feat(blog): add tofu provider iteration with for_each

content/blog/terraform/tofu_for_each_providers.md

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+---
+title: 'Opentofu provider iteration with `for_each`'
+description: 'a much anticipated feature'
+date: '2025-01-25'
+tags:
+- AWS
+- OpenTofu
+---
+
+## Introduction
+
+The latest release of OpenTofu came with a much anticipated feature: provider
+iteration with `for_each`!
+
+My code was already no longer compatible with terraform since OpenTofu added the
+much needed variable interpolation in provider blocks feature, so I was more
+than ready to take the plunge.
+
+## Usage
+
+A good example will be to rewrite the lengthy code from my [Securing AWS default
+vpcs]({{< ref "blog/aws/defaults.md" >}}#iterating-through-all-the-default-regions)
+article a few months ago. It now looks like:
+
+``` hcl
+locals {
+  aws_regions = toset([
+    "ap-northeast-1",
+    "ap-northeast-2",
+    "ap-northeast-3",
+    "ap-south-1",
+    "ap-southeast-1",
+    "ap-southeast-2",
+    "ca-central-1",
+    "eu-central-1",
+    "eu-north-1",
+    "eu-west-1",
+    "eu-west-2",
+    "eu-west-3",
+    "sa-east-1",
+    "us-east-1",
+    "us-east-2",
+    "us-west-1",
+    "us-west-2",
+  ])
+}
+
+provider "aws" {
+  alias = "all"
+  default_tags { tags = { "managed-by" = "tofu" } }
+  for_each = concat(local.aws_regions)
+  profile  = "common"
+  region   = each.key
+}
+
+module "default" {
+  for_each = local.aws_regions
+  providers = { aws = aws.all[each.key] }
+  source    = "../modules/defaults"
+}
+```
+
+Note the use of the `concat()` function in the `for_each` definition of the
+providers block. This is needed to silence a warning that tells you it is a bad
+idea to iterate through your providers using the same expression in provider
+definitions and module definitions.
+
+Though I understand the reason (to allow for resources destructions when the
+list we are iterating on changes), it is not a bother for me in this case.
+
+## Modules limitations
+
+The main limitation at the moment is the inability to pass down the whole
+`aws.all` to a module. This leads to code that repeats itself a bit, but it is
+still better than before.
+
+For example, when creating resources for multiple aws accounts, a common pattern
+is to have your DNS manged in a specific account (for me it is named `core`)
+that you need to pass around. Let's say you have another account named `common`
+with for example monitoring stuff and here is how some module invocation can
+look like:
+
+``` hcl
+module "base" {
+  providers = {
+    aws          = aws.all["${var.environment}_${var.region}"]
+    aws.common   = aws.all["common_us-east-1"]
+    aws.core     = aws.all["core_us-east-1"]
+  }
+  source = "../modules/base"
+
+  ...
+}
+```
+
+It would be nice to be able to just pass down aws.all, but alas we cannot yet.
+
+## Cardinality limitation
+
+Just be warned that you cannot go too crazy with this mechanism. I tried to
+iterate through a cross-product of all AWS regions and a dozen AWS accounts and
+it does not go well: OpenTofu slows down to a crawl and it starts taking a dozen
+minutes just to instantiate all providers in a folder, before planning any
+resources!
+
+This is because providers are instantiated as separate processes that OpenTofu
+then talks to. This model does not scale that well (and consumes a fair bit of
+memory), as least for the time being.
+
+## Conclusion
+
+I absolutely love this new feature!