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.
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-
https://kafka.apache.org/documentation/#consumerconfigs
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.
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I have a consumer polling from subscribed topic. It consumes each message and does some processing (within seconds), pushes to different topic and commits offset.
There are totally 5000 messages,
before restart - consumed 2900 messages and committed offset
after restart - started consuming from offset 0.
Even though consumer is created with same consumer group, it started processing messages from offset 0.
kafka version (strimzi) > 2.0.0
kafka-python == 2.0.1
We don't know how many partitions you have in your topic but when consumers are created within a same consumer group, they will consume records from different partitions ( We can't have two consumers in a consumer group that consume from the same partition and If you add a consumer the group coordinator will execute the process of Re-balancing to reassign each consumer to a specific partition).
I think the offset 0 comes from the property auto.offset.reset which can be :
latest: Start at the latest offset in log
earliest: Start with the earliest record.
none: Throw an exception when there is no existing offset data.
But this property kicks in only if your consumer group doesn't have a valid offset committed.
N.B: Records in a topic have a retention period log.retention.ms property so your latest messages could be deleted when your are processing the first records in the log.
Questions: While you want to consume message from one topic and process data and write them to another topic why you didn't use Kafka Streaming ?
Suppose:
Kakfa consumer reads a message M from its assigned partition P
It gets network-partitioned away from the broker
Kafka detects this and reassigns P to a consumer on another machine.
Network-partition is healed, and the first consumer tries to commit the offset for message M
Will there be any exception thrown in step 4? Is there a check which detects that the first consumer is no longer assigned partition P, and so shouldn't be committing offsets for it?
In this case, the first consumer can not commit off set anymore. It's assigned partition will be revoked and when it join consumer group again, a rebalancing process will be trigger. So the answer is yes.
https://cwiki.apache.org/confluence/display/KAFKA/A+Guide+To+The+Kafka+Protocol
The offsets for a given consumer group are maintained by a specific broker called the group coordinator. i.e., a consumer needs to issue its offset commit and fetch requests to this specific broker
Since one group coordinator is responsible and it know which consumers have partition assigned and which consumers have no longer the partition assigned
When they rejoin, they are treated in the same way as a brand new consumer joining the consumer group. They will get one or more partitions assigned by rebalancing (which may be totally different from the partitions they were reading from last time) and the consumer_offset topic will inform them where to start reading from.
I have a use case where i have 2 consumers in different consumer groups(cg1 and cg2) subscribing to same topic(Topic A) with 4 partitions.
What happens if both consumers are reading from same partition and one of them failed and other one commited the offset?
In Kafka the offset management is done by Consumer Group per Partition.
If you have two consumer groups reading the same topic and even partition a commit from one consumer group will not have any impact to the other consumer group. The consumer groups are completely discoupled.
One consumer of a consumer group can read data from a single topic partition. A single consumer can't read data from multiple partitions of a topic.
Example Consumer 1 of Consumer Group 1 can read data of only single topic partition.
Offset management is done by the zookeeper.
__consumer_offsets: Every consumer group maintains its offset per topic partitions. Since v0.9 the information of committed offsets for every consumer group is stored in this internal topic (prior to v0.9 this information was stored on Zookeeper).
When the offset manager receives an OffsetCommitRequest, it appends the request to a special compacted Kafka topic named __consumer_offsets. Finally, the offset manager will send a successful offset commit response to the consumer, only when all the replicas of the offsets topic receive the offsets.
simultaneously two consumers from two different consumer groups(cg1 and cg2) can read the data from same topic.
In kafka 1: Offset management is taken care by zookeeper.
In kafka 2: offsets of each consumer is stored at __Consumer_offsets topic
Offset used for keeping the track of consumers (how much records consumed by consumers), let say consumer-1 consume 10 records and consumer-2 consume-20 records and suddenly consumer-1 got died now whenever the consumer-1 will up then it will start reading from 11th record onward.
I am writing a kafka consumer application. I have a topic with 4 partitions - 1 is leader and 3 are followers. Producer uses key to identify a partition to push a message.
If I write a consumer and run it on different nodes or start 4 instances of same consumer, how message consuming will happen ? Does all 4 instances will get same messages ?
What happens in the case of multiple consumer(same group) consuming a single topic?
Do they get same data?
How offset is managed? Is it separate for each consumer?
I would suggest that you read at least first few chapters of confluent's definitive guide to kafka to get a priliminary understanding of how kafka works.
I've kept my answers brief. Please refer to the book for detailed explanation.
How offset is managed? Is it separate for each consumer?
Depends on the group id. Only one offset is managed for a group.
What happens in the case of multiple consumer(same group) consuming a single topic?
Consumers can be multiple - all can be identified by the same or different groups.
If 2 consumers belong to the same group, both will not get all messages.
Do they get same data?
No. Once a message is sent and a read is committed, the offset is incremented for that group. So a different consumer with the same group will not receive that message.
Hope that helps :)
What happens in the case of multiple consumer(same group) consuming a single topic?
Answer: Producers send records to a particular partition based on the record’s key here. The default partitioner for Java uses a hash of the record’s key to choose the partition. When there are multiple consumers in same consumer group, each consumer gets different partition. So, in this case, only single consumer receives all the messages. When the consumer which is receiving messages goes down, group coordinator (one of the brokers in the cluster) triggers rebalance and then that partition is assigned to one of the available consumer.
Do they get same data?
Answer: If consumer commits consumed messages to partition and goes down, so as stated above, rebalance occurs. The consumer who gets this partition, will not get messages. But if consumer goes down before committing its then the consumer who gets this partition, will get messages.
How offset is managed? Is it separate for each consumer?
Answer: No, offset is not separate to each consumer. Partition never gets assigned to multiple consumers in same consumer group at a time. The consumer who gets partition assigned, gets offset as well by default.
I have a single Topic with 5 partitions.
I have 5 threads, each creating a Consumer
All consumer are with the same consumer group using group.id.
I also gave each consumer a different and unique client.id
I see that 2 consumers are reading the same message to process
Should kafka handle this?
How do I troubleshoot it?
Consumers within the same group should not receive the same messages. The partitions should be split across all consumers and at any time Kafka's consumer group logic ensures only 1 consumer is assigned to each partition.
The exception is if 1 consumer crashes before it's able to commit its offset. In that case, the new consumer that gets assigned the partition will re-consume from the last committed offset.
You can use the consumer group tool kafka-consumer-groups that comes with Kafka to check the partitions assigned to each consumer in your group.