An operator resized a CloudFormation-managed instance in the console during an incident. The template still lists the smaller type. What will a drift-aware change set (REVERT_DRIFT mode) do to that instance if the template is not edited?
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Short Explanation
The template is the plan, and a drift-aware change set enforces it against the live state. You changed the live instance, and the plan still says small, so the change set puts small back. CloudFormation will not quietly rewrite the template to bless the console.
Full Explanation
A standard change set or stack update only compares the new template with the last deployed template. With an unchanged template it would not touch the instance, so the console resize would simply stay as drift. A drift-aware change set, created with the REVERT_DRIFT deployment mode, adds a third input, the actual state of the resource, and brings drifted resources back in line with the template. The console change to instance type is drift, so the change set shows the instance type being modified back to the value in the template, which undoes the incident resize. CloudFormation does not edit the template to match the console, and it does not skip a resource it manages. A type change is an update, not a requirement to delete the instance first, though some property changes replace resources, so read the change set before you execute it. Caveat: if the larger size must stay, change the template to that type and update on purpose, so the next person does not fight the same drift. Operational check: create the drift-aware change set, read the property diff that marks the instance type as drifted, then either execute it or edit the template first.