node-appstate

Application state handler without dependency

---

npm i @vanioinformatika/appstate

Initialization without callback (logger):

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

Initialization with a simple logger:

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.

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

Changing application state.

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

Checking application state (recommended).

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

Reading application state.

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

Listing state values.

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:

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:

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:

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()
    // ...
  },
)