Impure function

Q: How deeply does Terraform in Depth: Infrastructure as Code with Terraform and OpenTofu cover impure function?

Defines purity, explains deployment consequences, and stops short of implementing function internals.

As explained in Terraform in Depth: Infrastructure as Code with Terraform and OpenTofu

An impure function is a Terraform function whose result is not guaranteed to be known during planning or to remain fixed across evaluations.

An impure function can return a different value each time it runs. Common examples include random functions and functions that return the current time. Terraform makes this distinction operationally important because configuration values must remain stable across planning and application. section 4.3.3

Using an impure function in a resource configuration can make Terraform believe the resource needs updating after every apply. A configuration containing uuid() or timestamp() can therefore produce continual changes and apparent state drift. section 4.3.3

Account for impure functions in two important situations:

Avoid using timestamp or uuid where a value must be stable between runs. section 4.3.3
Treat impure functions as unsuitable for for_each or count values when Terraform cannot know them during the early planning phase. section 4.8.4
Use random-provider resources when generated data should persist, changing only when keepers change. section 6.8.1

A UUID that forces continual changes

In this resource, uuid() generates a new tag value during evaluation. Terraform consequently sees the instance configuration as changing after each apply. section 4.3.3

Listing 4.16 Example of UUID function forcing constant changes

resource "aws_instance" "example" {
  tags = {
    id = uuid()                                               #1
  }
}

Inspect the resource configuration that demonstrates how uuid() causes repeated changes. section 4.3.3

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Also explained in

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Write Powerful Rust MacrosAn impure function is a function whose behavior or result can depend on or produce effects beyond its input parameters.

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Related concepts

pure functioncontrasts with · A pure function is a function whose output is determined solely by its explicit inputs and whose evaluation has no externally observable side effects.
actionequivalent to · An action is a choice or operation selected or performed by an actor that can affect a system or environment or produce an outcome.
pseudo-random number generatorincludes · A pseudorandom number generator is a deterministic computational process that uses an initial seed or internal state to produce a sequence with random-like statistical properties.
functionkind of · A function is a callable program unit that performs an operation, may accept inputs, and can produce a result or side effect.
terraform resourceused by · A Terraform resource is a declarative configuration construct that describes an infrastructure object and its settings for Terraform to manage.
static methodcontrasts with · A static method is a function associated with a type or enclosing scope rather than an object instance, and callable without an instance receiver.

This explainer was generated by Manning's knowledge graph from the text of Terraform in Depth: Infrastructure as Code with Terraform and OpenTofu. The section links point to the parts of the book it draws on.