What are Terraform data sources and how do they differ from resources?

A resource creates, updates, or destroys infrastructure. A data source reads existing infrastructure that is managed outside of your current Terraform code — it is read-only.

# Data source — reads an existing VPC by tag, does not create it
data "aws_vpc" "main" {
  tags = {
    Environment = "production"
  }
}

# Use the data source output
resource "aws_subnet" "app" {
  vpc_id = data.aws_vpc.main.id
  ...
}

Data sources are essential for referencing shared infrastructure managed by a different team or Terraform root module.