Component instances progress through four major lifecycle events: create, start, stop, and destroy.
Component instances may retain isolated persistent state on a storage medium while they are not running which helps them maintain the illusion of continuity across restarts.
Creating a component instance
When a component instance is created, the component frameworks assigns a unique identity to the instance, adds it to the component topology, and makes its capabilities available for other components to use.
Once created, a component instance can then be started or destroyed.
Starting a component instance
Starting a component instance loads and runs the component's program and provides it access to the capabilities that it requires.
Every component runs for a reason. The component framework only starts a component instance when it has work to do, such as when another component requests to use its instance's capabilities.
Once started, a component instance continues to run until it is stopped.
Stopping a component instance
Stopping a component instance terminates the component's program but preserves its persistent state so that it can continue where it left off when subsequently restarted.
The component framework may stop a component instance for a variety of reasons, such as:
- When all of its clients have disconnected.
- When its parent is being stopped.
- When its package needs to be updated.
- When there are insufficient resources to keep running the component.
- When other components need resources more urgently.
- When the component is about to be destroyed.
- When the system is shutting down.
A component can implement a lifecycle handler (example) to be notified of its impending termination and other events on a best effort basis. Note that a component can be terminated involuntarily and without notice in circumstances such as resource exhaustion, crashes, or power failure.
Components can stop themselves by exiting. The means by which a component exits depend on the runner that runs the component.
Once stopped, a component instance can then be restarted or destroyed.
Destroying a component instance
Destroying a component instance permanently deletes all of its associated state and releases the system resources it consumed.
Once destroyed, a component instance ceases to exist and cannot be restarted. New instances of the same component can still be created but they will each have their own identity and state distinct from all prior instances.