async-service-helper

async-service-helper

Maintain state of a class that needs to asynchronously start and stop.

I've often found myself re-implementing logic for services that asynchronously start and stop and I need to manage state and wait for them to start before calling other methods, and I always make mistakes with race conditions. This helper class adds some basic state management for starting and stopping asynchronously that follows these rules:

• 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 _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 _start() again, but will resolve once the service has started
• Calling start() when the service is "started" will resolve immediately and do nothing.
• If _start() or _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

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

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

Table of Contents

• Install
• Usage
• API
• Maintainers
• Contributing
• License

Install

npm install async-service-helper

Usage

const AsyncService = require('async-service-helper')

class MyService extends AsyncService {
  async _start() {
    await new Promise(res => setTimeout(res, 100))
  }
  async _stop() {
    await new Promise(res => setTimeout(res, 100))
  }
  async doAfterStart() {
    await this.started()
    // Guaranteed that _start() has resolved and _stop() has not been called
  }
}

const service = new MyService()
service.start() // no need to await, because we have a guard in doAfterStart()
doAfterStart().then(() => {})

API

_start()

Override this method in your implementing class. Must return a Promise.

_stop()

Override this method in your implementing class. Must return a Promise.

start()

Start service. If the service is starting or started, will resolve when the service is started, and will not call _start() more than once. If the service is in the process of stopping, will wait until it stops before starting and will not call _stop() more than once. Any parameters are passed through to _start().

Returns [Promise][35]<void> Resolves when service is started

stop()

Stop the service.

Returns [Promise][35]<void> Resolves when service is stopped

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][35]<void>

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][35]<void>

addStateListener(listener)

Attach a listener function to changes of state. listener will be calls with the current service state (see below)

removeStateListener(listener)

Remove a listener to changes of state.

Maintainers

@gmaclennan

Contributing

PRs accepted.

Small note: If editing the README, please conform to the standard-readme specification.

License

MIT © 2022 Gregor MacLennan