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Rollback & Kill Switches

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Rolling back code

• Verify you can roll back to the previous release without rebuilding it.
• Verify you have rolled back at least once, deliberately, rather than only planned to.
• Verify how long a rollback takes, measured.
• Verify previous build artifacts are retained long enough to roll back to more than one release ago.
• Verify a rollback does not require the person who deployed.
• Verify rolling back the application does not leave the database in a state the old code cannot read — see scale/03-database.md.
• Verify a release that cannot be rolled back is identified as such before it ships, not after.

Switches that must already exist

• Verify every risky new feature ships behind a flag that can be turned off without a deploy.
• Verify the flag system itself does not depend on the thing that might be broken.
• Verify a flag defaults to the safe value if the flag service is unreachable.
• Verify there is a kill switch for each expensive or externally-facing capability: outbound fetching, email, SMS, model calls, signups, uploads.
• Verify each kill switch has been exercised in production at least once.
• Verify you can put the whole service into read-only mode, if the data model allows it.
• Verify you can disable a single tenant or account without affecting others.
• Verify turning something off is reversible and that turning it back on is also tested.

Undoing data and config

• Verify configuration changes are versioned and revertible, including anything changed in a provider's console.
• Verify infrastructure changes go through code where possible, so the previous state is recoverable.
• Verify a DNS or edge configuration change can be reverted quickly, and that you know the propagation delay.
• Verify a bad bulk operation can be undone — either it was reversible, or a backup was taken first.
• Verify someone reviews destructive one-off scripts before they run against production, even if that someone is you an hour later.