I did not use a partition to publish to Kafka topic.
ProducerRecord(String topic, K key, V value)
In the consumer, I would like to go to the beginning.
seekToBeginning(Collection partitions)
Is it possible to seek to beginning without using a partition? Does Kafka assign a default partition?
https://kafka.apache.org/0102/javadoc/org/apache/kafka/clients/producer/ProducerRecord.html
https://kafka.apache.org/0102/javadoc/org/apache/kafka/clients/consumer/KafkaConsumer.html
When producing, if you don't explicitely specify a partition, the producer will pick one automatically from your topic.
In your consumer, if your are subscribed to your topic, you can seek to the start of all the partitions your consumer is currently assigned to using:
consumer.seekToBeginning(consumer.assignment())
Related
I have few questions on Apache Kafka.
Can a single partition be assigned to more than one consumer from the same group?
Where is the offset stored? Is it in the partition or at the consumer.
Just like the producer always post the record to the lead partition and the records gets replicated to other partitions, Does Kafka consumer reads the data from the lead partition?
Lets say, that a consumer is reading from a partition and the consumer is running a long process. In this case, the rate at which the producer is updating the partition will be faster than the rate at which the consumer is consuming from the same partition. Is there a way we can speed up the consumption from that partition?
Can we create a checkpoint in the commit log on the partition so that the consumer can start processing from that specific checkpoint? This would be useful, if I want to perform the audit from a specific checkpoint onward?
Can a single partition be assigned to more than one consumer from the same group?
No, one partition can be consumed at most from one consumer within the same consumer group as described here: "This is achieved by assigning the partitions in the topic to the consumers in the consumer group so that each partition is consumed by exactly one consumer in the group."
Where is the offset stored? Is it in the partition or at the consumer.
The offsets for each consumer group is stored in an internal kafka topic called __consumer_offsets as described here: "The coordinator of each group is chosen from the leaders of the internal offsets topic __consumer_offsets, which is used to store committed offsets."
Just like the producer always post the record to the lead partition and the records gets replicated to other partitions, Does Kafka consumer reads the data from the lead partition?
Yes it does. The leader partition is the only "client-facing" partition as described here: "'leader' is the node responsible for all reads and writes for the given partition.".
EDIT:
Is there a way we can speed up the consumption from that partition?
The measure to speed up consumption is to increase the partitions of the topic so you can have more consumer threads reading from that topic and process the data in parallel. At the same time you need to make sure that your data is evenly distributed accross partitions.
I know that all the messages (or offset) in a Kafka Queue Partition has its offset number and it takes care of the sequence of offsets.
But if I have a Kafka Consumer Group (or single Kafka Consumer) which is reading particularly the Kafka Topic Partition then how it maintains up to which offset messages are read and who maintains this offset counter?
If the consumer goes down then how a new consumer will start reading the offset from the next unread (or not acknowledged) offset.
The information about Consumer Groups is all stored in the internal Kafka topic __consumer_offsets. Whenever a new group tries to read data from a topic it checks its offset position in that internal topic which has a deletion policy set to compact. The compaction keeps this topic small.
Kafka comes with a command line tool kafka-consumer-groups.sh that helps you understand which information is stored for each consumer group.
More information is given in the Kafka Documentation on offset tracking.
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.
Just wanna understand the basics properly.
Let's say I've a topic called "myTopic" that has 3 partitions P0, P1 & P2.
Each of these partitions will have a leader and the data (messages) for this topic is distributed across these partitions.
1. Producer will always writes to the leader of the partition in a round robin fashion based on the load on the broker. Is that right?
2. How do the producer know the leader of the partition?
3. Consumer reading a particular topic should read all partitions of that topic? Is that correct?
Appreciate your help.
Producer will always writes to the leader of the partition in a round robin fashion based on the load on the broker. Is that right?
By default, yes.
That said, a producer can also decide to use a custom partitioning scheme, i.e. a different strategy to which partitions data is being written to.
How do the producer know the leader of the partition?
Through the Kafka protocol.
Consumer reading a particular topic should read all partitions of that topic? Is that correct?
By default, yes.
That said, you can also implement e.g. consumer applications that implement custom logic, e.g. a "sampling" consumer that only reads from 1 out of N partitions.
Producer will always writes to the leader of the partition
Yes, always.
in a round robin fashion based on the load on the broker
No. If a partition is explicitly set on a ProducerRecord then that partition is used. Otherwise, if a custom partitioner implementation is provided, that determines the partition. Otherwise, if the msg key is not null, the hash of the key will be used to consistently send msgs with the same key to the same partition. If the msg key is null, only then the msg will indeed be sent to any partition in a round-robin fashion. However, this is irrespective of the load on the broker.
How do the producer know the leader of the partition?
By periodically asking the broker for metadata.
Consumer reading a particular topic should read all partitions of that topic? Is that correct?
Consumers form consumer groups. If there are multiple consumer instances in a consumer group, each consumes a subset of the partitions. But the consumer group as a whole consumes from all partitions. That is, unless you decide to go "low-level" and manage that yourself, which you can do.
I want any information/explanation on how Kafka maintains a message sequence when messages are written to topic with multiple partition.
For e.g. I have multiple message producer each producing messages sequentially and writing on the Kafka topic with more than 1 partition. In this case, how consumer group will work to consume messages.
Kafka only provides a total order over records within a partition, not between different partitions in a topic. Even within one partition, you still could encounter the out-of-order events if retries is enabled and max.in.flight.requests.per.connection is larger than 1.
Work-around is create a topic with only one partition although it means only one consumer process per consumer group.
Kafka will store messages in the partitions according to the message key given to the producer. If none is given, then the messages will be written in a round-robin style into the partitions. To keep ordering for a topic, you need to make sure the ordered sequence has the same key, or that the topic has only one partition.