Updated February 2023
In this article we provide succinct answers to common Apache Kafka consumer questions. You will learn the basics of how Kafka consumer groups work, how many consumers you can have per topic, and other important Kafka consumer facts.
Answers to commonly asked questions about Kafka consumers
Can a Kafka consumer read from multiple topics?
Yes, a single consumer or consumer group can read from multiple Kafka topics. We recommend using different consumer IDs and consumer groups per topic.
How do consumer groups work in Kafka?
One or more consumers can belong to a consumer group. All consumers within the consumer group will share a group id.
If a consumer group is made up of just one consumer, then it is referred to as an exclusive consumer. An exclusive consumer subscribes to all of the topic partitions it needs.
For consumer groups with two or more consumers, the workload is divided. Only one consumer within the group will subscribe to an individual topic partition.
If a consumer drops off or a new partition is added, then the consumer group will automatically rebalance the workload.
How does a Kafka consumer work?
Kafka consumers are the applications that read data from Kafka servers. Kafka consumers will subscribe to specific topics or topic partitions and retrieve messages from those topics in real-time.
Click here to learn more about how to optimize a Kafka consumer.
How many consumers can Kafka have?
Kafka can have multiple consumers subscribed to the same topic. When multiple consumers are subscribed to a topic, Kafka behaves like a pub/sub model with messages being received by multiple consumers. You can read more about this structure in our post on How Kafka Uses Consumer Groups for Scaling.
As far as the limit to the number of consumers, you should only have as many consumers of a topic as there are partitions of a topic. A partition can only send data to a single consumer. If there are 20 consumers in a single consumer group for a topic that has only 10 partitions, then only ten of the consumers will receive data.
The reason why the number of consumers is always equal to or less than the number of partitions is due to how Kafka guarantees message order. Learn more about Kafka consumers’ role in guaranteeing message order.
How to check if a Kafka consumer is running?
On the application side, if the consumer is pulling data, then you can tell that the consumer is running.
On the Kafka broker side, you can tell consumers are running by looking at the consumer group lag monitoring data. If consumer group lag is near zero, then the consumers are running. If the consumer group lag is falling further and further behind, then the consumers are either not running or are processing data slower than data is being queued in Kafka.
How to check Kafka consumer lag?
You will want to set up real-time monitoring of your Kafka implementation to monitor its status, including consumer lag. We have an article that covers how to set up Kafka monitoring using either Elasticsearch or OpenSearch.
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