Understanding Terraform's depends_on

Misunderstanding explicit dependencies in Terraform can create noisy, redundant and potentially confusing code. In this article you will learn when and how to use Terraform's depends_on meta-argument

As a reminder, dependencies help Terraform know in which order the resources must be created and deleted.

#What is an implicit dependency?

Implicit dependencies are automatically inferred by Terraform when a resource or module accesses the data of another resource or module.

In this example employee implicitly depends on employer because it accesses its name.

resource "employer" "monsters_inc" {
  name = "Monsters, Inc."
}

resource "employee" "mike" {
  name = "Mike"
  employer_name = employer.monsters_inc.name
}

#What is an explicit dependency?

Since driver doesn't access any of team's data, there is no implicit dependency.

resource "team" "ferrari" {
  name = "Ferrari"
}

resource "driver" "carlos" {
  name = "Carlos"
}

If we want to enforce an explicit dependency between team and driver we can use the depends_on meta-argument.

resource "team" "ferrari" {
  name = "Ferrari"
}

resource "driver" "carlos" {
  depends_on = [team.ferrari]
  name = "Carlos"
}

This guarantees that team will be created before driver.

#When should I use an explicit dependency?

Explicitly specifying a dependency using the depends_on meta-argument is only necessary when a resource relies on some other resource without accessing any of its data.

#Examples

In this example there is no dependency, implicit or explicit. It is dangerous because the employee could be created before the employer.

# 👎 DANGEROUS

local {
  employer_name = "Monsters, Inc."
}

module "employer" {
  name = locals.employer_name
}

module "employee" {
  employer_name = locals.employer_name
}

In this example there is an implied dependency, re-declared explicitly. This is redundant and potentially confusing.

# 👎 BAD

local {
  employer_name = "Monsters, Inc."
}

module "employer" {
  name = locals.employer_name
}

module "employee" {
  depends_on = [module.employer]
  employer_name = module.employer.name
}

This last example illustrates the ideal approach.

# 👍 IDEAL

local {
  employer_name = "Monsters, Inc."
}

module "employer" {
  name = locals.employer_name
}

module "employee" {
  employer_name = module.employer.name
}