Log driven development

Everyone knows attempting to figure out what's happening by resorting to print statements is desperation. Everyone knows you should use TDD, BDD or some xDD to write code well, don't they? I am desperate. I suggest PDD, print driven development is slow, error prone but sometimes helpful.

I recently shoe-horned a new feature into a large existing code base. It's hard to make it run a small set of work and I wanted to check a config change create my new class via the dependency injection framework. Now, I could put in a break point and wait a while for it to get hit, or not. Fortuenately, the code writes logs. By adding a log line to the constructor, I could tell much more quickly that I'd got the config correct. Write the code, leave it running, have a look later. I am tempted to name this Log Driven Development.

On one hand, adding the moral equivalents of prints is a bad thing. Teedy Deigh mused on logging in Overload 126, saying

It is generally arbitrary and unsustainable and few people are ever clear what the exact requirements are, so it spreads like a cat meme.

And yet, if I add informative logging as things happen, I can see how the whole system hangs together. Jonathan Boccara mentioned finding logs when trying to understand an unfamiliar code base, in his talk at the ACCU Conference. Existing logs can help you understand some code. Looking at this from a different direction begs the question are my logs useful? As a first step seeing a log entry when I wired in my class felt like a win. It told me what I wanted to know. I must make sure the logs I write out as my class starts doing something useful are themselves useful.

This is a step in the direction of a walking skeleton style approach - get things running end to end, outside-in, then fill in the details. I'm much more comfortable with a TDD approach, but keeping an eye on the whole system and how it hangs together is important. Building up the new feature by writing useful logs means the logs will be useful when it's released. LDD - log driven development. Why not?