Executor

Within each worker process, there can be multiple threads executing parts of the topology. Each of these threads is called an executor. An executor can execute only one of the components, that is, any spout or bolt in the topology.

Each executor, being a single thread, can execute only tasks assigned to it serially. The number of executors defined for a spout or bolt can be changed dynamically while the topology is running, which means that you can easily control the degree of parallelism of various components in your topology.