How to Use depends_on in Terraform for Resource Dependencies

In Terraform, use the depends_on argument inside a resource or module block to explicitly specify dependencies on other resources. This ensures Terraform creates or updates resources in the correct order when implicit dependencies are not enough.

Syntax

The depends_on argument takes a list of resource or module references that the current resource depends on. Terraform waits for these dependencies to be created or updated before processing the current resource.

• depends_on: A list of resource or module addresses.
• Placed inside a resource or module block.

terraform

resource "aws_instance" "example" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"

  depends_on = [aws_security_group.sg]
}

resource "aws_security_group" "sg" {
  name        = "example-sg"
  description = "Allow SSH"
  ingress {
    from_port   = 22
    to_port     = 22
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

Example

This example shows an AWS EC2 instance that depends on a security group. Terraform ensures the security group is created before the instance.

terraform

resource "aws_security_group" "web_sg" {
  name        = "web-sg"
  description = "Allow HTTP"

  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

resource "aws_instance" "web_server" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  depends_on = [aws_security_group.web_sg]

  tags = {
    Name = "WebServer"
  }
}

Output

Terraform will create the aws_security_group.web_sg first, then create aws_instance.web_server after the security group is ready.

Common Pitfalls

Common mistakes when using depends_on include:

• Using depends_on unnecessarily when Terraform already detects dependencies from resource references, which can slow down your plan and apply.
• Referencing resources incorrectly inside depends_on (must use full resource address).
• Trying to use depends_on inside variable or output blocks, which is not supported.

terraform

resource "aws_instance" "example" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"

  # Wrong: depends_on with a string instead of a list
  depends_on = aws_security_group.sg
}

# Correct usage:
resource "aws_instance" "example" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"

  depends_on = [aws_security_group.sg]
}

Quick Reference

Use depends_on to:

• Force resource creation order when implicit dependencies are missing.
• Control module execution order.
• Prevent race conditions in complex infrastructure.

Remember:

• depends_on accepts a list of resource or module addresses.
• It is optional and should be used only when needed.
• Terraform automatically detects dependencies from resource references.

Key Takeaways

Use depends_on to explicitly declare resource dependencies in Terraform.

Place depends_on inside resource or module blocks as a list of references.

Avoid unnecessary use of depends_on to keep Terraform plans efficient.

Terraform automatically detects dependencies from resource references, so use depends_on only when implicit detection is insufficient.

Do not use depends_on in variables or outputs; it only works in resources and modules.