What is a partition in Kafka and why are they important?

A topic is partitioned: it is spread over a number of buckets located on different brokers, and each new event is appended to one of the topic's partitions. This lets clients read and write many brokers at once, so partitions are how a topic scales. Each partition is a totally ordered log, and events with the same key go to the same partition, so ordering holds within a partition but not across the whole topic. Each partition is consumed by exactly one consumer within each subscribing consumer group at any given time, so the partition count limits how many consumers in a group can be active at once: extra consumers get no partitions. Partitions can be added to a topic later, but adding them does not change the partitioning of existing data, and Kafka does not support reducing their number.

Expected key concepts: partition, brokers, appended, totally ordered, same key, consumer group, exactly one consumer, scales, adding them