Through A Glass, Darkly

Although I get paid to write software, most of my time is spent understanding other people's software. I find that difficult: the available information is usually fragmentary, inconsistent, and more than I can hold in my head at one time anyway. It's like trying to read a mural on the side of a building, through the holes in a construction fence, as I drive by. I get little snapshots of partial information and I have to try to piece them together in my head. Sometimes the big picture I develop is entirely wrong, or missing big chunks.

Example: I've been working with Hibernate for more than a month now, but I still don't really understand exactly how it actually works. I only discovered tonight that it creates proxies to camouflage and control all my objects. This is sort of like wondering why your friends are acting a bit odd and then discovering that everyone on the planet except you has been replaced by space aliens.

Java has so dang many layers of obscurity that it is really hard to figure out precisely what code is actually being executed. The application code you write is just the tip of the iceberg. What with Spring, Hibernate, Terracotta, JUnit, the Hotspot compiler, and all the other frameworks and code enhancement tools we use, the program code itself is almost just a hint to the system. Maybe we're getting closer to the holy grail of being able to tell the computer what we want done, rather than how to do it.

Maven Continues to Suck

Maven is supposed to be a tool to support collaborative software development.

Maven is based on the concept of software modules, each of which is versioned, and which have interdependencies. Let's say version 1.0.2 of MyThing depends on version 2.4.1 of YourThing, for instance. Now, what if I want to make some changes to both YourThing and MyThing? Well, there's a special version called "SNAPSHOT" - as in 2.4.1-SNAPSHOT - that means "the latest and greatest up-to-the-minute version of YourThing version 2.4.1". Setting aside the discussion of how the concept of a "version of a version" is inherently flawed, this introduces a big problem: it does not distinguish between changes I make and changes somebody else makes.

Case in point. Another developer at my company, and I, both are working on modifications to a module that our other stuff depends on. My modifications won't ever be shared with the rest of the world, they're just for my temporary purposes, but nonetheless I need them. The other developer, however, made some changes and committed them to version control. When I went to do a build of MyThing, Maven checked dependencies and noticed that there was a new SNAPSHOT of YourThing.

In a sane world, a collaborative software tool would notice the conflict, and perhaps notify me: "a newer version of YourThing is available, but it conflicts with your local changes - do you want to upgrade and lose your changes, or keep the stale but familiar stuff you've got?" The default, of course, would be to keep what I've got; after all, if I made some local changes, it was presumably for a reason.

Not Maven, though. Because I said SNAPSHOT (so that I could make changes locally), Maven silently and transparently discards my local version and updates me to somebody else's changes, at a random time of its deciding (typically, in the first build I do after midnight, of any project that happens to depend on YourThing).

Fortunately, the other developer's changes contained a serious bug. I say fortunately, because otherwise I might not have noticed that my changes had been silently discarded, and I might have spent a lot of time trying to figure out why my code, that used to work, no longer did.

What kind of collaborative software development tool is it that can't gracefully handle the simple case of two people working on a shared module?

Maven is not really a collaborative software development tool, it seems. Maven is a tool for letting one person develop a single module of code, with external dependencies on an otherwise-static world. That does not describe any software project I have ever worked on.

I keep hoping to see the light, and discover that I'm wrong about Maven. But it keeps on sucking.

Code Reviews

A spare set of eyes is always useful, so most software teams have some practice of code reviews. There is huge variation from team to team in how code reviews are actually done, and in my experience how any given team does it is an axiom of the corporate culture; suggesting a different approach is met with the same sort of response as if you'd suggested, say, using an abacus instead of a personal computer. (I haven't tried this at my current employer, Terracotta, yet.)

The range of practices I've been directly involved with includes:

• No review at all. I haven't seen this in a long time.


• Self-initiated. If you want a review, you find a peer and beg some of their time. Surprisingly, this has been my experience working on Eclipse, and at a number of other teams; it may reflect resource constraints or perhaps just the fact that I've been in the industry for a long time and people foolishly think I'm less likely to write bad code.


• Recommended review for major work, especially for new developers. This too is pretty common in my experience, and is the approach taken at Terracotta.


• Mandatory review of all code before it's committed to version control. This approach was taken by a team I worked on in the '90s at Microsoft, and resulted in some of the best code I've ever written or seen. (The project made it through beta but was then axed by management; it conflicted with corporate strategy.)


• Pair programming, two sets of eyeballs at all times. I've only tried this a little bit; it did seem like we wrote pretty good code, but boy was it exhausting.

