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JavaScript-to-JavaScript code rewriter for taming async-callback-style code

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README.md

tamejs

This package is a source-to-source translator that outputs JavaScript. The input dialect looks a lot like JavaScript, but introduces the twait primitive, which allows asynchronous callback style code to work more like straight-line threaded code. tamejs is written in JavaScript.

One of the core powers of the tamejs rewriting idea is that it's fully compatible with existing vanilla-JS code (like node.js's libraries). That is, existing node.js can call code that's been output by the tamejs rewriter, and conversely, code output by the tamejs rewriter can call existing node.js code. Thus, tamejs is incrementally deployable --- you can keep all of your old code and just write the new bits in tamejs! So try it out and let us know what you think.

Code Examples

Here is a simple example that prints "hello" 10 times, with 100ms delay slots in between:

for (var i = 0; i < 10; i++) {
    twait { setTimeout (mkevent (), 100); }
    console.log ("hello");
}

There is one new language addition here, the twait { ... } block, and also one new primitive function, mkevent. The two of them work in concert. Within the context of a twait block, mkevent returns anonymous callback functions associated with that block. A function must "wait" at the close of a twait block until all callbacks made by mkevent in that twait block are called. In the code above, there is only one callback produced in each iteration of the loop, so after it's called by setTimer in 100ms, control continues past the twait block, onto the log line, and back to the next iteration of the loop. The code looks and feels like threaded code, but is still in the asynchronous idiom (if you look at the rewritten code output by the tamejs compiler).

This next example does the same, while showcasing power of the twait{..} language addition. In the example below, the two timers are fired in parallel, and only when both have returned (after 100ms), does progress continue...

for (var i = 0; i < 10; i++) {
    twait { 
        setTimeout (mkevent (), 100); 
        setTimeout (mkevent (), 10); 
    }
    console.log ("hello");
}

Now for something more useful. Here is a parallel DNS resolver that will exit as soon as the last of your resolutions completes:

var dns = require("dns");

function do_one (ev, host) {
    var err, ip;
    twait { dns.resolve (host, "A", mkevent (err, ip));}
    if (err) { console.log ("ERROR! " + err); } 
    else { console.log (host + " -> " + ip); }
    ev();
}

function do_all (lst) {
    twait {
        for (var i = 0; i < lst.length; i++) {
            do_one (mkevent (), lst[i]);
        }
    }
}

do_all (process.argv.slice (2));

You can run this on the command line like so:

node src/13out.js yahoo.com google.com nytimes.com okcupid.com tinyurl.com

And you will get a response:

yahoo.com -> 72.30.2.43,98.137.149.56,209.191.122.70,67.195.160.76,69.147.125.65
google.com -> 74.125.93.105,74.125.93.99,74.125.93.104,74.125.93.147,74.125.93.106,74.125.93.103
nytimes.com -> 199.239.136.200
okcupid.com -> 66.59.66.6
tinyurl.com -> 195.66.135.140,195.66.135.139

If you want to run these DNS resolutions in serial (rather than parallel), then the change from above is trivial: just switch the order of the twait and for statements above:

function do_all (lst) {
    for (var i = 0; i < lst.length; i++) {
        twait {
            do_one (mkevent (), lst[i]);
        }
    }
}

Slightly More Advanced Example

We've shown parallel and serial work flows, what about something in between? For instance, we might want to make progress in parallel on our DNS lookups, but not smash the server all at once. A compromise is windowing, which can be achieved in tamejs conveniently in a number of different ways. The 2007 academic paper on tame suggests a technique called a rendezvous. A rendezvous is implemented in tamejs as a pure JS construct (no rewriting involved), which allows a program to continue as soon as the first event fires (rather than the last):

function do_all (lst, windowsz) {
    var rv = new tame.Rendezvous ();
    var nsent = 0;
    var nrecv = 0;

    while (nrecv < lst.length) {
        if (nsent - nrecv < windowsz && nsent < n) {
            do_one (rv.mkev (nsent), lst[nsent]);
            nsent++;
        } else {
            var evid;
            twait { rv.wait (mkevent (evid)); }
            console.log ("got back lookup nsent=" + evid);
            nrecv++;
        }
    }
}

This code maintains two counters: the number of requests sent, and the number received. It keeps looping until the last lookup is received. Inside the loop, if there is room in the window and there are more to send, then send; otherwise, wait and harvest. Rendezvous.mkev makes an event much like the mkevent primitive, but it also takes a first argument that associates an idenitifer with the event fired. This way, the waiter can know which event he's getting back. In this case we use the variable nsent as the event ID --- it's the ID of this event in launch order. When we harvest the event, rv.wait fires its callback with the ID of the event that's harvested.

Note that with windowing, the arrival order might not be the same as the issue order. In this example, a slower DNS lookup might arrive after faster ones, even if issued before them.

Composing Serial And Parallel Patterns

In Tame, arbitrary composition of serial and parallel control flows is possible with just normal functional decomposition. Therefore, we don't allow direct twait nesting. With inline anonymous JavaScript functions, you can consicely achieve interesting patterns. The code below launches 10 parallel computations, each of which must complete two serial actions before finishing:

function f(cb) {
    twait {
        for (var i = 0; i < n; i++) {
            (function (cb) {
                twait { setTimeout (mkevent (), 5*Math.random ()); }
                twait { setTimeout (mkevent (), 4*Math.random ()); }
                cb();
             })(mkevent ());
        }
    }
    cb();
}

Installing and Using

Install via npm:

npm install -g tamejs

You can their either use the tamejs compiler on the command line:

tamejs -o <outfile> <infile>
node <outfile> # or whatever you want

Or as an extension to node's module import system:

require ('tamejs').register (); // register the *.tjs suffix
require ("mylib.tjs");          // then use node.js's import as normal

ToDos

See the github issue tracker for the more immediate issues.

