sync-threads

Make asynchronous calls in Node.js synchronously using worker threads and Atomics mutexes.

Especially useful when you need to resolve promises at require-time, for example in an AWS Lambda function when using provisioned concurrency.

NOTE: you don't need this library if you're happy with the overhead of creating a Node.js subprocess, see the Appendix for details.

Example

This example shows how we can synchronously retrieve secrets from AWS SSM at require-time – that is, before the init stage of Lambda has finished.

index.js:

const { createSyncFn } = require('sync-threads')

// Create our synchronous function
const getSsmSecretSync = createSyncFn('./worker.js')

// Call it at init (require) time, no async needed!
const secret = getSsmSecretSync('/my-secret')

// Logged at init time
console.log({ secret, source: 'init' })

exports.handler = async () => {
  // This value can be used in our handler without needing to resolve anything async
  console.log({ secret, source: 'handler' })
}

worker.js:

const SSM = require('aws-sdk/clients/ssm')
const { runAsWorker } = require('sync-threads')

const ssm = new SSM()

runAsWorker(async (ssmParamName) => {
  const {
    Parameter: { Value },
  } = await ssm.getParameter({ Name: ssmParamName, WithDecryption: true }).promise()
  return Value
})

You can see this example, as well as how you'd bundle it if you're using webpack or similar, in the examples directory.

API

sync-threads exports two main functions: createSyncFn and runAsWorker

createSyncFn(filename[, bufferSize])

Returns a synchronous function that will run the specified file as a worker, pass in any arguments you give it, and wait for the result.

runAsWorker(workerAsyncFn)

To be called from inside your worker code. It will run the given asynchronous function with the given arguments from the parent and share the result.

Installation

With npm do:

npm install sync-threads

Appendix

You can achieve something very similar to this library using the spawnSync/execSync functions from the child_process module in Node.js.

The main difference is that this library performs the async work in a thread, without creating a separate node process, which makes it a little faster.

Here's a simple example of how you'd do it without needing this library:

index.js:

const { execSync } = require('child_process')

const { secret } = JSON.parse(execSync('node worker.js /my-secret', 'utf8').trim().split('\n').pop())

console.log({ secret, source: 'init' })

exports.handler = async () => {
  console.log({ secret, source: 'handler' })
}

worker.js:

const SSM = require('aws-sdk/clients/ssm')

const ssm = new SSM()

;(async () => {
  const {
    Parameter: { Value },
  } = await ssm.getParameter({ Name: process.argv[2], WithDecryption: true }).promise()
  console.log('%j', { secret: Value })
})()