Along with that range is the question of how, exactly, the code gets reviewed. Approaches I've tried include:

• Over my shoulder. The reviewer looks over my shoulder as I walk through the code on my monitor, explaining it.


• Over the reviewer's shoulder. Same, except that the reviewer is the one driving.


• Independent review, meet with feedback. Each reviewer is given the code to review, and given some time (a day, perhaps) to review and come up with comments; we then meet to collect together the feedback. The meeting is typically moderated by someone other than the developer, and there may also be a designated scribe to collect the comments.


• Collective review. Like "over my shoulder", except that there is more than one reviewer. Having multiple reviewers makes it psychologically harder for any one reviewer to change the flow of the review: if someone wants to go back and look at a previous file, they have to interrupt someone else.

And then, there's the question of media and presentation. If code is being given to reviewers, it may be presented as a set of changes to the existing code, or as a finished document; it may be presented as hard copy or as files. The review may be conducted in person, over a video link (like WebEx or Elluminate), by audio conference, or by email.

The costs and benefits vary a lot depending on approach. I've gotten the best reviews out of the "independent review" approach: it creates a social dynamic where there is some shame for any reviewer who fails to find a particular bug that others do find, so there's incentive for thoroughness, but it also lets each reviewer bring their skills to bear in whatever way and at whatever pace works best for them. The collective feedback (whether in a meeting or in group email) is a good way to learn advanced techniques, subtle errors, and so forth.

But this approach is also expensive - I used to budget half a day to participate in a review like that, between the reviewing and the feedback meeting, and if three or four people have to spend half a day for each developer who wants to check something in, it's easy to spend all your time reviewing. Also, this strategy works poorly for small changes or even large refactorings of existing code, because reviewers tend to get distracted by things that really should be fixed, but that have been that way forever and is not in the scope of the current changes.

The opposite end of the curve is probably the "go borrow someone's eyeballs if you need them" sort of review. This misses a lot of errors, but sometimes you get so deep into a problem that you lose track of the obvious. A second set of eyes can see that you failed to close an open stream, or that you're doing something that a library routine already exists for. A second set of eyes will probably miss subtleties like locking on the wrong object.

Personally, I'm afraid that of all the possibilities, I get the very least out of collective review over video conference. The audio and video quality is generally very poor, so it's hard for me to follow what anyone else is saying; if my attention wanders for a moment, it's socially awkward to ask people to back up, so I tend to lose track of crucial details; and since it's the code owner who drives the review, any assumptions they have about what is and is not important become shared. Often bugs lurk precisely within those assumptions.

Finally there's the question of what sort of comments matter. In the most formal review process I was part of, we prioritized comments exactly the same way we'd have prioritized bugs:

• pri 1 was a bug that would make the code fail significantly or a design flaw that would prevent the code from working with the rest of the product


• pri 2 was a more minor bug or design infelicity, or possible performance improvements


• pri 3 was a bug that wouldn't actually affect the code as it stood but that might cause problems during maintenance or if someone else tried to use it in a different way


• pri 4 was typographical stuff, like use of whitespace, naming of internal variables, and the like.

Using a structure like that, we went through each module of code, collecting and discussing comments in priority order. Pri 1 bugs had to be fixed before the code could be checked in, and generally another review meeting would be scheduled to review the fix, depending on the developer's track record. Pri 2 bugs were usually fixed depending on the amount of effort required. Pri 3 bugs might be deferred or ignored. Generally priority-4 bugs would be relegated to email, that is, the scribe would just collect them from the individual reviewers and tack them onto the followup email without them ever being discussed in person, to save time and avoid religious wars; the developer would be expected to make a personal decision about whether to implement the sugggested change. Doing it this way also let us directly compare the relative effectiveness of reviewing versus testing.

I'm against any review process that takes a lot of people's time to do a shallow review. Either review thinly and cheaply, or deeply and expensively, but don't fool yourself into thinking that a lightweight review process is finding all the interesting bugs.

Imbalanced outputs

Although I earn most of my living writing software, I also work on audio whenever I get the chance. A few days ago one of my clients, a bass player, came to me with a piece of equipment that was causing a buzz.

The equipment in question was powered by an AC wall adapter, and it had a balanced output for sending the signal to the PA system. Balanced signal transmission is used in professional audio in order to reduce induced noise problems: the idea is that the signal is sent along two wires simultaneously, but with opposite polarity. At the receiving end, one voltage is subtracted from the other, eliminating any common noise that might have crept into the cables and leaving only the intended signal. The problem my client had was that whenever he used this output, it caused a terrible buzz in the PA system - rather counterproductive.

