Kafka: The Definitive Guide

Chapter 4. Kafka Consumers: Reading Data from Kafka

Applications that need to read data from Kafka use a KafkaConsumer to subscribe to Kafka topics and receive messages from these topics. Reading data from Kafka is a bit different than reading data from other messaging systems, and there are few unique concepts and ideas involved. It is difficult to understand how to use the consumer API without understanding these concepts first. We’ll start by explaining some of the important concepts, and then we’ll go through some examples that show the different ways consumer APIs can be used to implement applications with varying requirements.

Kafka Consumer Concepts

In order to understand how to read data from Kafka, you first need to understand its consumers and consumer groups. The following sections cover those concepts.

Consumers and Consumer Groups

Suppose you have an application that needs to read messages from a Kafka topic, run some validations against them, and write the results to another data store. In this case your application will create a consumer object, subscribe to the appropriate topic, and start receiving messages, validating them and writing the results. This may work well for a while, but what if the rate at which producers write messages to the topic exceeds the rate at which your application can validate them? If you are limited to a single consumer reading and processing the data, your application may fall farther and farther behind, unable to keep up with the ...