Consumer group

As explained in Troubleshooting Java, Second Edition

A consumer group is a set of consumers that cooperatively processes the same Kafka topic stream.

A consumer group is a set of consumers that cooperatively processes the same Kafka topic stream. Kafka assigns each consumer a portion of the topic, typically a partition, so services can divide message processing. section 13.2.2

Consumer groups help investigate missing messages by exposing offsets: bookmarks showing how far a service has progressed. An unchanged offset can cause reprocessing, an advanced offset can skip earlier messages, and an invalid topic or partition can produce no messages. section 13.2.2

Diagnosing missing welcome emails

An email service belongs to a consumer group reading user events. Audit logs and the database confirm UserRegistered events, but the service receives none. Checking the group configuration reveals that a deployment selected the wrong topic. Correcting the topic in a local test environment and replaying missed events with Kafka console tools validates the fix. section 13.2.2

Figure 13.6 A developer can choose to replay events to troubleshoot a certain scenario.

Use the event-replay diagram to connect consumer-group diagnosis with safely replaying missed Kafka events. section 13.2.2

How Troubleshooting Java, Second Edition covers it

How deep it goes

Explains cooperative processing, partition assignment, offset behavior, and configuration diagnosis, but not consumer-group implementation code.

Examples

Uses a missing welcome-email incident, a misconfigured topic, consumer-group offsets, and replayed UserRegistered events.

What it assumes

Assumes basic familiarity with Kafka topics and messages while defining consumer groups, partitions, and offsets during the investigation.

What you can do after

Diagnose missing Kafka events by checking consumer-group offsets and topic configuration, then validate recovery through event replay.

How it connects

Connects consumer groups with Kafka topics, partitions, offsets, event logs, message replay, and event-driven architecture.

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Also explained in

• 

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  Kafka for ArchitectsA shared consumer group is a Kafka consumption mode that permits multiple consumers in a group to receive records from the same partition.
• 

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  Apache Kafka in ActionA consumer group is a set of consumers that collaboratively consume topic partitions, with each partition assigned to one consumer in the group.
• 

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  Kafka Streams in Action, Second EditionA consumer group is a set of Kafka consumers that jointly consume a topic's partitions, with each partition assigned to at most one group member at a time.

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Related concepts

• group.idconfigured by · group.id is a Kafka consumer configuration property that identifies the consumer group whose partition assignments and consumption progress the consumer participates in.
• offsetuses · An offset is a position or displacement measured relative to a reference point, such as within an ordered sequence, memory region, or layout.
• partitionuses · A partition is a distinct subdivision of a larger data, storage, or work unit that can be stored, managed, or processed separately.
• Apache Kafkaused by · Apache Kafka is a distributed event-streaming platform for publishing, storing, processing, and consuming streams of records.
• publish/subscribe modelimplements · The publish-subscribe model is a messaging pattern in which publishers send messages without addressing specific recipients, and subscribers receive messages according to their interests or subscriptions.

This explainer was generated by Manning's knowledge graph from the text of Troubleshooting Java, Second Edition. The section links point to the parts of the book it draws on.