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 OnStart hooks. These run in the order they were appended.
• Shutdown hooks, also referred to as OnStop hooks. 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