arrays.asList(topicPartition));
                        Iterator<Map.Entry<TopicPartition, Long>> itr3 = mapEnd.entrySet().iterator();
                        long lastOffset = 0;
                        while(itr3.hasNext()) { 
                            Map.Entry<TopicPartition, Long> tmpEntry2 = itr3.next();
                            lastOffset = tmpEntry2.getValue();
                        }
                        long expectedOffSet = lastOffset - COUNT;
                        expectedOffSet = expectedOffSet > 0? expectedOffSet : 1;
                        System.out.println("Leader of partitionId: " + partitionId + " is " + node + ". expectedOffSet:"+ expectedOffSet
                        + "， beginOffset:" + beginOffset + ", lastOffset:" + lastOffset);
                        consumer.commitSync(Collections.singletonMap(topicPartition, new OffsetAndMetadata(expectedOffSet -1 )));
                    });
                }
    
                consumer.subscribe(Arrays.asList(topic));
                while (true) { 
                    ConsumerRecords<String, String> records = consumer.poll(100);
                    for (ConsumerRecord<String, String> record : records) { 
                        System.out.printf("read offset =%d, key=%s , value= %s, partition=%s\n",
                                record.offset(), record.key(), record.value(), record.partition());
                    }
                }
            } catch (Exception ex) { 
                ex.printStackTrace();
                System.out.println("when calling kafka output error." + ex.getMessage());
            } finally { 
                adminClient.close();
                consumer.close();
            }
        }
    }

# 运行效果 #

效果图解释：  
我们只读取topic01每个分区最近的30条消息，如果该partition没有30条，那就读取全部的。 可以看到partition 0有39条消息，因此从第8条开始读取。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3J1c3NsZQ_size_16_color_FFFFFF_t_70]

[Link 1]: https://github.com/yqbjtu/KafkaTutorials/blob/master/KafkaCLI/src/main/java/com/yq/consumer/ReceiveLatestMessageMain.java
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