How to drain/delete/expire existing messages in Kafka

Some time you might have a bad record in Kafka topic that you want to delete. Kafka does not provide direct option to delete specific record. Only way to delete records is to expire them. You can achieve this by setting data retention to say 1 second that expires all the old messages. You can follow these steps

First check the topic to find out value of retention.ms config parameter for the topic

1. 
   kafka_2.11-0.11.0.1 spatil$ bin/kafka-configs.sh --zookeeper localhost:2181 --describe --entity-name my-topic --entity-type topics
   Configs for topic 'my-topic' are retention.ms=86400000
   

2. Change value of retention.ms to 1 which means all messages older than 1 ms will be expired

   
   kafka_2.11-0.11.0.1 spatil$ bin/kafka-configs.sh --zookeeper localhost:2181 --alter --entity-name my-topic --entity-type topics --add-config retention.ms=1
   Completed Updating config for entity: topic 'my-topic'
   

3. Wait for few seconds and monitor logs of the Kafka server to make sure that messages from the topic are deleted
4. Now change the value of retention.ms back to its original value which was 86400000 (7 days)

   
   kafka_2.11-0.11.0.1 spatil$ bin/kafka-configs.sh --zookeeper localhost:2181 --alter --entity-name my-topic --entity-type topics --add-config retention.ms=86400000
   Completed Updating config for entity: topic 'my-topic'.
   

5. You can verify that your changes are saved by running this command

   
   kafka_2.11-0.11.0.1 spatil$ bin/kafka-configs.sh --zookeeper localhost:2181 --describe --entity-name my-topic --entity-type topics
   Configs for topic 'my-topic' are retention.ms=86400000
   

Kafka how to reset number of partitions in a topics

I wanted to figure out how to reset number of partitions in my topic in Kafka and I followed these steps

1. I did create a sample topic called my-topic with single partition

   
   spatil$ bin/kafka-topics.sh --create --zookeeper localhost:2181 --topic my-topic --replication-factor 1 --partitions 1
   Created topic "my-topic".
   

2. I used describe command to verify that my topic has actually single partition

   
   spatil$ bin/kafka-topics.sh --zookeeper localhost:2181 --describe --topic my-topic
   Topic:my-topic PartitionCount:1 ReplicationFactor:1 Configs:
    Topic: my-topic Partition: 0 Leader: 0 Replicas: 0 Isr: 0
   

3. Then I did execute alter command on my partition and changed number of partitions from 1 to 3

   
   spatil$ bin/kafka-topics.sh --zookeeper localhost:2181 --alter --topic my-topic --partitions 3
   WARNING: If partitions are increased for a topic that has a key, the partition logic or ordering of the messages will be affected
   Adding partitions succeeded!
   

4. I did execute describe command on my topic to verify that it actually has 3 topics

   
   spatil$ bin/kafka-topics.sh --zookeeper localhost:2181 --describe --topic my-topic
   Topic:my-topic PartitionCount:3 ReplicationFactor:1 Configs:
    Topic: my-topic Partition: 0 Leader: 0 Replicas: 0 Isr: 0
    Topic: my-topic Partition: 1 Leader: 0 Replicas: 0 Isr: 0
    Topic: my-topic Partition: 2 Leader: 0 Replicas: 0 Isr: 0
   

How to reset consumer group offset

First run describe on topic to check what it the current LAG its zero in this case

bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group user.kafkaconsumer

Now run following command to just preview what will be the next offset if you reset

bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --group user.kafkaconsumer --reset-offsets --to-earliest --all-topics

Shutdown the consumer so that you can restart the consumer Now go back/reset the offset so that it goes back to first message 3bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --group user.kafkaconsumer --reset-offsets --to-earliest --all-topics --execute Go back and verify that the consumer offset actually went back by executing following command

bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group user.kafkaconsumer

You should be able to see the offset set back to 22000 which is start of first message in Kafka.