Count vs for_each decision in Terraform - Hands-On Comparison

Count vs for_each decision

📖 Scenario: You are setting up cloud resources using Terraform. You want to create multiple virtual machines (VMs) in a cloud environment. Some VMs are identical, and some have unique names and settings.

🎯 Goal: Learn when to use count and when to use for_each in Terraform to create multiple resources efficiently.

📋 What You'll Learn

Create a variable with a list of VM names

Create a variable with the number of identical VMs

Use count to create identical VMs

Use for_each to create uniquely named VMs

💡 Why This Matters

🌍 Real World

Cloud engineers often need to create multiple resources that are either identical or unique. Knowing when to use <code>count</code> or <code>for_each</code> helps manage infrastructure efficiently.

💼 Career

Understanding <code>count</code> and <code>for_each</code> is essential for Terraform users to write scalable and maintainable infrastructure code.

1

Create a list variable with VM names

Create a Terraform variable called vm_names with the list ["web1", "web2", "db1"].

Terraform

# Create a variable called vm_names with the list ["web1", "web2", "db1"]
# Your code here

Hint

Use variable block with type = list(string) and default set to the list.

2

Create a variable for identical VM count

Create a Terraform variable called vm_count with the default value 3.

Terraform

variable "vm_names" {
  type    = list(string)
  default = ["web1", "web2", "db1"]
}

# Create a variable called vm_count with default 3
# Your code here

Hint

Use a variable block with type = number and default = 3.

3

Use count to create identical VMs

Create a resource block resource "azurerm_virtual_machine" "identical_vm" that uses count = var.vm_count and sets name = "vm-${count.index + 1}".

Terraform

variable "vm_names" {
  type    = list(string)
  default = ["web1", "web2", "db1"]
}

variable "vm_count" {
  type    = number
  default = 3
}

# Create resource azurerm_virtual_machine identical_vm with count = var.vm_count
# Set name = "vm-${count.index + 1}"
# Your code here

Hint

Use count = var.vm_count and name = "vm-${count.index + 1}" inside the resource block.

4

Use for_each to create uniquely named VMs

Create a resource block resource "azurerm_virtual_machine" "unique_vm" that uses for_each = toset(var.vm_names) and sets name = each.key.

Terraform

variable "vm_names" {
  type    = list(string)
  default = ["web1", "web2", "db1"]
}

variable "vm_count" {
  type    = number
  default = 3
}

resource "azurerm_virtual_machine" "identical_vm" {
  count = var.vm_count

  name = "vm-${count.index + 1}"
  # Other required properties would go here
}

# Create resource azurerm_virtual_machine unique_vm with for_each = toset(var.vm_names)
# Set name = each.key
# Your code here

Hint

Use for_each = toset(var.vm_names) and name = each.key inside the resource block.

Practice

1. Which Terraform argument should you use when you want to create 3 identical resources without unique identifiers?

easy

A. count = 3

B. for_each = {}

C. for_each = toset(["a", "b", "c"])

D. count = 0

Solution

1. Step 1: Understand the purpose of count

   count creates multiple copies of a resource by number, perfect for identical resources.

2. Step 2: Compare with for_each

   for_each is used when you want unique keys or values to identify each resource, not just copies.

3. Final Answer:

   count = 3 -> Option A

4. Quick Check:

   Simple copies = count [OK]

Hint: Use count for number copies, for_each for unique keys [OK]

Common Mistakes:

• Using for_each when only number copies are needed
• Setting count to zero expecting resources
• Confusing sets with counts

2. Which of the following is the correct syntax to create resources using for_each with a map in Terraform?

easy

A. resource "aws_instance" "example" { for_each = ["a", "b"] }

B. resource "aws_instance" "example" { for_each = "string" }

C. resource "aws_instance" "example" { count = 2 }

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

Solution

1. Step 1: Identify valid for_each types

   for_each accepts maps or sets, not lists or strings directly.

2. Step 2: Check each option

   resource "aws_instance" "example" { for_each = { key1 = "val1", key2 = "val2" } } uses a map with keys and values, which is valid syntax for for_each.

3. Final Answer:

   resource with for_each map -> Option D

4. Quick Check:

   for_each needs map or set [OK]

Hint: for_each needs map or set, not list or string [OK]

Common Mistakes:

• Using list instead of map/set for for_each
• Confusing count with for_each syntax
• Passing string directly to for_each

3. Given this Terraform snippet:

resource "aws_s3_bucket" "buckets" {
  for_each = { bucket1 = "private", bucket2 = "public-read" }
  bucket = each.key
  acl = each.value
}

What will be the names of the created buckets?

medium

A. ["each.key", "each.value"]

B. ["private", "public-read"]

C. ["bucket1", "bucket2"]

D. No buckets created due to syntax error

Solution

1. Step 1: Understand for_each map keys and values

   The keys are bucket1 and bucket2, values are ACL values.

2. Step 2: Check bucket assignment

   The bucket name is set to each.key, so buckets named "bucket1" and "bucket2" are created.

3. Final Answer:

   ["bucket1", "bucket2"] -> Option C

4. Quick Check:

   Bucket names = map keys [OK]

Hint: for_each keys become resource identifiers [OK]

Common Mistakes:

• Using values as bucket names instead of keys
• Assuming syntax error without checking
• Confusing each.key with each.value

4. You wrote this Terraform resource:

resource "aws_instance" "example" {
  count = length(var.instances)
  name  = var.instances[count.index]
}

But Terraform throws an error about count.index. What is the likely cause?

medium

A. count is used but for_each should be used for maps

B. var.instances is a map, but count expects a list

C. You cannot use count.index inside resource blocks

D. count must be a fixed number, not a variable

Solution

1. Step 1: Identify variable type mismatch

   var.instances is likely a map, but count works with numbers and lists indexed by numbers.

2. Step 2: Choose correct iterator

   For maps, for_each is the correct choice to iterate over keys and values.

3. Final Answer:

   Use for_each for maps, not count -> Option A

4. Quick Check:

   Maps need for_each, not count [OK]

Hint: Use for_each for maps, count for lists [OK]

Common Mistakes:

• Using count.index with maps
• Assuming count must be fixed number
• Ignoring variable data type

5. You want to create multiple AWS IAM users with unique usernames and policies from this map:

users = {
  alice = "admin",
  bob = "read-only",
  carol = "power-user"
}

Which Terraform resource block correctly uses count or for_each to create these users with unique names?

hard

A. resource "aws_iam_user" "user" {
count = length(keys(var.users))
name = keys(var.users)[count.index]
policy = var.users[name]
}

B. resource "aws_iam_user" "user" {
for_each = var.users
name = each.key
policy = each.value
}

C. resource "aws_iam_user" "user" {
count = 3
name = var.users[count.index]
policy = var.users[count.index]
}

D. resource "aws_iam_user" "user" {
for_each = toset(var.users)
name = each.value
policy = each.key
}

Solution

1. Step 1: Analyze data structure

   The users variable is a map with usernames as keys and policies as values.

2. Step 2: Match iteration method

   for_each works well with maps to create uniquely identified resources using keys and values.

3. Step 3: Check each option

   resource "aws_iam_user" "user" {
     for_each = var.users
     name     = each.key
     policy   = each.value
   }

   correctly uses for_each = var.users and assigns name = each.key and policy = each.value.

4. Final Answer:

   Use for_each with map for unique users -> Option B

5. Quick Check:

   Unique keys need for_each [OK]

Hint: Use for_each with maps for unique resource keys [OK]

Common Mistakes:

• Using count with maps incorrectly
• Mixing keys and values in wrong order
• Using toset on maps causing errors