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Equation #15 · Chapter 6

Reversibility Ratio R_r

Rr=ImpactblastSpeedrollbackR_r = \frac{Impact_{blast}}{Speed_{rollback}}

Reversibility Ratio

R_r · Ch 6

Blast radius ÷ rollback speed. Build the Kill Switch before the feature.

Worked example — A change to the pricing engine

If it's wrong, every invoice that month is wrong — blast radius 60. Rollback is a config flag: seconds, no deploy, score it 80.

It came out at 0.75. Green — ship it.

The same change without the flag, where rollback means a redeploy and a data fix — score 15 — reads 4.0: red, canary first. The feature did not get riskier. The kill switch is the entire difference, which is why you build it before you build the feature.

Your turn — change one number
Result0.75

Green. Ship it. Hand the CIO the kill switch and say 'press it.'