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title: Hooks layout: documentation documentation: true

Task hooks

Storm provides hooks with which you can insert custom code to run on any number of events within Storm. You create a hook by extending the BaseTaskHook class and overriding the appropriate method for the event you want to catch. There are two ways to register your hook:

1. In the open method of your spout or prepare method of your bolt using the TopologyContext method.
2. Through the Storm configuration using the “topology.auto.task.hooks” config. These hooks are automatically registered in every spout or bolt, and are useful for doing things like integrating with a custom monitoring system.

Worker hooks

Storm also provides worker-level hooks that are called during worker startup, before any bolts or spouts are prepared/opened. You can create such a hook by extending BaseWorkerHook (an implementation of IWorkerHook) and overriding the methods you want to implement. You can register your hook via TopologyBuilder.addWorkerHook.
The IWorkerHook#start(Map, WorkerUserContext) lifecycle method exposes WorkerUserContext which provides a way to set application-level common resources via setResource(String, Object) method. This resource can then be retrieved by tasks, both spouts (via open(Map, TopologyContext, SpoutOutputCollector) and bolts (via prepare(Map, TopologyContext, OutputCollector), by calling TopologyContext#getResource(String).

Shared State amongst components and hooks

Storm provides ways to share resources across different components via the following ways:

1. taskData: this pertains to the task level data and can be written and read by task and task hooks in their corresponding lifecycle methods (open for spout and prepare for bolt and task hook).
   1. write access: TopologyContext#setTaskData(String, Object)
   2. read access: TopologyContext#getTask(String)
2. executorData: this pertains to executor level data and is shared across tasks and task hooks which are managed by the concerned executor. Similar to above it is accessible to spouts via open and to bolts and task hooks via prepare lifecycle method.
   1. write access: TopologyContext#setExecutorData
   2. read access: TopologyContext#getExecutorData(String)
3. userResources: this pertains to worker level data and is shared across executors, tasks, worker hooks and task hooks which are managed by the concerned worker. Unlike others it can only be written by worker hooks.
   1. write access: WorkerUserContext#setResource(String, Object)
   2. read access: WorkerTopologyContext#getResouce(String) or TopologyContext#getResource(String)