How to fix Error: Cycle: resource dependency in Terraform

Use the terraform graph command to see the dependency relationships. For detailed cycle detection, use:

terraform graph -draw-cycles | dot -Tsvg > graph.svg

This creates a visual representation where cycles are highlighted. You'll need Graphviz installed (apt-get install graphviz on Linux or brew install graphviz on macOS).

Examine the error message and the graph to find which resources are referencing each other. For example:

Error: Cycle: aws_instance.web, aws_security_group.web

This means the EC2 instance and security group are referencing each other.

Replace direct resource references with data sources where possible. Instead of:

resource "aws_security_group" "web" {
  vpc_id = aws_vpc.main.id
}

resource "aws_instance" "web" {
  security_groups = [aws_security_group.web.id]
  vpc_id          = aws_vpc.main.id
}

Use a data source to look up the security group by name instead of creating a new reference:

data "aws_security_group" "web" {
  name = "existing-sg"
  vpc_id = aws_vpc.main.id
}

resource "aws_instance" "web" {
  security_groups = [data.aws_security_group.web.id]
  vpc_id          = aws_vpc.main.id
}

In some cases, you can break cycles by explicitly defining the creation order with depends_on. However, this only works if you're referencing specific outputs rather than attributes:

resource "aws_network_interface" "example" {
  subnet_id = aws_subnet.example.id
}

resource "aws_instance" "example" {
  depends_on = [aws_network_interface.example]
  # Don't reference the network interface directly
}

For complex cycles that can't be broken with data sources, split the configuration into multiple Terraform projects:

1. Create resource A and output its ID
2. In a separate Terraform configuration, use data sources to reference the ID from step 1
3. Create resource B using the data source
4. If needed, manually update resource A in a third Terraform run

This multi-step approach respects the DAG requirement while still managing complex infrastructure.

After refactoring, run:

terraform validate
terraform plan

Ensure the plan shows the resources in the correct order and no cycles remain.