---
lang: en
title: 'Life cycle events and observers'
keywords: LoopBack 4.0, LoopBack 4
sidebar: lb4_sidebar
permalink: /doc/en/lb4/Life-cycle.html
---

## Overview

A LoopBack application has its own life cycles at runtime. There are two methods
to control the transition of states of `Application`.

- start(): Start the application
- stop(): Stop the application

It's often desirable for various types of artifacts to participate in the life
cycles and perform related processing upon `start` and `stop`. Good examples of
such artifacts are:

- Servers

  - start: Starts the HTTP server listening for connections.
  - stop: Stops the server from accepting new connections.

- Components

  - A component itself can be a life cycle observer and it can also contribute
    life cycle observers

- DataSources

  - connect: Connect to the underlying database or service
  - disconnect: Disconnect from the underlying database or service

- Custom scripts
  - start: Custom logic to be invoked when the application starts
  - stop: Custom logic to be invoked when the application stops

## The `LifeCycleObserver` interface

To react on life cycle events, a life cycle observer implements the
`LifeCycleObserver` interface.

```ts
import {ValueOrPromise} from '@loopback/context';

/**
 * Observers to handle life cycle start/stop events
 */
export interface LifeCycleObserver {
  start?(): ValueOrPromise<void>;
  stop?(): ValueOrPromise<void>;
}
```

Both `start` and `stop` methods are optional so that an observer can opt in
certain events.

## Register a life cycle observer

A life cycle observer can be registered by calling `lifeCycleObserver()` of the
application. It binds the observer to the application context with a special
tag - `CoreTags.LIFE_CYCLE_OBSERVER`.

```ts
app.lifeCycleObserver(MyObserver);
```

Please note that `app.server()` automatically registers servers as life cycle
observers.

Life cycle observers can be registered via a component too:

```ts
export class MyComponentWithObservers implements Component {
  /**
   * Populate `lifeCycleObservers` per `Component` interface to register life
   * cycle observers
   */
  lifeCycleObservers = [XObserver, YObserver];
}
```

## Discover life cycle observers

The `Application` finds all bindings tagged with `CoreTags.LIFE_CYCLE_OBSERVER`
within the context chain and resolve them as observers to be notified.

## Notify life cycle observers of start/stop related events by order

There may be dependencies between life cycle observers and their order of
processing for `start` and `stop` need to be coordinated. For example, we
usually start a server to listen on incoming requests only after other parts of
the application are ready to handle requests. The stop sequence is typically
processed in the reverse order. To support such cases, we introduce
two-dimension steps to control the order of life cycle actions.

### Observer groups

First of all, we allow each of the life cycle observers to be tagged with a
group. For example:

- datasource (connect/disconnect)

  - mongodb
  - mysql

- server
  - rest
  - gRPC

We can then configure the application to trigger observers group by group as
configured by an array of groups in order such as `['datasource', 'server']`.

For example,

```ts
app
  .bind('observers.MyObserver')
  .toClass(MyObserver)
  .tag({
    [CoreTags.LIFE_CYCLE_OBSERVER_GROUP]: 'g1',
  })
  .apply(asLifeCycleObserver);
```

The observer class can also be decorated with `@bind` to provide binding
metadata.

```ts
import {bind, createBindingFromClass} from '@loopback/context';
import {CoreTags, asLifeCycleObserver} from '@loopback/core';

@bind(
  {
    tags: {
      [CoreTags.LIFE_CYCLE_OBSERVER_GROUP]: 'g1',
    },
  },
  asLifeCycleObserver,
)
export class MyObserver {
  // ...
}

app.add(createBindingFromClass(MyObserver));
```

Or even simpler with `@lifeCycleObserver`:

```ts
import {createBindingFromClass} from '@loopback/context';
import {lifeCycleObserver} from '@loopback/core';

@lifeCycleObserver('g1')
export class MyObserver {
  // ...
}

app.add(createBindingFromClass(MyObserver));
```

The order of observers is controlled by a `orderedGroups` property of
`LifeCycleObserverRegistry`, which receives its options including the
`orderedGroups` from `CoreBindings.LIFE_CYCLE_OBSERVER_OPTIONS`.

```ts
export type LifeCycleObserverOptions = {
  /**
   * Control the order of observer groups for notifications. For example,
   * with `['datasource', 'server']`, the observers in `datasource` group are
   * notified before those in `server` group during `start`. Please note that
   * observers are notified in the reverse order during `stop`.
   */
  orderedGroups: string[];
  /**
   * Notify observers of the same group in parallel, default to `true`
   */
  parallel?: boolean;
};
```

Thus the initial `orderedGroups` can be set as follows:

```ts
app
  .bind(CoreBindings.LIFE_CYCLE_OBSERVER_OPTIONS)
  .to({orderedGroups: ['g1', 'g2', 'server']});
```

Or:

```ts
const registry = await app.get(CoreBindings.LIFE_CYCLE_OBSERVER_REGISTRY);
registry.setOrderedGroups(['g1', 'g2', 'server']);
```

Observers are sorted using `orderedGroups` as the relative order. If an observer
is tagged with a group that is not defined in `orderedGroups`, it will come
before any groups included in `orderedGroups`. Such custom groups are also
sorted by their names alphabetically.

In the example below, `orderedGroups` is set to
`['setup-servers', 'publish-services']`. Given the following observers:

- 'my-observer-1' ('setup-servers')
- 'my-observer-2' ('publish-services')
- 'my-observer-4' ('2-custom-group')
- 'my-observer-3' ('1-custom-group')

The sorted observer groups will be:

```ts
{
  '1-custom-group': ['my-observer-3'], // by alphabetical order
  '2-custom-group': ['my-observer-4'], // by alphabetical order
  'setup-servers': ['my-observer-1'], // by orderedGroups
  'publish-services': ['my-observer-2'], // orderedGroups
}
```

The execution order of observers within the same group is controlled by
`LifeCycleObserverOptions.parallel`:

- `true` (default): observers within the same group are notified in parallel
- `false`: observers within the same group are notified one by one. The order is
  not defined. If you want to have one to be invoked before the other, mark them
  with two distinct groups.

## Add custom life cycle observers by convention

Each application can have custom life cycle observers to be dropped into
`src/observers` folder as classes implementing `LifeCycleObserver`.

During application.boot(), such artifacts are discovered, loaded, and bound to
the application context as life cycle observers. This is achieved by a built-in
`LifeCycleObserverBooter` extension.

## CLI command to generate life cycle observers

To make it easy for application developers to add custom life cycle observers,
we introduce `lb4 observer` command as part the CLI.

```sh
$ lb4 observer
? Observer name: test
? Observer group: g1
   create src/observers/test.observer.ts
   update src/observers/index.ts

Observer test was created in src/observers/
```

See [Life cycle observer generator](Life-cycle-observer-generator.md) for more
details.
