For_each for map-based instances in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does for_each do in Terraform when used with a map?

for_each lets you create multiple resources, one for each item in the map. Each resource gets a unique key from the map.

beginner

How do you access the key and value inside a for_each block for a map?

Use each.key for the map key and each.value for the map value inside the resource block.

intermediate

Why is using a map with for_each better than a list for some cases?

<p>Maps let you name each instance with a key, making it easier to identify and update specific resources.</p>

intermediate

What happens if you change the keys in the map used by for_each?

Terraform will see the old keys as removed and new keys as added, so it will destroy and create resources accordingly.

beginner

Show a simple example of for_each with a map to create multiple AWS EC2 instances.

resource "aws_instance" "example" {
  for_each = {
    web1 = "t2.micro"
    web2 = "t2.small"
  }

  ami           = "ami-123456"
  instance_type = each.value
  tags = {
    Name = each.key
  }
}

What does each.key represent in a for_each loop over a map?

A The index number of the item

B The key of the current map item

C The value of the current map item

D The total number of items

each.key is the key from the map for the current resource instance.

If you want to create resources with names from a map, which Terraform feature helps you do this easily?

A for_each

B count

C variable interpolation

D locals

for_each lets you create one resource per map entry, using keys as names.

What happens if you use a list instead of a map with for_each?

A Terraform will fail to apply

B Terraform will create only one resource

C Terraform will use list indexes as keys

D Terraform will ignore the list

Terraform uses list indexes as keys when for_each is given a list.

Why should you avoid changing keys in a map used by for_each after deployment?

A It automatically updates resources without changes

B It improves performance

C It makes Terraform ignore the resources

D It causes Terraform to recreate resources

Changing keys makes Terraform think old resources are removed and new ones added, causing recreation.

In the example for_each = { web1 = "t2.micro", web2 = "t2.small" }, what is each.value for the resource named web2?

A "t2.small"

B null

C "t2.micro"

D "web2"

each.value is the value from the map for the current key, so for web2 it is "t2.small".

Explain how for_each works with maps in Terraform and why it is useful.

Think about how maps have keys and values and how Terraform uses them.

Describe what happens if you rename a key in the map used by for_each after resources are created.

Consider how Terraform tracks resources by keys.

Practice

1. What does for_each do when used with a map in Terraform?

easy

A. Creates resources only for the first key in the map

B. Deletes all resources in the map

C. Combines all map entries into a single resource

D. Creates one resource for each key-value pair in the map

Solution

1. Step 1: Understand the role of for_each with maps

   for_each iterates over each key-value pair in a map to create multiple resources.

2. Step 2: Recognize resource creation behavior

   Each key-value pair results in one resource instance, using each.key and each.value inside the resource block.

3. Final Answer:

   Creates one resource for each key-value pair in the map -> Option D

4. Quick Check:

   for_each with map = one resource per pair [OK]

Hint: Remember: for_each maps keys to separate resources [OK]

Common Mistakes:

• Thinking for_each merges all map entries into one resource
• Assuming for_each deletes resources
• Believing for_each only uses the first map key

2. Which of the following is the correct syntax to use for_each with a map in a Terraform resource?

easy

A. resource "aws_instance" "example" { for_each = { key1 = "val1", key2 = "val2" } }

B. resource "aws_instance" "example" { for_each = ("key1", "key2") }

C. resource "aws_instance" "example" { for_each = "key1" }

D. resource "aws_instance" "example" { for_each = ["one", "two"] }

Solution

1. Step 1: Identify valid for_each types

   for_each accepts a map or set of strings, but here we focus on map usage.

2. Step 2: Check syntax correctness

   resource "aws_instance" "example" { for_each = { key1 = "val1", key2 = "val2" } } uses a map with key-value pairs correctly inside curly braces, which is valid Terraform syntax.

3. Final Answer:

   resource "aws_instance" "example" { for_each = { key1 = "val1", key2 = "val2" } } -> Option A

4. Quick Check:

   Map syntax with curly braces = correct for_each [OK]

Hint: Use curly braces for maps in for_each [OK]

Common Mistakes:

• Using square brackets for maps instead of curly braces
• Passing a single string instead of a map
• Using parentheses which are invalid here

3. Given the Terraform snippet:

variable "servers" {
  default = {
    web = "10.0.0.1"
    db  = "10.0.0.2"
  }
}

resource "aws_instance" "example" {
  for_each = var.servers
  ami           = "ami-123456"
  instance_type = "t2.micro"
  tags = {
    Name = each.key
    IP   = each.value
  }
}

What will be the value of the tag Name for the instance with IP 10.0.0.2?

medium

A. web

B. 10.0.0.2

C. db

D. example

Solution

1. Step 1: Map keys to IP addresses

   The map has keys 'web' and 'db' with values '10.0.0.1' and '10.0.0.2' respectively.

2. Step 2: Match IP to key

   The IP '10.0.0.2' corresponds to the key 'db', so the tag Name will be 'db'.

3. Final Answer:

   db -> Option C

4. Quick Check:

   each.key for IP 10.0.0.2 = db [OK]

Hint: Match IP value to map key for Name tag [OK]

Common Mistakes:

• Confusing key and value in the map
• Using IP as the Name tag instead of the key
• Assuming static tag values

4. What is wrong with this Terraform resource using for_each on a map?

resource "aws_instance" "example" {
  for_each = { "one" = "10.0.0.1", "two" = "10.0.0.2" }
  ami           = "ami-123456"
  instance_type = "t2.micro"
  tags = {
    Name = each.value
  }
  count = 2
}

medium

A. Using both for_each and count together causes an error

B. The map syntax is invalid

C. Missing each.key in tags

D. AMI ID is incorrect

Solution

1. Step 1: Check resource meta-arguments

   Terraform does not allow using both for_each and count in the same resource block.

2. Step 2: Identify the error cause

   The presence of count = 2 alongside for_each causes a configuration error.

3. Final Answer:

   Using both for_each and count together causes an error -> Option A

4. Quick Check:

   for_each and count cannot coexist [OK]

Hint: Never mix for_each and count in one resource [OK]

Common Mistakes:

• Thinking map syntax is wrong
• Believing missing each.key causes error
• Assuming AMI ID is the problem

5. You have a map of server roles and IPs:

servers = {
  app1 = "10.0.1.1"
  app2 = ""
  app3 = "10.0.1.3"
  app4 = null
}

You want to create AWS instances only for servers with valid IPs using for_each. Which expression correctly filters the map to exclude empty or null IPs?

hard

A. for_each = var.servers

B. for_each = { for k, v in var.servers : k => v if v != "" && v != null }

C. for_each = { for k, v in var.servers : k => v if v == "" || v == null }

D. for_each = [ for v in var.servers : v if v != "" ]

Solution

1. Step 1: Understand filtering with for expressions

   Use a for expression with an if condition to filter out empty strings and null values from the map.

2. Step 2: Check each option

   for_each = { for k, v in var.servers : k => v if v != "" && v != null } correctly filters keys where value is neither empty string nor null, producing a valid filtered map for for_each.

3. Final Answer:

   for_each = { for k, v in var.servers : k => v if v != "" && v != null } -> Option B

4. Quick Check:

   Filter map with if condition to exclude empty/null [OK]

Hint: Filter map with for and if to exclude empty/null [OK]

Common Mistakes:

• Not filtering out empty or null values
• Using list comprehension instead of map
• Filtering for empty/null instead of excluding them