Check blocks for assertions in Terraform - Step-by-Step Execution

Process Flow - Check blocks for assertions

Start Terraform Plan

↓

Evaluate Check Block

↓

Run Assertion Condition

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Pass Check

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Continue

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Apply Infrastructure

Terraform runs check blocks during plan to verify conditions. If assertions pass, plan continues; if not, it stops.

Execution Sample

Terraform

check "example_check" {
  condition = length(var.names) > 0
  error_message = "names list must not be empty"
}

This check block asserts that the variable 'names' list is not empty during terraform plan.

Process Table

Step	Action	Condition Evaluated	Result	Plan Behavior
1	Start terraform plan	N/A	N/A	Plan starts
2	Evaluate check block 'example_check'	length(var.names) > 0	True	Plan continues
3	Apply infrastructure	N/A	N/A	Resources created
4	End plan	N/A	N/A	Plan successful

💡 Check block condition is true, so plan proceeds and applies resources

Status Tracker

Variable	Start	After Check Evaluation	Final
var.names	["app", "db"]	["app", "db"]	["app", "db"]
check.condition	N/A	True	True

Key Moments - 2 Insights

What happens if the check block condition is false?

If the condition is false (see execution_table step 2), Terraform stops the plan and shows the error_message.

Does the check block run during apply?

No, check blocks run only during terraform plan to validate conditions before applying.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of the check condition at step 2?

A True

B False

C Error

D Not evaluated

💡 Hint

Refer to the 'Condition Evaluated' and 'Result' columns in execution_table row 2

At which step does Terraform stop the plan if the check fails?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Plan Behavior' column in execution_table for when the plan stops

If var.names was empty, how would the execution_table change at step 2?

A Result would be True and plan continues

B Condition would not be evaluated

C Result would be False and plan stops

D Plan would skip check block

💡 Hint

Refer to the key_moments explanation about false condition behavior

Concept Snapshot

Terraform check blocks run during plan to assert conditions.
Syntax: check "name" { condition = <expr> error_message = "msg" }
If condition is false, plan stops with error.
If true, plan continues and applies resources.
Check blocks help catch config errors early.

Full Transcript

Terraform check blocks are special blocks that run during the terraform plan phase. They evaluate a condition expression. If the condition is true, the plan continues normally. If the condition is false, Terraform stops the plan and shows the error message defined in the check block. This helps catch configuration mistakes before applying infrastructure. Check blocks do not run during apply, only during plan. In the example, the check asserts that the variable 'names' list is not empty. Since it is not empty, the plan passes and resources are created.