Sunday, December 15, 2013

Principle of Exposure and Transparency - POET

The Visibility Problem

We have a problem in programming. The majority of our work is invisible. It's invisible to the end user. It's invisible to us. We can only see ~50 lines at a time of the thousands and millions of lines of code in our programs. Imagine a miopic Leonardo da Vinci who could only focus on a square inch of canvas trying to paint the Mono Lisa or even the Sistine Chapel.

Most modern debuggers can only show a dozen variables and a stack trace. Generously, that represents no more than a few hundred bytes of memory. Our running programs are measured in megabytes if not gigabytes. In anything but the most trivial program, we can't see more than a small fraction of a percent of our paused runtime state. When our creation is actually running, we are blind.

Two Challenges of Programming

All difficulties in programming can be separated into two categories. There are the problems due to core computer-science limitations or limitations of our personal understanding. These are legitimate challenges which require real engineering, design and exploration. All other problems are due to a lack of understanding of what our creation is actually doing.

One way to measure a programmer's skill is how well they can simulate programs in their head. This can be measured concretely by how much code can they write without error without running or compiling. Mental program simulation is the only way we can mange the complete lack of visibility in running programs. The best programmers have developed an intuitive sense about running programs over years and decades.

Simulating programs in our head is a major part of modern programming. This is broken. The computer is infinitely better at running programs. There is a monumentous opportunity to improve programming by developing better tools for inspecting, exposing and making our running programs transparent.

The Principle of Exposure and Transparency (POET)

POET: All bugs are trivial once their cause is exposed; given enough runtime transparency, all bugs are trivial.

The vast majority of bugs only require a few lines to fix. Most of our debugging time is spent trying to understand bugs well enough to know what two lines to change. POET embodies this observation: All this wasted time can be reclaimed with better instruments for understanding our running programs.

Linus's Law

Given enough eyeballs, all bugs are shallow. - Eric Raymond, The Cathedral and the Bazaar

POET is a generalization of Linus's Law. Getting more people looking at the code - "enough eyeballs" - is one way to better expose the code's runtime properties. However, it is a very inefficient method. It is akin to saying "with enough miopic da Vincis, we can paint the whole Mono Lisa." While perhaps true, a better solution would be repair his vision so Leonardo can do it without assistance.


  1. Pay attention the next time you run your code. Why are you running it? What information are you attempting to gain? How much time does it take to gather this information? Be sure to include the time it takes you to context switch, type in commands, wait for the run to start and do whatever interaction it takes to bring the program to the required state. How might you gain that information quicker with better tools? LiveReload plugins for web-page development is one such example.
  2. When you are working on a bug or performance problem, note how much time you spend understanding the problem vs how many lines of code were changed in the final fix. If you could send that patch back in time, how much time would you have saved? Note the key insight along your discovery path led to the final solution. How could you have gotten that insight sooner? ( Spark Inspector was just that tool for us in a recent debug jam session for our iPad app. )


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