Outstanding

A task register for facilitating graceful shutdown.

In the world of continuous deployment applications are being started and stopped more frequently than ever. Stopping a process is normally achived by sending a 'SIGINT' or 'SIGTERM' signal. Your node application can listen for this and perform a gracesful shutdown by stopping listeners and closing connections, ensuring that new http requests are rejected, but in-flight ones can complete. But what happens to asynchronous processes that are not part of a request/response workflow, e.g.

setInterval(() => {
    fs.writeFile('file.txt', res.body, function(err) {

        // If SIGINT or SIGTERM is triggered now your application will shutdown
        // before the file is deleted

        fs.unlink('file.txt', function() {
            console.log('done')
        })
    })
}, 5000)

In truth there's nothing you can do to completely prevent this scenario. SIGKILL will kill your application instantly and cannot be listened for, but since most deployment mechanism try SIGTERM first and wait for a bit before invoking SIGKILL why not be a good citizen and wait until the asynchronous operations are complete?

This is where outstanding comes in handy. Outstanding assists with graceful shutdown of node.js applications by providing a way to track asynchronous tasks and block shutdown until they complete or an optional timeout expires.

TL;DR

const Outstanding = require('outstanding')
const outstanding = new Outstanding({ timeout: '5s' })
const signals = ['SIGINT', 'SIGTERM']
signals.forEach((signal) => {
    process.on(signal, () => {
        outstanding.shutdown((err, incomplete) => {
            if (err) {
                console.log(err.message, incomplete)
                process.exit(1)
            } else {
                process.exit(0)
            }
        })
    })
})

outstanding.run(someAsynchronousTask, (err, result) => {
    if (err) console.error(err)
    else console.log('The result of some asychronous task was', result)
})

Advanced Usage

Wrapping tasks

Sometimes you want to pass a function wrapped with outstanding to an executor.

var wrapped = outstanding.wrap(someAsynchronousTask)
async.times(3, wrapped)

Labeling tasks

When you run or wrap an asynchronous task, outstanding keeps track of the function name so you can at least log incomplete tasks on shutdown. If you use anonymous functions or prefer a custom label you can add an extra argument to both these functions, e.g.

outstanding.run('my label', someAsynchronousTask, (err, result) => {
    if (err) console.error(err)
    else console.log('The result of some asychronous task was', result)
})

Getting current tasks

Just call outstanding.list(). This will return a map of tokens to task details, e.g.

{
  "74a16a3b-129a-4921-bb71-897f2b6e64b7": { "name": "foo", "registered": 1472713271233 },
  "9e351837-421d-4785-bf7a-93f0aeed51b1": { "name": "bar", "registered": 1472713349045 }
}

Checking whether outstanding has been shutdown

Use outstanding.isClosed(). Returns false until outstanding.shutdown(cb) has been invoked, after which it returns true

Low Level API

The simplest way to use outstanding is to run or wrap tasks, but you can also use the api synchronously and asynchronously. In this case you need to explicitly use a label.

Asynchronous Usage

outstanding.register('my label', (err, token) => {
    if (err) return console.log('Cannot register task - the system is shutting down')
    someAsynchronousTask((err, result) => {
        outstanding.clear(token, () => {
            // Clear never returns an error
            if (err) console.error(err)
            else console.log('The result of some asychronous task was', result)
        })
    })
})

Synchronous Usage

const token = tasklist.register('my label')
if (token) {
    someAsynchronousTask((err, result) => {
        taskList.clear(token)
        if (err) console.error(err)
        else console.log('The result of some asychronous task was', result)
    })
} else {
    console.log('Cannot register task - the system is shutting down')
}