Fail Fast

The statement in a test that must hold, and the only part that can actually fail.

Over-trusting an automated recommendation, including against available contrary evidence.

Finding a value in a sorted range by repeatedly discarding half of it.

The proportion of conditional outcomes exercised — the strictest ordinary coverage metric, and the most informative.

Deliberately breaking parts of a running system to prove it survives failures that will eventually happen anyway.

Time-based scheduling of recurring jobs, and the reasons scheduled jobs quietly stop working.

A substituted value that papers over a missing input and hides the misconfiguration that produced it.

Writing code that anticipates misuse and bad input, checking assumptions and handling the unexpected rather than trusting callers, along with the debate over when defense should give way to failing loudly.

Specifying what each function requires and guarantees, as checkable preconditions, postconditions, and invariants, so a violation says exactly whose bug it is.

A named value in a process's environment, inherited by its children — the standard configuration channel.

Writing and placing the messages that tell people something went wrong, so they understand what happened and how to recover without feeling blamed.

The small number every process returns when it ends, zero for success and anything else for failure, and the conventions that let shells, CI systems, and init systems act on it.

A design principle in which a system that breaks falls into a safe state rather than a dangerous one, as when a railway signal drops to stop or a train's brakes apply if their air line is cut.

Scripts Git runs automatically at points such as committing or pushing, used to check formatting and messages locally, and on servers to enforce policy or trigger deploys.

When a measure becomes a target, it stops being a good measure.

Requiring a person's judgment at chosen points in an otherwise automated process.

The operating system kernel deliberately halting when it detects an error it can't safely recover from, the Unix version of the Blue Screen of Death, and how crash dumps and watchdogs turn it into a fast reboot.

The time spent debating an item is inversely proportional to its actual stakes.

A wrapper type with a way to lift a value in and a way to chain functions that return more wrappers.

How much of a system's internal state can be inferred from what it emits.

The kernel's last resort when memory is exhausted, killing a process chosen by heuristic, and the userspace daemons that act earlier with better judgment.

Pressure applied to a confined fluid is transmitted undiminished throughout it.

Be conservative in what you produce, liberal in what you accept — a design rule for interoperating systems.

Checking data against a declared shape at runtime, and using the result as the typed value.

Storing, distributing, and rotating credentials without embedding them in code or history.

A failure that produces no signal, so the absence of an error reads as success.