# Application lifecycle
The lifecycle of an Fx application has two high-level phases: initialization and execution. Both of these, in turn are comprised of multiple steps.
During initialization, Fx will,
- register all constructors passed to
fx.Provide - register all decorators passed to
fx.Decorate - run all functions passed to
fx.Invoke, calling constructors and decorators as needed
During execution, Fx will,
- run all startup hooks appended to the application by providers, decorators, and invoked functions
- wait for a signal to stop running
- run all shutdown hooks appended to the application
# Lifecycle hooks
Lifecycle hooks provide the ability to schedule work to be executed by Fx, when the application starts up or shuts down. Fx provides two kinds of hooks:
- Startup hooks, also referred to as
OnStarthooks. These run in the order they were appended. - Shutdown hooks, also referred to as
OnStophooks. These run in the reverse of the order they were appended.
Typically, components that provide a startup hook also provide a corresponding shutdown hook to release the resources they acquired at startup.
Fx runs both kinds of hooks with a hard timeout enforcement. Therefore, hooks are expected to block only as long as they need to schedule work. In other words,
- hooks must not block to run long-running tasks synchronously
- hooks should schedule long-running tasks in background goroutines
- shutdown hooks should stop the background work started by startup hooks