Kafka parallel consuming

For very large topics when you need more parallelism (especially on the insert side) you may use several tables with the same pipeline (pre ClickHouse® 20.9) or enable kafka_thread_per_consumer (after 20.9).

kafka_num_consumers = N,
kafka_thread_per_consumer=1

Notes:

• the inserts will happen in parallel (without that setting inserts happen linearly)
• enough partitions are needed.
• kafka_num_consumers is limited by number of physical cores (half of vCPUs). kafka_disable_num_consumers_limit can be used to override the limit.
• background_message_broker_schedule_pool_size is 16 by default, you may need to increase if using more than 16 consumers

Before increasing kafka_num_consumers with keeping kafka_thread_per_consumer=0 may improve consumption & parsing speed, but flushing & committing still happens by a single thread there (so inserts are linear).