Looping Through Object Collections in Terraform

Terraform doesn't have a built-in loop specifically for object collections. However, you can achieve loop-like behavior using a combination of techniques like for_each with expressions and conditional statements.

1. Using for_each with Expressions

This approach leverages the for_each meta-argument with expressions to iterate through an object collection stored in a variable. Here's the basic idea:

variable "servers" {
  type = map(object({
    name = string
    type = string
  }))
}

resource "null_resource" "configure_server" {
  for_each = { for server in var.servers : server.name => server }

  provisioner "local-exec" {
    command = <<EOF
      echo Configuring server: ${each.key} (type: ${each.value.type})
    EOF
  }
}

Explanation:

• variable "servers" defines a map containing server configurations.
• resource "null_resource" defines a resource with a for_each argument.
• The for_each expression uses a nested loop:
  • The outer loop iterates through each key-value pair in var.servers.
  • The inner expression creates a new object with the server name as the key and the entire server configuration object as the value.
• The local-exec provisioner demonstrates accessing the server name and type within the loop using each.key and each.value.type.

2. Looping with Conditional Statements

This approach iterates through the object collection using a counter and conditional statements to access specific server configurations.

variable "servers" {
  # ... same definition as above
}

resource "null_resource" "configure_server" {
  count = length(var.servers)

  provisioner "local-exec" {
    when = count.index < length(var.servers)
    command = <<EOF
      server_name=${var.servers[count.index].name}
      server_type=${var.servers[count.index].type}
      # ... commands using server variables
    EOF
  }
}

Explanation:

• resource "null_resource" defines a resource with count set to the length of the servers map.
• The local-exec provisioner uses a when condition to ensure it runs only for valid indices within the loop (up to count.index less than length(var.servers)).
• The command accesses server details using bracket notation with the current index (count.index) to retrieve the corresponding server configuration object from the servers map.

Choosing the Right Approach:

• The first approach using for_each with expressions is generally more concise and easier to read for simple iteration and accessing object properties.
• The second approach with conditional statements might be preferable if you need additional logic within the loop or want to avoid the nested expression for complex scenarios.

Additional Considerations

• While these methods enable iterating through object collections, Terraform doesn't natively support complex object manipulation within loops.
• For advanced object processing or transformations, consider using external tools or scripting languages that can be integrated with Terraform using the local-exec provisioner.

By understanding these techniques, you can effectively process and use data stored in object collections within your Terraform configurations.