@vanioinformatika/appstate/README.md

Application state handler

# node-appstate

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Application state handler without dependency

---

`npm i @vanioinformatika/appstate`

Initialization without callback (logger):

```javascript
const appState = require('@vanioinformatika/appstate')()
```

Initialization with a simple logger:

```javascript
const appState = require('@vanioinformatika/appstate')((appState, newAppState) => {
  console.log(`App state has changed from ${appState} to ${newAppState}`)
})
```

This example is always logging the application state change.

You have two variables:

- appState: application state

- newAppState: new application state

You can use any logger library, for example pino.

```javascript
let logger = require('pino')()
const appState = require('@vanioinformatika/appstate')((appState, newAppState) => {
  logger.warn(`App state has changed from ${appState} to ${newAppState}`)
})
```

Changing application state.

```javascript
const appState = require('@vanioinformatika/appstate')()
appState.init()
appState.running()
appState.stopped()
appState.error()
appState.fatal()
```

Checking application state (recommended).

```javascript
const appState = require('@vanioinformatika/appstate')()
appState.isInit()
appState.isRunning()
appState.isStopped()
appState.isError()
appState.isFatal()
```

Reading application state.

```javascript
const appState = require('@vanioinformatika/appstate')()
let applicationState = appState.get()
```

Listing state values.

```javascript
const appState = require('@vanioinformatika/appstate')()
let applicationStateValues = appState.list()
```

**Application state values are 'INIT', 'ERROR', 'RUNNING', 'STOPPED', 'FATAL'**

## Debug

Turn on debugging with env. variable: `DEBUG=appState`

Debug messages are:

```javascript
debug('info: appState has already set: ' + newAppState)
debug('warn: invalid state changes from ' + appState + ' to ' + newAppState)
debug('warn: unknow appState: ' + newAppState)
```

## State machine

States:

- **INIT** - Default state, application is starting, initialization: starting phase (app doesn't handle request)
- **RUNNING** - application is running
- **STOPPED** - application is running, but programmatically stopped
- **ERROR** - application is running, but has a critical error (e.g.: DB connection error): app doesn't serve requests
- **FATAL** - application doesn't serve request, and never comes to RUNNING state, all other state changes ignored

State machine:

- INIT -> [INIT, RUNNING, STOPPED, ERROR, FATAL]

- RUNNING -> [INIT, RUNNING, STOPPED, ERROR, FATAL]

- STOPPED -> [INIT, RUNNING, STOPPED, ERROR, FATAL]

- ERROR -> [INIT, RUNNING, STOPPED, ERROR, FATAL]

- FATAL -> [FATAL]

## Best practice

Turn on DEBUG on test environment and check debug messages.

Invalid state changes doesn't throw error, but ignored and logged.

Use a /health endpoint for load-balancers, and set to UP, if `appState.isRunning()`, else DOWN.

You can change anytime the application state, for example under initialization process: persistent DB connection error => appState.error()

## TypeScript example

1. Creating a module, for example appState.ts:

```javascript
import * as AppState from '@vanioinformatika/appstate'
import { AppStateInstance } from '@vanioinformatika/appstate'
import * as Pino from 'pino'

// init application state handler with logger
export const init = (logger: Pino.Logger): AppStateInstance => {
  return AppState(
    (appState: string, newAppState: string): void => {
      if (newAppState === AppState.state.ERROR || newAppState === AppState.state.FATAL) {
        logger.error(`appstate ${appState} to ${newAppState}`)
      } else {
        logger.warn(`appstate ${appState} to ${newAppState}`)
      }
    },
  )
}
```

2. Import to, and using in index.ts:

```javascript
import { AppStateInstance } from '@vanioinformatika/appstate'
import * as appState from './appstate'

// initialized with logger
const appStateInstance: AppStateInstance = appState.init(logger)

// ... and later you can use it anywhere
process.on(
  'uncaughtException',
  (err): void => {
    appStateInstance.fatal()
    // ...
  },
)
```