Count vs for_each decision in Terraform - Quick Revision & Key Differences

Recall & Review

beginner

What is the main difference between count and for_each in Terraform?

count creates multiple instances based on a number, while for_each creates instances based on each item in a map or set.

beginner

When should you prefer for_each over count?

Use for_each when you want to create resources with unique keys or names, especially when working with maps or sets.

beginner

Can count be used with maps or sets directly?

No, count only accepts a number. To use maps or sets, for_each is required.

intermediate

How does resource addressing differ between count and for_each?

count uses numeric indexes (e.g., resource[0]), while for_each uses keys from the map or set (e.g., resource["key"]).

intermediate

What is a common pitfall when using count with conditional expressions?

Changing count dynamically can cause resource replacement because indexes shift, leading to unexpected resource destruction and recreation.

Which Terraform argument allows creating resources based on keys from a map or set?

A for_each

B count

C depends_on

D lifecycle

for_each iterates over maps or sets to create resources with unique keys.

What type of value does count accept?

A Number

B Map

C Set

D List

count requires a numeric value to specify how many instances to create.

If you want to create three identical resources, which is simpler to use?

A for_each with a map of three keys

B count = 3

C depends_on

D provider block

count is simpler for creating multiple identical resources by number.

What happens if you change the order of items in a for_each map?

A Terraform throws an error

B Resources get replaced due to index shift

C Resources keep their keys and do not get replaced

D Resources are duplicated

for_each uses keys, so order changes do not cause resource replacement.

Which method is better for resources that need unique names or identifiers?

A depends_on

B count

C variable

D for_each

for_each allows assigning unique keys, making it better for unique resource names.

Explain when and why you would choose count versus for_each in Terraform.

Think about whether you need unique keys or just a number of copies.

Describe how resource addressing differs between count and for_each and why this matters.

Consider how Terraform tracks each resource instance.

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