Splat expressions in Terraform - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Understanding Splat Expression Output

Given the following Terraform resource block, what will be the output of output.instances_ips?

Terraform

resource "aws_instance" "example" {
  count         = 3
  ami           = "ami-123456"
  instance_type = "t2.micro"
  tags = {
    Name = "example-${count.index}"
  }
}

output "instances_ips" {
  value = aws_instance.example[*].public_ip
}

A ["ami-123456", "ami-123456", "ami-123456"]

B [null, null, null]

C ["example-0", "example-1", "example-2"]

D ["t2.micro", "t2.micro", "t2.micro"]

💡 Hint

The public_ip attribute is only set after the instance is created and if it has a public IP assigned.

The splat expression aws_instance.example[*].public_ip collects the public_ip attribute from all instances. Since these instances do not have an associated public IP by default, the output is a list of nulls.

❓ Configuration

intermediate

Correct Use of Splat Expression in Resource Arguments

Which option correctly uses a splat expression to assign all subnet IDs from a list of subnets to a security group rule's subnet_ids argument?

Terraform

resource "aws_subnet" "example" {
  count = 2
  vpc_id     = "vpc-123456"
  cidr_block = "10.0.${count.index}.0/24"
}

resource "aws_security_group_rule" "example" {
  type        = "ingress"
  from_port   = 80
  to_port     = 80
  protocol    = "tcp"
  security_group_id = "sg-123456"
  subnet_ids  = ???
}

A aws_subnet.example.id

B aws_subnet.example.id[*]

C aws_subnet.example[*].id

D aws_subnet.example[*].subnet_id

💡 Hint

The splat expression syntax is resource_name[*].attribute to get a list of attribute values.

Option C correctly uses the splat expression to get a list of all subnet IDs from the aws_subnet.example resources. Other options either have incorrect syntax or reference non-existent attributes.

❓ Architecture

advanced

Splat Expressions in Complex Module Outputs

You have a module that creates multiple EC2 instances and outputs their private IPs as private_ips. Which option correctly references all private IPs from the module using a splat expression?

Terraform

module "web_servers" {
  source = "./modules/ec2_instances"
  instance_count = 3
}

output "all_private_ips" {
  value = ???
}

A module.web_servers.private_ips

B module.web_servers.private_ips[*]

C module.web_servers[*].private_ips

D module.web_servers[*].private_ips[*]

💡 Hint

Modules are referenced by name and their outputs are accessed directly without splat unless the module is called multiple times with count or for_each.

Since the module is called once, its output private_ips is already a list of IPs. Using splat expressions like module.web_servers[*].private_ips is invalid because the module is not a list.

❓ security

advanced

Splat Expression Impact on Security Group Rules

You want to create security group rules for multiple instances using their private IPs. Which splat expression usage will cause a Terraform plan error?

Terraform

resource "aws_instance" "app" {
  count         = 2
  ami           = "ami-abc123"
  instance_type = "t3.micro"
}

resource "aws_security_group_rule" "allow_app" {
  type              = "ingress"
  from_port         = 8080
  to_port           = 8080
  protocol          = "tcp"
  security_group_id = "sg-abc123"
  cidr_blocks       = ???
}

A aws_instance.app.private_ip[*]

B aws_instance.app[*].private_ip

C aws_instance.app.private_ip

D aws_instance.app[*].private_ip[*]

💡 Hint

Check the correct syntax for splat expressions on resource attributes.

Option A is invalid because aws_instance.app.private_ip is not a list, so applying [*] causes a syntax error. The correct splat expression is aws_instance.app[*].private_ip.

🧠 Conceptual

expert

Behavior of Nested Splat Expressions

Consider a resource with nested blocks and a splat expression used to extract a nested attribute list. What is the output of the following expression?

Terraform

resource "aws_lb" "example" {
  name               = "example-lb"
  internal           = false
  load_balancer_type = "application"

  subnet_mapping {
    subnet_id     = "subnet-123"
    allocation_id = "eipalloc-123"
  }

  subnet_mapping {
    subnet_id     = "subnet-456"
    allocation_id = "eipalloc-456"
  }
}

output "allocation_ids" {
  value = aws_lb.example[*].subnet_mapping[*].allocation_id
}

A [["subnet-123", "subnet-456"]]

B ["eipalloc-123", "eipalloc-456"]

C [null, null]

D [["eipalloc-123", "eipalloc-456"]]

💡 Hint

Nested splat expressions produce nested lists reflecting the structure of the nested blocks.

The expression aws_lb.example[*].subnet_mapping[*].allocation_id returns a list of lists because subnet_mapping is a nested block list inside each load balancer resource. Since there is one load balancer, the outer list has one element which is a list of allocation IDs.