atlas-concurrent-queue

Async job queue that limits the number of concurrent jobs.

install

npm install --save atlas-concurrent-queue

why

I was writing a totally legal file downloader and I needed to run the downloads in parallel, but not all at once otherwise we'd run into performance and spamming problems.

examples

single queue

Let's assume we have some file downloading API and we're trying to upload the downloaded files to our personal server. The queue's API is dead simple -- you instantiate a queue and then push jobs onto it:

const ConcurrentQueue = require("atlas-concurrent-queue");
const downloadFile = require("./my-file-downloader");
const uploadFile = require("./my-file-uploader");
const urls = require("./url-list");
const destinationUrl = require("./dest-url")

const concurrency = 10
const queue = new ConcurrentQueue(concurrency);

// urls.length === 2000
for (let i = urls.length; i--;){
  queue.push(done => {
    downloadFile(urls[i], contents => {
      done();
      uploadFile(`${destinatonUrl}?index=${i}`, contents, () => {
        // no-op, don't care about result of write
      })
    })
  })
}

In the example above, we have 2000 download jobs, but no more than 10 are running at any given time. This helps keep us under the radar and prevents us from overloading our system. You might notice that we called done() before we started uploading the files to our server. This means that the uploading isn't actually limited in concurrency; we could easily have more than 10 uploads being attempted at once if our personal server is weak. This could be fixed by calling done in the uploadFile callback, but then we run into potential problems if the download server and upload server operate at different speeds.

multiple queues

The example above can run into problems because we aren't pacing the upload jobs, so let's fix it by adding a second queue:

...
const downloadConcurrency = 10;
const uploadConcurrency = 5;
const downloadQueue = new ConcurrentQueue(downloadConcurrency);
const uploadQueue = new ConcurrentQueue(uploadConcurrency);

// urls.length === 2000
for (let i = urls.length; i--;){
  downloadQueue.push(downloadDone => {
    downloadFile(urls[i], contents => {
      downloadDone();
      uploadQueue.push(uploadDone => {
        uploadFile(`${destinatonUrl}?index=${i}`, contents, uploadDone)        
      })
    })
  })
}

Now, we won't be running more than 5 upload jobs at any given time, in addition to limiting the concurrency of the download jobs.

todo

dynamic concurrency

It might be interesting to implement a dynamic concurrency that can react to changes in bandwidth. For example, we might want to run only N downloads at a given time based on network factors.

capturing errors and data

Should this be implemented? See caveats below.

caveats

capturing errors and data

There's no way to capture errors or results through the done callback. I wanted this queue to do as little work as possible. If you need to capture errors or results, do it at the scope you're writing your jobs in.

done callback

Don't forget to wrap your async functions with a done callback acceptor, because that's how the queue knows when to spin up the next job in the line.

streams

You might have noticed we aren't using streams in the examples above. This is for simplicity. With tasks like this, it's better to use streams to limit your memory usage.