What is the test pyramid?

What are the three layers?

• Unit tests check one function or class in isolation and run in milliseconds.
• Integration tests check that parts work together, for example code and a database.
• End-to-end tests drive the whole system the way a user would, and are the slowest and most fragile.

Why is it a pyramid and not a rectangle?

Every layer you climb costs more to run, more to maintain and more to debug when it fails. A suite that is mostly end-to-end tests is slow and flaky, so people stop trusting it. Many cheap tests at the bottom give fast feedback and pinpoint failures.

What is a flaky test, and why is it a problem?

A flaky test sometimes passes and sometimes fails with no code change. It teaches the team to ignore red builds, which hides real failures. Fix it or remove it quickly, and find the cause, which is often timing, shared data or a network call.

Does the pyramid mean never write end-to-end tests?

No. Keep a few for the journeys that matter most, such as signing up and paying. The pyramid is about proportion, not about banning a layer.

How does it relate to continuous integration?

Fast tests make it practical to run checks on every change before it merges, which is the core of CI and CD. A suite that takes an hour defeats the purpose.

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