When we have multiple consumer reading from the topic with single partition Is there any possibility that all the consumer will get all the message. I have created the two consumers with manual offset commit.started the first consumer and after 2 mins started 2nd consumer . The second consumer is reading from the message from where the 1st consumer stopped reading. Is there any possibility that the 2nd consumer will read all the message from beginning.I'm new to kafka please help me out.
0
votes
2 Answers
1
votes
In your consumer, you would be using commitSync which commits offset returned on last poll. Now, when you start your 2nd consumer, since it is in same consumer group it will read messages from last committed offset.
Messages which your consumer will consumes depends on the ConsumerGroup it belongs to. Suppose you have 2 partitions and 2 consumers in single Consumer Group, then each consumer will read from different partitions which helps to achieve parallelism.
So, if you want your 2nd consumer to read from beginning, you can do one of 2 things:
a) Try putting 2nd consumer in different consumer group. For this consumer group, there won't be any offset stored anywhere. At this time, auto.offset.reset config will decide the starting offset. Set auto.offset.reset to earliest(reset the offset to earliest offset) or to latest(reset the offset to latest offset).
b) Seek to start of all partitions your consumer is assigned by using: consumer.seekToBeginning(consumer.assignment())
Documentation: https://kafka.apache.org/11/javadoc/org/apache/kafka/clients/consumer/KafkaConsumer.html#seekToBeginning-java.util.Collection-
0
votes
Partition is always assigned to unique consumer in single consumer group irrespective of multiplpe consumers. It means only that consumer can read the data and others won't consume data until the partition is assigned to them. When consumer goes down, partition rebalance happens and it will be assigned to another consumer. Since you are performing manual commit, new consumer will start reading from committed offset.
