Core Storm Concepts

Developing a Storm application requires an understanding of the following basic concepts.

Table 1. Storm Concepts
Storm Concept	Description
Tuple	A named list of values of any data type. A tuple is the native data structure used by Storm.
Stream	An unbounded sequence of tuples.
Spout	Generates a stream from a realtime data source.
Bolt	Contains data processing, persistence, and messaging alert logic. Can also emit tuples for downstream bolts.
Stream Grouping	Controls the routing of tuples to bolts for processing.
Topology	A group of spouts and bolts wired together into a workflow. A Storm application.
Processing Reliability	Storm guarantee about the delivery of tuples in a topology.
Workers	A Storm process. A worker may run one or more executors.
Executors	A Storm thread launched by a Storm worker. An executor may run one or more tasks.
Tasks	A Storm job from a spout or bolt.
Parallelism	Attribute of distributed data processing that determines how many jobs are processed simultaneously for a topology. Topology developers adjust parallelism to tune their applications.
Process Controller	Monitors and restarts failed Storm processes. Examples include supervisord, monit, and daemontools.
Master/Nimbus Node	The host in a multi-node Storm cluster that runs a process controller (such as supervisord) and the Storm nimbus, ui, and other related daemons. The process controller is responsible for restarting failed process controller daemons on slave nodes. The Nimbus node is a thrift service that is responsible for distributing code around the cluster, assigning tasks to machines, and monitoring for failures.
Slave Node	A host in a multi-node Storm cluster that runs a process controller daemon, such as supervisor, as well as the worker processes that run Storm topologies. The process controller daemon is responsible for restarting failed worker processes.

The following subsections describe several of these concepts in more detail.