Cross-resource dependencies

Cross-Resource Dependencies in Terraform

Why Cross-Resource Dependencies Matter

Cross-resource dependencies are fundamental to Infrastructure as Code because real-world infrastructure components often rely on each other. A web server needs a network to exist before it can be deployed, a database security group must be created before the database instance references it, and load balancers require target instances to be running. Understanding how Terraform handles these dependencies is crucial for building reliable, predictable infrastructure.

What Are Cross-Resource Dependencies?

Cross-resource dependencies define the order in which Terraform creates, updates, or destroys resources. When Resource B depends on Resource A, Terraform ensures that Resource A is fully provisioned before attempting to create Resource B. Similarly, during destruction, Resource B is removed before Resource A.

Terraform supports two types of dependencies:

1. Implicit Dependencies
These are automatically detected by Terraform when you reference one resource's attributes within another resource's configuration. Terraform analyzes these references and builds a dependency graph.

Example:
resource "aws_instance" "web" {
  subnet_id = aws_subnet.main.id
}

Here, Terraform understands that the EC2 instance depends on the subnet because it references aws_subnet.main.id.

2. Explicit Dependencies
When there's no attribute reference but a dependency still exists, you use the depends_on meta-argument to explicitly declare the relationship.

Example:
resource "aws_instance" "web" {
  depends_on = [aws_iam_role_policy.example]
}

How Cross-Resource Dependencies Work

Terraform builds a dependency graph during the planning phase. This directed acyclic graph (DAG) determines:

• The order of resource operations
• Which resources can be created in parallel
• Which resources must wait for others

When you run terraform plan or terraform apply, Terraform:

1. Parses all configuration files
2. Identifies all resource references and explicit dependencies
3. Constructs the dependency graph
4. Determines the optimal execution order
5. Executes operations respecting the dependency order

You can visualize this graph using terraform graph, which outputs a DOT-formatted representation of the dependency relationships.

Best Practices for Managing Dependencies

• Prefer implicit dependencies - They're self-documenting and automatically maintained
• Use depends_on sparingly - Only when Terraform cannot infer the relationship
• Avoid circular dependencies - Resource A cannot depend on B if B depends on A
• Use data sources carefully - They can create hidden dependencies
• Consider module dependencies - Modules inherit dependencies from their internal resources

Common Scenarios Requiring Explicit Dependencies

• IAM policies that must exist before resources use them
• Custom provisioners that depend on external resources
• Resources that have side effects not captured in their attributes
• Timing-sensitive deployments where order matters beyond attribute references

Exam Tips: Answering Questions on Cross-Resource Dependencies

Key Concepts to Remember:

• Implicit dependencies are created through resource attribute references
• Explicit dependencies use the depends_on meta-argument
• depends_on accepts a list of resources or modules
• The dependency graph is a directed acyclic graph (DAG)
• terraform graph visualizes dependencies

Common Exam Question Patterns:

1. Questions asking how to ensure Resource A is created before Resource B - look for references between resources first, then consider depends_on

2. Questions about parallel execution - resources with no dependencies between them can be created simultaneously

3. Scenarios where depends_on is necessary - typically involve IAM, provisioners, or resources with non-obvious relationships

4. Questions about destruction order - dependencies are reversed during destroy operations

Watch Out For:

• Answer choices suggesting manual ordering of resources in config files - Terraform does not respect file order
• Options mentioning requires or after - these are not valid Terraform arguments
• Circular dependency scenarios - these will cause Terraform to fail
• Using depends_on when an implicit dependency would suffice - while functional, it's not best practice