It turns out that a lot of what the industry calls "balanced outputs" really aren't. There's a popular belief amongst equipment designers that to make a balanced output, what you need is two signals in opposition - that is, you split the intended signal, send it unaltered onto one wire, and then flip its polarity (that is, multiply the voltage by -1) and send that to the other wire.

This is malarkey, as has been pointed out by luminaries like Douglas Self and Bill Whitlock. Having two voltages in opposition is irrelevant; if that mattered, then when the signal was zero (dead quiet), noise would no longer be eliminated. What actually matters is that the impedance on the two legs is balanced, so that any induced common-mode noise is the same on the two legs.

This is pretty old news, but has been largely ignored in the industry. So I was not surprised to discover, on tracing the circuit in my client's equipment, the following "balanced" output stage (I've eliminated a few unimportant details, like DC blocking capacitors):



It does just what it was meant to - the voltage on pin 3 will always be -1 times the voltage on pin 2. But look at what happens when the AC adapter is plugged into the wall:



There's always a little bit of leakage between the windings in a power transformer, perhaps a few picofarads. 120V from the wall leaks through that capacitance, into the power supply, into the signal ground. From there, I've traced the two main current paths to the output. Notice that one path goes through about 21.8k of resistance before getting to the output, while the other sees only 1k.

This 22:1 imbalance, with the tiny leakage through the transformer, was enough to generate a couple mV of differential signal - a lot, in the terms of audio signals, which rarely exceed one volt. By replacing this output stage with one that was truly impedance-balanced (based on an SSM2142 chip), I was able to reduce the noise signal by a factor of 30 - enough to get it below the noise floor of the unit.

The cost of the extra components was about $8 retail. But frankly, if cost were an issue, it might have been just as good to have gotten rid of the entire inverting amp stage and simply connected pin 3 with a 1k resistor to ground. The impedances would be balanced. The differential voltage would only be half as big as before, but the common-mode noise voltage would be reduced by much more than half, so signal to noise ratio would be better than with the "balanced" output the designer came up with.

Many software errors come from using a common design pattern without understanding the problem it's aimed at. This was the same mistake in hardware.

Comments

I often hear that comments in source code are A Bad Thing. Comments are evil because they don't accurately describe the code they apply to; because the code gets modified and the comment doesn't; because good code is self-documenting and therefore doesn't need comments; and because you need to read the code anyway to understand what it does and so comments just get in the way.

Horseshit.

This is roughly like saying that synchronization is evil, because it's often done incorrectly, because it gets broken when people update the code without fixing the synchronization, and because good code uses constructs like immutability that don't require synchronization.

If we treated comments as being as important as synchronization, they'd live up to their end of the deal just fine. There is nothing inherent in the idea of a comment that renders it impotent. Think of comments as being like error-handling code or synchronization: bulky, hard to write, even harder to test, but crucial to reliability.

I think the real reason so little code is commented is simply that most code is written in short bursts of effort by highly productive individuals, and while they're writing it, they understand their own assumptions well enough to not need the comments themselves, and they're in too much of a hurry to worry about the next fellow. And because this is what new programmers then see all around them, this is how they in turn learn to program.

If we built buildings this way, instead of having architectural drawings, the carpenters would come in after the foundations were poured, look at where the rebar was sticking out, and frame the walls where it looked like they should probably go. The resulting buildings would be ugly, short, and tend to collapse in a few years. Much like software, in fact.

If I were to design a programming language, any uncommented abstract definition (for instance, a method definition in an interface class) would be a compiler error. Yes, people would work around it by putting in useless comments, but it would be a start.

Control Theory

In the last post I mentioned that we programmers don't generally get training in control theory. This is too bad, because I think learning to recognize the behavioral modes of feedback-controlled systems can have a lot of practical benefit for us.

Suppose that you have the following two Java classes, B and Main. Without running this, can you tell how it will behave?

If you do run it, you'll see that the output is spread across a range of numbers. Without changing class B, how would you make that range smaller? What properties of the code determine the range?

