Terraform: locals vs data

[hoangquochung1110]

The main differences between locals and data sources in Terraform are:

Locals

• Purpose: Define reusable values within a Terraform configuration
• Syntax: Defined using the locals block
• Behavior: Calculated during planning phase
• Scope: Limited to the module where they're defined
• Use case: For transforming or combining values, reducing repetition

locals {
  common_tags = {
    Environment = var.environment
    Project     = var.project_name
    Owner       = "DevOps Team"
  }
}

resource "aws_instance" "example" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
  tags          = local.common_tags
}

Data Sources

• Purpose: Fetch or compute data from external sources or providers
• Syntax: Defined using the data block
• Behavior: Query external systems for information
• Scope: Available within the module where they're defined
• Use case: For importing existing infrastructure information

data "aws_vpc" "existing" {
  filter {
    name   = "tag:Name"
    values = ["main-vpc"]
  }
}

resource "aws_subnet" "example" {
  vpc_id     = data.aws_vpc.existing.id
  cidr_block = "10.0.1.0/24"
}