Introducing Servant, a Clojurescript library for using web workers

01 Oct 2013

Concurrent Programming

Javascript by default is single threaded, but web workers introduce OS level threads. Concurrent programming is hard enough (in imperative languages), so the webworker designers decided to circumvent a bunch of concurrency problems by forbidding any shared data between threads. There are better ways of doing this (read immutability), but we work with what we got.

Problems with web workers

I've done a couple projects with web workers. The biggest project being, which uses webworkers to efficiently encrypt/decrypt large (GBs) files, and parallel {down,up}load file chunks. Here are problems I've stumbled across:

  • Everything about the web worker needs to be asynchronous, meaning callback hell
  • You need to think in a separate context for the web worker, you can't call any functions defined with the rest of your code.
  • Distributing workload effectively.
  • The problems only gets worse the more web workers you bring in.

Enter Servant

Servant is a super small (literally ~100 lines) library that solves all the problems above, allowing you to write clean, multithreaded, ClojureScript. Even though it's small, it does a lot.

  • It allows you to define servant functions alongside the rest of your code, even using functions already defined in your namespace.
  • It automatically balances work across webworkers.
  • It provides simple ways to do a normal (copy of arguments) or efficient (arraybuffer transfer) call to webworkers, easily.

Sharing functions, and predefined variables

This was the trickiest part of the library. I wanted the ability to define useful functions, and use them in the webworker without having to copy it over to a separate worker.js file. I solved it by using the same exact file for both the main page (browser context) and the web worker. That, however, came with one problem; you have to explicitly declare code that should run on the webworker and code that runs in the browser. Like so:

(ns servant.demo
  (:require [servant.core :as servant] 
            [servant.worker :as worker]))

(defn window-load [] (.log js/console "this runs in the browser"))

(if (servant/webworker?)
    (worker/bootstrap) ;;Sets the worker up to receive messages
    (set! (.-onload js/window) window-load))

As part of that caveat, the webworker can only see code that it can get to. Anything defined in window-load would not be visible to the webworker. Now let's take a look at how we can define a servant function, using the defservantfn macro.

We need to use a special function, defservantfn to define functions that will serve as our "access points" to the web worker.

(ns servant.demo
  (:require-macros [servant.macros :refer [defservantfn]]))

(defn make-it-funny [not-funny]
  (str "Hahahah:" not-funny))

(defservantfn servant-with-humor [your-joke]
    (make-it-funny your-joke))

The defservantfn macro simply calls a defn with the same arguments, and registers that function with a hashmap atom for the webworker. The key is the hash of the function and the value is the function itself. The webworker needs to be able to know what function the browser is
referring in a message, so I use the function's hash as a token that the browser context and webworker can both agree on. The function's .toString() value could have worked just as well.

I should also mention, for efficiency reasons, Servant keeps a pool of N webworkers (you specify N) alive (until you explicitly kill them) so you only pay for the webworkers once. You control when the webworkers are created with servant/spawn-servants.

Workload Balancing

Core.async is simply amazing, it took this tricky problem and made it trivial. The solution is 4 lines. The solution for Servant is:

  • spawn N number of workers and place them in a buffered (of size N) channel.
  • Take workers from the channel as you use them.
  • Put them back when you get your result.

This is so beautifully simple. I just write the behavior I want, and core.async handles the messy state. If all the webworkers are busy the code will "block" until a webworker is free. What this means for you as a user, is you don't have to think about which worker is available to run your code.

Configurable message types

Now the whole point of using webworkers is to be as fast as possible. Sometimes you can't even afford copying data to the webworker (especially if the data is big, like at Servant provides a way to access webworkers' nifty arraybuffer transfer context ability. Take for example:

(defservantfn get-first-4bytes-as-str [arraybuffer]
  (let [d (js/DataView. arraybuffer)]
      (.toString (.getUint32 d 0) 16)))

That function expects an arraybuffer and returns a string. If we wanted to be efficient about it (and didn't care about getting rid of the arraybuffer) we can make the call using the servant/array-buffer-message-standard-reply fn instead of the servant/standard-message. So the efficient result would be:

(def arraybuffer (js/ArrayBuffer. 10))
(def d (js/DataView. arraybuffer))
(.setUint32 d 0 0xdeadbeef)
(def result-channel 
    get-first-4bytes-as-str arraybuffer [arraybuffer]))

The arguments to servant-thread are:

  • servant-channel - channel that contains the available workers
  • servant/array-buffer-message-standard-reply - A function that defines how the .postMessage function will be called (a.k.a mesage-fn)
  • get-first-4bytes-as-str - The servant function we defined earlier
  • arraybuffer - our argument to the function
  • [arraybuffer] - a vector of arraybuffers that are going to be transferred

The message-fn can be anything, but I think servant has you covered with:

  • standard-message : Copies all the data both ways
  • array-buffer-message : Can transfer the context both ways
  • array-buffer-message-standard-reply : Can transfer the context when making the call, won't transfer the context coming back

There is a reason why array-buffer-message isn't just the standard. You need to explicitly tell the postMessage call that you want to transfer arraybuffers. So to transfer context you need an additional argument, an array of arraybuffers. You also need to make sure the defservantfn returns a vector of results and an array of arraybuffers [result [arraybuffer1]] if you want to transfer the arraybuffer from the worker to the browser context. I figured if you wanted that you could use it and deal with the extra argument, if you didn't you could write your functions how you normally would.


I wrote two examples using the servant library:

  • The first is a simple demo showing several use cases.
  • The next is more featured demo that can encrypt/decrypt large files efficiently using webworkers.

Last thoughts

I used to curse the name webworkers. They brought gifts of speed at the cost of complexity. Servant is different, it doesn't sacrifice simplicity or efficiency. I'm pretty excited at the ease of using webworkers with servant, and I hope you have fun making an amazing, multithreaded Clojurescript application!

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