  • Documentation
    • Change mkevent to something else?
  • Optimizations
    • Can passThrough blocks in a tamed function that don't have twaits, so can get more aggressive here --- in progress, but can still seek out some more optimizations....
  • Parsing
    • Switch to uglify's parser

How It's Implemented In JavaScript

The key idea behind the tamejs implementation is Continuation-Passing Style (CPS) compilation. That is, elements of code like for, while and if statements are converted to anonymous JavaScript functions written in continuation-passing style. Then, twait blocks just grab those continuations, store them away, and call them when the time is right (i.e., when all relevant events have completed).

For example, the simple program:

if (true) { twait { setTimeout (mkevent (), 100); } }

Is rewritten to something like the following (which has been hand-simplified for demonstration purposes):

var tame = require('tamejs').runtime;
var f0 = function (k) {
    var f1 = function (k) {
        var __ev = new tame.Event (k);
        setTimeout ( __ev.mkevent(), 100 ) ;
    };
    if (true) {
        f1 (k);
    } else {
        k();
    }
};
f0 (tame.end);

That is, the function f0 is the rewrite of the if statement. Function f0 takes as a parameter the continuation k, which signifies "the rest of the program". In the case of this trivial program, the rest of the program is just a call to the exit function tame.end. Inside the if statement, there are two branches. In the true branch, we call into f1, the rewrite of the twait block, and in the false branch, it's just go on with the rest of the program by calling the continuation k. Function f1 is doing something a little bit different --- it's passing its continuation into the pure JavaScript class tame.Event, which will hold onto it until all associated events (like the one passed to setTimeout) have been called. When the last event is fired (here after 100ms), then the tame.Event class calls the continuation k, which here refers to tame.end.

The tamejs implementation uses other CPS-conversions for while and for loops, turning standard iteration into tail-recursion. If you are curious to learn more, examine the output of the tamejs compiler to see what your favorite JavaScript control flow is translated to. The translation of switch is probably the trickiest.

As you might guess, the output code is less efficient than the input code. All of the anonymous functions add bloat. This unfortunate side-effect of our approach is mitigated by skipping CPS compilation when possible. Functions with no twait blocks are passed through unmolested. Similarly, blocks within tamed functions that don't call twait can also pass through.

Another concern is that the use of tail recursion in translated loops might overflow the runtime callstack. That is certainly true for programs like the following:

while (true) { twait { i++; } }

...but you should never write programs like these! That is, there's no reason to have a twait block unless your program needs to wait for some asynchronous event, like a timer fired, a packet arrival, or a user action. Programs like these:

while (true) { twait { setTimeout (mkevent (), 1); i++; } }

will not overflow the runtime stack, since the stack is unwound every iteration through the loop (via setTimeout). And these are the types of programs that you should be using twait for.

History

The Tame rewriting idea come about at OkCupid in 2006. Until that time, the website was written in an entirely asynchronous-callback-based style with OKWS in C++. This serving technology was extremely fast, and led to huge cost savings in hardware and hosting, but as the site's code grew, it became increasingly unmanageable. Simple serial loops with network access, like the sequential DNS example above, required "stack-ripping" into multiple mutually recursive calls. As more employees began to work the code, and editted code that they didn't write, development slowed to a crawl.

Chris Coyne, OkCupid's director of product, demanded that something be done. The requirements were manifold. The new solution had to be compatible with existing code; it had to be incrementally deployable, so that the whole codebase wouldn't need to be rewritten at once; it had to be nearly as fast as the status quo; it had to clean the code up, so that it was readable; it had to speed up and simplify development.

The answer that emerged was Tame for C++. It's a source-to-source translator that mapped C++ with a few language additions into regular C++, which is then compiled with a standard compiler (like gcc). The key implementation ideas behind Tame C++ are: (1) generate a heap-allocated "closure" for each tamed function; (2) use labels and goto to jump back into tamed function as asynchronous events fired. Once Tame was brought to bear on OkCupid's code, it offered almost all of the flexibilty and performance of hand-crafted asynchronous-callback-passing code without any of the stack-ripping headaches. New employees picked it right up, and contributed to the incremental effort to modernize OkCupid's code to the Tame dialect.

OkCupid to this day runs Tame and OKWS in C++ to churn out high-performance, parallel applications, without worrying about traditional thread-based headaches, like deadlock and race-conditions. Our goal with tamejs is to bring these benefits to JavaScript and the node.js platform.

See our "Glossy Page"

See tamejs.org for documentation and information on tamejs.

Related Projects & Plugs

pubjs is yet another a node.js templating engine. But it allows arbtirarily nested code and output sections. Check it out, if you think that:

{% for (var i = 0; i < 10; i++) {{
   Counting ... %{i}
   {% if (i % 2 == 0) {{ ... is even ... }}
      else            {{ ... is odd .....}}
   %}
}} %}

is better than:

<% for (var i = 0; i < 10; i++) { %>
   Counting ... <%= i %>
   <% if (i % 2 == 0) { %> ... is even ...
   <% } else { %> ... is odd .. 
   <% } %>
<% } %>

Also Available In C++!

As described above, the Tame source-to-source translator was originally written for asynchronous C++ code. It's an actively maintained project, and it is in widespread use at OkCupid.com. See the sfslite/tame Wiki for more information, or read the 2007 Usenix ATC paper.

Authors

License

Copyright (c) 2011 Max Krohn for HumorRainbow, Inc., under the MIT license

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