Language Features

Expressions + Functions

Expressions:

• Template strings
• Operators (!, -, *, /, %, >, ==, etc…)
• Conditionals (cond ? true: false)
• For ([for o in var.list: o.id])
• Splat (var.list[*].id)
• Dynamic Blocks
• Constraints (Type & Version).

Functions:

• Numeric
• String
• Collection
• Encoding
• Filesystem
• Date & Time.
• Hash & Crypto
• IP Network
• Type Conversion

Meta-Arguments

depends_on

• Terraform automatically generates dependency graph based on references.
• If two resources depend on each other (but not each others data), depends_on specifies that dependency to enforce ordering.
• For example, if software on the instance needs access to S3, trying to create the aws_instance would fail if attempting to create it before the aws_iam_role_policy.

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

count

• Allows for creation of multiple resources/modules from a single block.
• Useful when the multiple necessary resources are nearly identical.

for_each

• Allows for creation of multiple resources/modules from a single block.
• Allows more control to customize each resource than count.

locals {
    subnet_ids = toset([
        "subnet-abcd",
        "subnet-1234"
    ])
}

resource "aws_instance" "server" {
    for_each  = local.subnet_ids
    subnet_id = each.key
}

lifecycle

• A set of meta arguments to control terraform behaviour for specific resources.
  • create_before_destroy can help with zero downtime deployments.
  • ignore_changes prevents Terraform from trying to revert metadata being set elsewhere.
  • prevent_destroy causes Terraform to reject any plan which would destroy this resource.

lifecycle {
    create_before_destroy = true
    ignore_changes = [
        tags
    ]
}

Provisioners

Perform action on local or remote machine.

• file
• local-exec
• remote-exec
• vendor
  • chef
  • puppet