# Functionality

Behavior matching what the software claims to do. The most-discussed
dimension in software quality and the one the testing tradition lives
inside. Two classes of method divide the work:
[example tests](https://quality.stereobooster.com/example-tests.md) pin the cases
written down, and generative methods (property-based, fuzzing,
differential, metamorphic) reach the input space enumeration can't
cover. A functionality verification program carries both.

## What goes wrong

- **Incorrect output for known inputs.** Example regression tests catch
  these, once written.
- **Incorrect output for inputs nobody thought to test.** Where
  example enumeration falls short by construction.
  Property-based, fuzzing, differential, and metamorphic testing
  target this gap.
- **Edge cases.** Off-by-one, null/empty/overflow, time zones,
  the boundary of integer ranges. Often invisible to spec writers
  because the spec doesn't enumerate them.
- **Concurrency and ordering.** Race conditions, lost updates,
  out-of-order delivery. [TLA+](https://lamport.azurewebsites.net/tla/tla.html) and deterministic simulation find
  these; example tests miss them.
- **API contract drift.** Two services that agreed to a spec but
  one drifted. Static schema-compatibility checks ([OpenAPI](https://www.openapis.org/) diffing,
  protobuf breaking-change detection) catch syntactic breaks; live
  integration tests catch semantic ones; consumer-driven contract
  tests catch semantic breaks without a live counterpart ([snapshot
  testing at the seam](https://quality.stereobooster.com/snapshot-testing.md#recorded-interaction)).

## What success looks like

- **Bug-fix commits include a test.** The regression ceiling
  means this is the only way to know the bug won't recur.
- **Diff-time signals fire before merge.** Coverage, linter, and
  types on touched files.
- **Property tests cover the input space that can be described.** A
  bug that falls outside that space marks the next gap.
- **The next bug class has a named oracle.** For input classes that
  can't be enumerated, a property, a reference implementation, or a
  metamorphic relation is the oracle to reach for next.

## Referenced by

- [Quality dimensions](https://quality.stereobooster.com/quality-dimensions.md) · Quality dimensions
- [Playwright](https://quality.stereobooster.com/playwright.md) · Recipes
- [Choosing methods](https://quality.stereobooster.com/choosing.md) · Overview