Reviewing a Plan Before You Apply

The message that precedes the costliest incidents is always the same, and it was on the screen: Plan: 3 to add, 1 to change, 1 to destroy. The resource being destroyed was the production database, and the plan said so, in plain text, fifteen lines earlier. Nobody read it. Not out of carelessness: the output was four hundred lines long, the previous twenty applies had destroyed nothing, and everyone had gotten into the habit of scrolling straight to the confirmation prompt.

A Terraform plan is the only artifact in the entire chain where a mistake still costs nothing. After apply, a deleted database can be restored from a backup, at best with some data loss and a service interruption. Knowing how to read a plan is therefore not a nice-to-have skill; it is the tool's main defensive skill. This lesson teaches you to decode the symbols, to spot destructive replacements in seconds even in a long output, and to take review out of the realm of individual goodwill and put it into a continuous integration pipeline.

The Symbols

Terraform lists its symbols at the top of the output, but only five really matter.