Partition ordering guarantees in Kafka - Commands & Configuration

Introduction

When sending messages in Kafka, it is important to know how the order of messages is kept. Partition ordering guarantees explain how Kafka keeps messages in the same order within a partition, so consumers read them in the same sequence they were sent.

When you want to process events in the exact order they happened, like user actions or transactions.

When you have multiple producers sending messages but need to keep order for each key or category.

When building systems that rely on ordered logs or event streams for accurate results.

When troubleshooting message order issues in your Kafka consumers.

When designing topics and partitions to ensure correct message flow.

Commands

Create a Kafka topic named 'example-topic' with 3 partitions to distribute messages and allow ordering within each partition.

Terminal

kafka-topics.sh --create --topic example-topic --bootstrap-server localhost:9092 --partitions 3 --replication-factor 1

Expected Output

Created topic example-topic.

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--partitions - Sets number of partitions for the topic

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--replication-factor - Sets how many copies of data are kept for fault tolerance

Start a producer that sends messages with keys separated by ':' to ensure messages with the same key go to the same partition, preserving order per key.

Terminal

kafka-console-producer.sh --topic example-topic --bootstrap-server localhost:9092 --property parse.key=true --property key.separator=:

Expected Output

No output (command runs silently)

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parse.key=true - Enables key parsing for messages

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key.separator=: - Defines the separator between key and message

Start a consumer that reads all messages from the beginning, showing keys to verify the order of messages per partition.

Terminal

kafka-console-consumer.sh --topic example-topic --bootstrap-server localhost:9092 --from-beginning --property print.key=true --property key.separator=:

Expected Output

key1:message1

key1:message2

key2:message3

key1:message4

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--from-beginning - Reads messages from the start of the topic

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print.key=true - Shows the key of each message in output

Key Concept

Kafka guarantees that messages with the same key are stored and read in order within the same partition.

Common Mistakes

Sending messages without keys expecting global ordering.

Without keys, messages are distributed across partitions randomly, so order is only guaranteed within each partition, not across the whole topic.

Always send messages with keys when order matters, so Kafka routes them to the same partition.

Using too few partitions and expecting high throughput.

Fewer partitions limit parallelism and throughput, but more partitions can split keys across partitions if keys are not used properly.

Balance the number of partitions and use keys to control ordering and throughput.

Summary

Create topics with multiple partitions to allow parallel processing and ordering within partitions.

Send messages with keys so Kafka routes them to the same partition, preserving order per key.

Consume messages from the beginning with keys shown to verify ordering guarantees.