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The weighted routing lesson covered the mechanism: a coin per request, the statistics of a small canary, and the fact that the denominator moves during an incident.

This lesson is about what has to be true before it is safe to use it.

Five rows with what each costs. Change the weights: one line of configuration, a minute. Make the cache keys distinguish versions: or one version serves the other's output, an afternoon. Make the session readable by both: or a user's cart breaks mid-flow, a week. Make the schema work in both directions: in both directions, both live, sometimes two releases. Make the message formats compatible: consumers of both versions exist, a design decision. — FigureFigure 1. The weight change takes a minute. Everything that makes it safe takes much longer
Running two versions at once means everything they share has to work for both, and caches, sessions, schemas and message formats are all shared by default.

The question that decides whether a split is ready is not whether the routing is configured. It is whether an old request and a new request, interleaved at random, both produce correct results against the same shared state.

A hand-drawn comparison. The old code against everything the new code has written. The new code against everything the old code has written. Two notes read: a deploy only needs the second one, a split needs both, and which is why splitting is harder than deploying, not easier. — SketchFigure 2. A split needs compatibility in both directions
New code reading old data is the direction everybody thinks about. Old code reading what the new version wrote is the one that gets missed.

Traffic Splitting

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The Weight Change Is the Easy Part

What Actually Collides