Local mode simulates a Storm cluster in process and is useful for developing and testing topologies. Running topologies in local mode is similar to running topologies on a cluster.

To create an in-process cluster, simply use the LocalCluster class. For example:

import backtype.storm.LocalCluster;

LocalCluster cluster = new LocalCluster();

You can then submit topologies using the submitTopology method on the LocalCluster object. Just like the corresponding method on StormSubmitter, submitTopology takes a name, a topology configuration, and the topology object. You can then kill a topology using the killTopology method which takes the topology name as an argument.

To shutdown a local cluster, simple call:

cluster.shutdown();

Common configurations for local mode

You can see a full list of configurations here.

1. Config.TOPOLOGY_MAX_TASK_PARALLELISM: This config puts a ceiling on the number of threads spawned for a single component. Oftentimes production topologies have a lot of parallelism (hundreds of threads) which places unreasonable load when trying to test the topology in local mode. This config lets you easy control that parallelism.
2. Config.TOPOLOGY_DEBUG: When this is set to true, Storm will log a message every time a tuple is emitted from any spout or bolt. This is extremely useful for debugging.