The depends_on meta-argument in Terraform is a powerful configuration option that allows you to explicitly define dependencies between resources when Terraform cannot automatically detect them. By default, Terraform analyzes your configuration and builds a dependency graph based on references betwe…The depends_on meta-argument in Terraform is a powerful configuration option that allows you to explicitly define dependencies between resources when Terraform cannot automatically detect them. By default, Terraform analyzes your configuration and builds a dependency graph based on references between resources. However, there are situations where implicit dependencies are not sufficient.
The depends_on meta-argument accepts a list of resource or module references, telling Terraform that the current resource relies on the specified resources being created first. This ensures proper ordering during apply and destroy operations.
Syntax example:
resource "aws_instance" "example" {
ami = "ami-12345678"
instance_type = "t2.micro"
depends_on = [aws_iam_role_policy.example]
}
Common use cases include:
1. Hidden dependencies: When a resource depends on another through means Terraform cannot see, such as IAM policies that must exist before an EC2 instance can assume a role.
2. Module dependencies: When one module must complete before another begins, even though there are no explicit references between them.
3. External system dependencies: When resources interact through external systems or APIs that Terraform does not manage.
Best practices for using depends_on:
- Use it sparingly, as explicit dependencies can make configurations harder to maintain
- Prefer implicit dependencies through resource references when possible
- Document why the explicit dependency is necessary
- Remember that depends_on affects both creation and destruction order
The depends_on meta-argument can be used with any resource type and also works with modules. When applied to a module, all resources within that module will wait for the specified dependencies.
Important considerations:
- depends_on creates a strict ordering requirement
- It can impact parallelism and slow down operations
- Overuse may indicate a need to refactor your configuration
Understanding depends_on is essential for managing complex infrastructure where resource relationships extend beyond simple attribute references.
The depends_on Meta-Argument in Terraform
What is the depends_on Meta-Argument?
The depends_on meta-argument is a special argument available in Terraform that allows you to explicitly specify dependencies between resources that Terraform cannot automatically detect. It forces Terraform to create or destroy resources in a specific order, ensuring that one resource is fully provisioned before another resource begins its creation.
Why is depends_on Important?
Terraform automatically builds a dependency graph based on references between resources. However, there are situations where dependencies exist that are not visible through resource attributes or references. These are called hidden dependencies. The depends_on argument addresses this limitation by:
• Ensuring correct resource ordering when implicit dependencies cannot be detected • Preventing race conditions between resources • Handling dependencies that exist outside of Terraform's knowledge (such as IAM policies that must exist before a resource can function) • Managing resources that have side effects affecting other resources
How depends_on Works
The depends_on meta-argument accepts a list of resource or module references. When specified, Terraform will complete all actions on the dependency object before performing actions on the object declaring the dependency.
• It takes a list of resources or modules as its value • It can be used with resources, data sources, and modules • It creates explicit dependencies that override automatic dependency detection • The dependency is only for ordering purposes and does not pass any data between resources
When to Use depends_on
You should use depends_on only when necessary, specifically when:
• A resource relies on another resource's behavior but does not reference its attributes • IAM policies or permissions must be in place before a resource can operate • Network configurations must be complete before launching instances • There are timing issues between resource provisioning
Best Practices
• Use depends_on sparingly as a last resort • Prefer implicit dependencies through resource attribute references when possible • Document why depends_on is necessary in your configuration • Be aware that depends_on creates a strict ordering that may slow down provisioning
Exam Tips: Answering Questions on The depends_on Meta-Argument
1. Remember that depends_on is for hidden dependencies: If a question describes a scenario where Terraform cannot detect a dependency through attribute references, depends_on is likely the correct answer.
2. Know the syntax: depends_on takes a list enclosed in square brackets, even for a single dependency. Questions may test whether you recognize valid syntax.
3. Understand when NOT to use it: If resources reference each other through attributes, Terraform handles dependencies automatically. Questions may present scenarios where depends_on is unnecessary.
4. Recognize applicable resource types: depends_on works with resources, data sources, and modules. Be prepared for questions testing this knowledge.
5. Order of operations: Resources listed in depends_on will be created first and destroyed last. Exam questions may test your understanding of this behavior.
6. Common exam scenarios include: • IAM roles and policies that must exist before EC2 instances • S3 bucket policies that must be in place before other resources access the bucket • Network resources that must be configured before compute resources
7. Differentiate from other meta-arguments: Know how depends_on differs from lifecycle, count, for_each, and provider meta-arguments.