More async: Using auto() for Parallel Operations

One of the straw men that people often cite when discussing event-driven programming, ala Node.js, is the fear that complex server-side behavior will take the form of unmanageably complex, deeply nested callbacks.

Although the majority of the code I've so far written with Node is very simple, usually involving only a single callback, my mental image of Node is of a request arriving, and kicking off a series of database operations and other asynchronous requests that all combine, in some murky fashion, into a single response. I visualize such a request as a pinball dropping into some bumpers and bouncing around knocking down targets, until shooting out, back down to the flippers.

Here's an example workflow from the sample application I'm building; I'm managing a set of images used in a slide show; so I have a SlideImage entity in MongoDB (using Mongoose), and each SlideImage references a file stored in Mongo's GridFS.

When it comes time to delete a SlideImage, it is necessary to delete the GridFS file as well. The pseudo-code for such an operation, in a non-event based system, might look something like:

def deleteSlideImageById(db, imageId)
  err, slideImage = db.readSlideImage(imageId)
  if err ...

  err, file = db.openGridFile(slideImage.fileId)
  if err ...

  err = file.delete()
  if err ...

  err = slideImage.delete()
  if err ...

Inside Node, where all code is event-driven and callback oriented, we should be able to improve on the pseudo-code by doing the deletes of the SlideImage document, and the GridFS file, in parallel. Well, that's the theory anyway, but I'd normally stick to a waterfall approach, as tracking when all operations have completed would be tedious and error prone, especially in light of correctly handling any errors that might occur.

Enter async's auto() function. With auto(), you define a set of tasks that have dependencies on each other. Each task is a function that receives a callback, and a results object. auto() figures out when each task is ready to be invoked.

When a task fails, it passes the error to the callback function. When a task succeeds, it passes null and a result value to the callback function. Later executing tasks can see the results of prior tasks in the result object, keyed on the prior tasks's name.

As with waterfall(), a single callback is passed the error object, or the final result object.

Let's see how it all fits together, in five steps:

• Find the SlideImage document
• Open the GridFS file
• Delete the GridFS file
• Delete the SlideImage document
• Send a response (success or failure) to the client

The granularity here is partly driven by the specific APIs and their callbacks.

The code for this is surprisingly tight:

app.delete "/api/images/pending/:id", (req, res) ->

    async.auto
      find: [(callback) ->
        schema.SlideImage.findById req.params.id, callback]
      remove: ["find", (callback, results) ->
        results.find.remove callback]
      openFile: ["find", (callback, results) ->
        new GridStore(mongoose.connection.db, results.find.file, "r").open callback]
      removeFile: ["openFile", (callback, results) ->
        results.openFile.unlink callback],
      (err) ->
        if err
          res.send "unable to delete SlideImage or File", 500
        else
          res.send result: "ok"

auto() is passed two values; an object that maps keys to arrays, and the final callback. Each array consists of the names of dependencies for the task, followed by the task function (you can just specify the function if a task has no dependencies, but I prefer the consistency of each entry being an array).

So find has no dependencies, and kicks of the overall process. I think it is really significant how consistent Node.js APIs are: the basic callback consisting of an error and a result makes it very easy to integrate code from many different libraries and authors (I think there's a kind of Monad hidden in there). In the code, the callback created by auto(), and passed to find, is perfectly OK to pass into findById. It's all low impedance: no need to write any kind of shim or adapter.

The later tasks take the additional results parameter; results.find is the SlideImage document provided by the find task.

The remove and openFile tasks both depend on find: they will run in no particular order after find; more importantly, their callbacks will be invoked in no predictable order, based on when the various asynchronous operations complete.

Only once all tasks have executed (or one task has passed an error to its callback), does the final callback get invoked; this is what sends a 500 error to the client, or a 200 success (with a { "result" : "ok" } JSON response.

I think this code is both readable, and concise; in fact, I can't imagine it being much more concise. My brain is starting to really go parallel: part of my brain is evaluating everything in terms of Java code and idioms while the rest is embracing JavaScript and CoffeeScript and Node.js idioms; the Java part is impressed by the degree to which these JavaScript solutions eschew complex APIs in favor of working on a specific "shape" of data; if I was writing something like this in Java, I'd be up to my ears in fluid interfaces and hidden implementations, with a ton of code to write and test.

I'm not sure that the application I'm writing will have any processing workflows significantly more complex than this bit, but if it does, I'm quite relieved to have auto() in my toolbox.