# Canary vs Blue-Green

Two rollout patterns: canary exposes a small slice first; blue-green flips all traffic between two full environments.

## Why the CCAR-P exam tests this

The exam checks whether you pick the rollout whose reversal cost and evidence gathering fit the risk of the change.

## Definition

A **canary** release routes a **small slice** of traffic to the new version first, watches the signals, and widens gradually. It gathers real evidence at low blast radius and lets you abort after affecting only a few users. A **blue-green** deployment stands up a full second environment (green) alongside the current one (blue) and **flips all traffic at once**, with the old environment kept warm for instant rollback.

The axis that separates them is **how you buy down reversal cost**. Canary reduces exposure *while learning* — ideal when the change is risky and you want progressive evidence. Blue-green makes the *switch itself* instantly reversible — ideal when you cannot tolerate a mixed-version state (e.g., a schema-coupled change) but can validate the whole environment before the flip.

Neither eliminates the need for a **parity eval before cutover**: blue-green's clean flip is only safe if green has been proven equivalent to blue first. Canary spreads that validation across live traffic instead.

## Exam trap

Choosing blue-green for its instant rollback while skipping the pre-cutover parity check is the trap — a fast flip to an unvalidated environment just fails faster.

## Commonly confused with

- **migration-parity-eval** — Parity eval is the validation that must pass before a blue-green flip; the rollout pattern is not a substitute for it.
- **poc-vs-pilot-vs-production** — These are release mechanics within production; PoC/pilot/production are the lifecycle stages around them.
- **reversal-cost** — Both patterns are tools for lowering reversal cost; reversal cost is the underlying property, not a rollout method.

## Related

- migration-parity-eval
- reversal-cost
- poc-vs-pilot-vs-production

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Source: Claude Cert Prep — an independent, unofficial CCAR-P study resource (domain P6). Not affiliated with Anthropic.