Start-stop-state-machine NPM

start-stop-state-machine

A simple state machine for managing a service that asynchronously starts and stops.

Install

npm install start-stop-state-machine

Usage

const StateMachine = require('start-stop-state-machine')

async function startService() {
  console.log('starting')
  await new Promise((res) => setTimeout(res, 200))
  console.log('started')
}

async function stopService() {
  console.log('stopping')
  await new Promise((res) => setTimeout(res, 200))
  console.log('stopped')
}

const sm = new StateMachine({ start: startService, stop: stopService })

;(async () => {
  sm.start()
  sm.start()
  await sm.started()
  // logs "starting" then "started", but only once
  await sm.stop()
  // lots "stopping", then "stopped"
})()

API

ServiceState

Type: ({value: Exclude\<ServiceStateValue, "error">} | {value: "error", error: Error})

ExternalEvents

Type: Object

StartStopStateMachine

Extends TypedEmitter

A state machine for managing a service that has asynchronous "start" and "stop" methods. Create an instance passing async opts.start() and opts.stop() methods. It manages state following some basic rules:

Most importantly: You can call start() and stop() multiple times, but the service will end in the state of the last call (e.g. if the last call was to stop() then it will end up stopped)
Calling start() when the service is "stopped" calls the opts.start() method and resolves when it completes.
Calling start() when the service is "starting" (e.g. start() has been called but has not completed) will not call opts.start() again, but will resolve once the service has started
Calling start() when the service is "started" will resolve immediately and do nothing.
If opts.start() or opts.stop() throw, then the service is left in an unrecoverable "error" state.
Calling start() or stop() when the service is in "error" state will throw with the error from the error state

Logic for calling stop() follows the inverse of start().

To wait for the service to be in the "started" state from other methods, use await stateMachine.started(). Note that if the services is "stopping" or "stopped" then this will await (e.g. queue) until next start

Parameters

opts Object? (optional, default {})
- opts.start (optional, default async()=>{})
- opts.stop (optional, default async()=>{})

state

Get the current state of the service.

Returns ServiceState

started

Will resolve when the service is in started state. E.g. to ensure an async method only runs when the service is in "started" state, use:

await this.started()

Will reject if the service is in "error" state.

Note: If the service is in "stopping" or "stopped" state this will queue until the next time the service starts. If this is not desirable behaviour, check this.#state.value first

Returns Promise\

stopped

Will resolve when the service is in stopped state. Less useful than started() E.g. to ensure an async method only runs when the service is in "stopped" state, use:

await this.stopped()

Will reject if the service is in "error" state.

Note: If the service is in "starting" or "started" state this will queue until the next time the service stops. If this is not desirable behaviour, check this.#state.value first

Returns Promise\

start

Start service. If the service is starting or started, will resolve when the service is started, and will not call opts.start() for than once. If the service is in the process of stopping, will wait until it stops before starting and will not call opts.stop() more than once