Smarter Async Batching in Javascript / Node.js

Keith A. Knight -

tldr; async-batch is a great way to manage complex task queues.

Promises and Async/Await have made asynchronous programming in Javascript / Node.js simpler and cleaner.  But I frequently find myself processes sets of Promises concurrently and having to decide the best approach.

Async/await is a syntactic wrapper around Promises.  So I will refer to Promises and Async/await interchangeably.

Below are several strategies with their pros and cons.

Strategy 1 - Simple For Loop

const arr = [1, 2, 3];
const asyncMethod = async (val) => { console.log(`Value: ${val}`); };

for (let i = 0, len = arr.length; i < len; i++) {
  await asyncMethod(arr[i]);
}
Simple Example with Async For Loop

Pros:

  • Simple
  • Run promises sequentially, avoid potential concurrency issues
  • Control load on external resources – ie. don't make too many queries at the same time
  • Easy debugging / monitoring

Cons:

  • Slower if you want concurrent execution

Strategy 2 - Promise.all with Array.map 

const arr = [1, 2, 3];
const asyncMethod = async (val) => { console.log(`Value: ${val}`); };

await Promise.all(arr.map((x) => asyncMethod(x)));
Simple Example with Promise.all with Array.map

Pros:

  • Concise code that can run all functions concurrently

Cons:

  • Assumes the number of concurrent Promises is small and there are sufficient resources to run concurrently.
  • Debugging becomes more complicated that a simple for loop if one of the promises fails

Strategy 3 - Async Batch

Async-Batch (GitHub, NPM) is an excellent dependency free library for managing concurrency in a set of promises.  It allows you to specify the number of parallel promises running.  As one completes, it starts the next in the queue.

Note: this library using ES6 syntax, so until Node.js fully supports it, you may need to use require().default.

const Parallelism = 2; 
const asyncBatch = require('async-batch').default;
const arr = [1, 2, 3];
const asyncMethod = async (val) => { console.log(`Value: ${val}`); };

await asyncBatch(arr, asyncMethod, Parallelism);
Simple Example Async-Batch

Pros:

  • Efficiently manages large sets of async processes
  • Relatively simple way to run complex task queues
  • Dependency free
  • Can scale the Parallelism from 1 to x dynamically, adjusting to available resources.
  • Debugging is simpler than Promise.all since you can set parallelism to 1.

Cons:

  • ? Possibly more complex to read than the For loop.

Conclusion:

Each method has its advantages, but I find myself using async-batch regularly for simple or complex scenarios.