Storm distinguishes between the following three main entities that are used to actually run a topology in a Storm cluster:
- Worker processes
- Executors (threads)
- Tasks
A worker process executes a subset of a topology. A worker process belongs to a specific topology and may run one or more executors for one or more components (spouts or bolts) of this topology. A running topology consists of many such processes running on many machines within a Storm cluster.
An executor is a thread that is spawned by a worker process. It may run one or more tasks for the same component (spout or bolt).
A task performs the actual data processing — each spout or bolt that you implement in your code executes as many tasks across the cluster. The number of tasks for a component is always the same throughout the lifetime of a topology, but the number of executors (threads) for a component can change over time.