https://jed.codes/tags/elixir/Recent content in Elixir on jed.codesHugoen-usWed, 01 Jul 2026 21:41:52 -0700https://jed.codes/posts/betzen-telemetry-driven-development/Wed, 01 Jul 2026 21:41:52 -0700https://jed.codes/posts/betzen-telemetry-driven-development/<p><img src="https://jed.codes/images/telemetry-driven-development.png" alt="Split illustration: on the left, an engineer in a lab coat working at a workbench with red/green/refactor monitors labeled &ldquo;MODEL: TDD — Predefined Behavior&rdquo;; on the right, a field-scientist figure walking through a landscape of telemetry span waterfalls and flame graphs, alongside a placard reading &ldquo;The purpose of a system is what it does — Stafford Beer&rdquo; and &ldquo;FIELD SCIENCE: TeDD — Actual Behavior&rdquo;."></p> <p>Noah Betzen&rsquo;s <a href="https://www.youtube.com/watch?v=irQicdafnyM">&ldquo;Telemetry Driven Development&rdquo;</a> talk from ElixirConf gave me a reason to write down a conviction I&rsquo;ve been carrying for a while: for most of us, Test Driven Development (TDD) is a substitute for understanding how the system behaves. A more honest description is that it <em>models</em> how the system behaves against a set of predefined behaviors and expectations. <strong>TDD is a model of behavior — not the behavior of the system at large.</strong></p>