What are partitions in Kafka and how do you choose their number?

In Kafka, partitions are logical divisions of a topic that allow for scalable message processing and provide parallelism. Each partition is an ordered, immutable log of messages.

The number of partitions affects performance and parallelism:

• A higher number of partitions allows more consumers to read data in parallel, increasing throughput.
• However, too many partitions can lead to management overhead and increased latency.

When choosing the number of partitions, consider:

• Expected load and processing speed.
• The number of consumers that will read the topic.
• Hardware resources of brokers.
• Message processing order requirements (messages within a partition are ordered, but order across partitions is not guaranteed).

It is recommended to start with a number of partitions equal to the number of processing threads or consumers and scale as load grows.