This program is an analog of an elementary problem in control theory, so basic that even my espresso machine implements a solution to it. But I suspect that most computer programmers, coming upon behavior like this while performance-tuning an application, wouldn't immediately recognize it.

public class B implements Runnable {
  private static final int ELEMENTS = 100;
  private double[] v = new double[ELEMENTS];
  private boolean state;
  private volatile boolean done = false;
  
  public void run() {
    while (!done) {
      synchronized(this) {
        v[0] = state ? 200.0 : 0.0; 
        // propagate gradually through the array
        int i;
        for (i = 1; i < v.length - 1; ++i) {
          v[i] = (v[i - 1] + v[i] + v[i + 1]) / 3.0;
        }
        v[i] = (v[i - 1] + v[i]) / 2.0;
      }
      try {
        Thread.sleep(1);
      } catch (InterruptedException e) {}
    }
  }

  public synchronized int get() {
    return (int)v[v.length - 1];
  }
  
  public synchronized void set(boolean state) {
    this.state = state;
  }
  
  public void finish() {
    done = true;
  }
}


public class Main {
  final Object l = new Object();
  final long end = System.currentTimeMillis() + 60000; // 1 minute
  
  public static void main(String[] args) {
    new Main().run();
  }
  
  private void run() {
    B b = new B();
    new Thread(b).start();
    
    while (System.currentTimeMillis() < end) {
      double t = b.get();
      boolean state = t < 50.0;
      System.out.println("t = " + t + 
        " - state is " + (state ? "true" : "false"));
      
      b.set(t < 50.0);
      try {
        Thread.sleep(1000);
      } catch (InterruptedException e) {}
    }
    b.finish();
  }
}

Self-reference and Statistical Density

Programming languages are ways of mapping between three things: a set of problems in some problem domain; a set of envisioned solutions to those problems in a computer programmer's mind; and a set of instructions that can be executed by a computer.

Computers are very stupid, and not at all lazy. Programmers are very lazy, and hopefully quite smart. So, the language has a big gap to close.

For example, suppose I have an object that stores some numbers "a", "b", ..., "y", and "z", and I want to provide "fudge" and "muddle" operations on these numbers. I could write functions called fudgeA, fudgeB, muddleA, muddleB, and so on. Boring! Once I've written the code for fudge and muddle for A, I'd really like to just say "and do the same thing for the rest". So I want a language with that sort of expressive power, a language that can contain concepts like "the rest" and "the same thing". A language, that is, that can refer to itself, not only to the entities in a problem domain.

Taken to the extreme, what this leads to is programs of maximal statistical density: that is, programs that contain no repetition, but lots of self-reference and automatic code generation. There may in fact be many nested layers of this stuff.

As with Zappa's opus of statistical density, The Black Page, the problem in practice is that this becomes devilishly hard to play. Highly self-modifying programs save the original programmer's time, but generally make life for everyone else worse. The goal of making code more concise is seriously flawed. The goal should be to make code more understandable, not more concise.

I say "generally", because there's one case where it works. That's when the self-referential features of the language fit with the way that programmers think about solutions. For instance, "do the same thing to all these numbers" is the way that a programmer would typically describe the solution; to have to spell out each function separately isn't just repetitive, it makes the program code be less like what's in the programmer's brain in the first place.

My point is that I think that language designers should learn more about how human minds work, and should prefer native constructs and shy away from constructs that require unusual intelligence and training to master. Computers do not think like people; so programming languages need to.

There is a lot of overlap between computer science and cognitive psychology, but so far as I know this premise has not been applied; quite the opposite, it seems that new programming languages are more often used as a vehicle for showing how smart the programmer is.

Specifically, there are certain constructs that are demonstrably hard for most people to reason about. The ability to reason about recursion, for instance, is often used as an interview test to separate the "real programmers" from the mediocre. Recursion is actually one of the few things that computers innately know how to do, so it's not surprising that it's in computer languages too; but if it makes it so that only really smart people can write good programs, then I think it's something we should be trying to get rid of, not use more of. If a car is hard to steer for people of normal strength, we give it power steering, we don't just assume that Driving is a job open only to the abnormally strong.

Of course, I'm blowing into the wind here, because cognitive psychology has not held up its end of the deal. Language designers can't learn much more about how the mind works, because cognitive scientists haven't figured it out yet either. We know very little about what processing contructs are "native" to the mind.

But as a starting point: three things that seem hard for most people are feedback loops (that is, control theory, home of things like proportional-integral-derivative algorithms); recursion (especially if the rules change depending on the depth); and n-dimensional geometry. Electrical and industrial engineers get advanced training in handling feedback loops; I've never met a computer scientist who has, and I've never met a non-specialist who has. Physicists and mathematicians work with n-dimensional manifolds, but EEs and programmers rarely go beyond three. Programmers get advanced training in recursion, but no one else does. In each case, the fact that training is required and that these are all exclusive fields says something about the cognitive difficulty.

The question is not how we can make programming more efficient for a vanishingly small number of people. The question is how we can make it more efficient for a larger number of people.