Working with Legacy code - Lag Time

 Lag Time

Changes often take a long time for another very common reason: lag time. Lag time is the amount of time that passes between a change that you make and the moment that you get real feedback about the change. At the time of this writing, the Mars rover Spirit is crawling across the surface of Mars taking pictures. It takes about seven minutes for signals to get from Earth to Mars. Luckily, Spirit has some onboard guidance software that helps it move around on its own. Imagine what it would be like to drive it manually from Earth. You operate the controls and find out 14 minutes later how far the rover moved. Then you decide what you want to do next, do it, and wait another 14 minutes to find out what happened. It seems ridiculously inefficient, right? Yet, when you think about it, that is exactly the way most of us work right now when we develop software. We make some changes, start a build, and then find out what happened later. Unfortunately, we don’t have software that knows how to navigate around obstacles in the build, things such as test failures. What we try to do instead is bundle a bunch of changes and make them all at once so that we don’t have to build too often. If our changes are good, we move along, albeit as slow as the Mars rover. If we hit an obstacle, we go even slower.

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The human mind has some interesting qualities. If we have to perform a short task (5-10 seconds long) and we can only take a step once every minute, we usually do it and then pause. If we have to do some work to figure out what to do at the next step, we start to plan. After we plan, our minds wander until we can do the next step. If we compress the time betwen steps down from a minute to a few seconds, the quality of the mental work becomes different. We can use feedback to try out approaches quickly. Our work becomes more like driving than like waiting at a bus stop. Our concentration is more intense because we aren’t constantly waiting for the next chance to do something. Most important, the amount of time that it takes us to notice and correct mistakes is much smaller.