Tuning Guide
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Chapter 2. General Tuning Suggestions

Note that there is currently no method for specifying the number of tasks from the number of executors in Flux topologies (enrichment, indexing). By default, the number of tasks will equal the number of executors. Logically, setting the number of tasks equal to the number of executors is sensible. Storm enforces num executors <= num tasks. The reason you might set the number of tasks higher than the number of executors is for future performance tuning and rebalancing without the need to bring down your topologies. The number of tasks is fixed at topology startup time whereas the number of executors can be increased up to a maximum value equal to the number of tasks.

When configuring Storm Kafka spouts, we found that the default values for poll.timeout.ms, offset.commit.period.ms, and max.uncommitted.offsets worked well in nearly all cases. As a general rule, it was optimal to set spout parallelism equal to the number of partitions used in your Kafka topic. Any greater parallelism will leave you with idle consumers since Kafka limits the maximum number of consumers to the number of partitions. This is important because Kafka has certain ordering guarantees for message delivery per partition that would not be possible if more than one consumer in a given consumer group were able to read from that partition.