Add documentation for OpenTofu remote state

.markdownlint.json

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+{
+  "no-duplicate-heading": {
+    "siblings_only": true
+  }
+}

docs/opentofu-remote-state.md

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+# OpenTofu remote state
+
+OpenTofu supports a number of [remote state backends](https://opentofu.org/docs/language/state/remote/)
+which can be used to persist state independently of where a deployment is run.
+This allows deployments to be made from anywhere that can access the state
+without corrupting or conflicting with any existing resources from previous
+deployments.
+
+Using remote state is therefore strongly recommended for environments which
+should only be instantiated once, e.g. `production` and `staging`.
+
+This page provides guidance for configuring remote states using backends
+commonly available on OpenStack deployments.
+
+> [!IMPORTANT]
+> In the below replace `$ENV` with the relevant environment name.
+
+## GitLab
+
+GitLab can be used with the [http backend](https://opentofu.org/docs/language/settings/backends/http/)
+to store separate states for each environment within the GitLab project.
+Access is protected by GitLab access tokens, which in the approach below are
+persisted to local files. Therefore each repository checkout will need to
+authenticate separately, using either a separate token or a shared token from
+some external secret store.
+
+The below is based on the [official docs](https://docs.gitlab.com/user/infrastructure/iac/terraform_state/)
+but includes some missing details and is modified for common appliance workflows.
+
+### Initial setup
+
+1. Create the backend file:
+
+   ```shell
+   cp environments/site/tofu/example-backends/gitlab.tf environments/$ENV/tofu
+   ```
+
+2. Modify `environments/$ENV/tofu/gitlab.tf` to set the default for the
+   project ID. This can be found by clicking the 3-dot menu at the top right of
+   the GitLab project page.
+
+   ```terraform
+   # environments/$ENV/tofu/backend.tf:
+   terraform {
+       backend "http" {}
+   }
+   ```
+
+3. Commit it.
+
+4. Follow the per-checkout steps below.
+
+### Per-checkout configuration
+
+1. Create an access token in the GitLab UI, using either:
+
+   a. If project access tokens are available, create one via
+   Project > Settings > Access tokens.
+   The token must have `Maintainer` role and `api` scope.
+
+   b. Otherwise create a personal access token via
+   User profile > Preferences > Access tokens.
+   The token must have `api` scope.
+
+   Copy the generated secret and set an environment variable:
+
+   ```shell
+   export TF_VAR_gitlab_access_token=$secret
+   ```
+
+2. If using a personal access token, set the GitLab username as an environment variable:
+
+   ```shell
+   export TF_VAR_gitlab_username=$your_username
+   ```
+
+3. With the environment activated, initialise OpenTofu.
+
+   If no local state exists run:
+
+   ```shell
+   cd environments/$ENV/tofu/
+   tofu init
+   ```
+
+   otherwise append `-migrate-state` to the `init` command to attempt to copy
+   local state to the new backend.
+
+OpenTofu is now configured to use GitLab to store state for this environment.
+
+Repeat for each environment needing remote state.
+
+> [!CAUTION]
+> The GitLab credentials are [persisted](https://opentofu.org/docs/language/settings/backends/configuration/#credentials-and-sensitive-data)
+> into a file `environments/$ENV/tofu/.terraform/terraform.tfstate` and any
+> plan files. These should therefore not be committed.
+
+### Token expiry
+
+If the project token expires repeat the per-checkout configuration, but using
+`opentofu init -reconfigure` instead.
+
+## S3
+
+For clouds with S3-compatible object storage (e.g. Ceph with [radosgw](https://docs.ceph.com/en/latest/radosgw/))
+the S3 backend can be used. This approach uses a bucket per environment and
+derives credentials from OpenStack credentials, meaning no backend-specific
+per-checkout configuration is required.
+
+### Initial setup
+
+1. Create an S3 bucket with a name `${cluster_name}-${environment_name}-tfstate`
+   where:
+
+   - `CLUSTER_NAME` is defined in `environments/$ENV/tofu/main.tf`
+   - `$ENVIRONMENT_NAME` is the name of the environment directory
+
+   e.g.
+
+   ```shell
+   openstack container create research-staging-tfstate
+   ```
+
+2. Create `ec2` credentials:
+
+   ```shell
+   openstack ec2 credentials create
+   ```
+
+   and make a note of the `access` field returned.
+
+3. Create the backend file:
+
+   ```shell
+   cp environments/site/tofu/example-backends/s3.tf environments/$ENV/tofu
+   ```
+
+4. Modify `environments/$ENV/tofu/s3.tf` to set the default for `s3_backend_endpoint`.
+   This is the radosgw address. If not known it can be determined by creating a
+   public bucket, and then getting the URL using
+   Project > Containers > (your public bucket) > Link
+   which provides a URL of the form `https://$ENDPOINT/swift/...`.
+
+5. Add the following to `environments/$ENV/activate`:
+
+   ```bash
+   export AWS_ACCESS_KEY_ID=$EC2_CREDENTIALS_ACCESS
+   export AWS_SECRET_ACCESS_KEY=$(openstack ec2 credentials show $AWS_ACCESS_KEY_ID -f value -c secret)
+   ```
+
+   replacing `$EC2_CREDENTIALS_ACCESS` with the `access` field of the created
+   credentials.
+
+   This avoids these credentials being persisted in local files.
+
+6. Copy the lines above into your shell to set them for your current shell.
+
+7. With the environment activated, initialise OpenTofu.
+
+   If no local state exists run:
+
+   ```shell
+   cd environments/$ENV/tofu/
+   tofu init
+   ```
+
+   otherwise append `-migrate-state` to the `init` command to attempt to copy
+   local state to the new backend.
+
+8. If this fails, try setting `use_path_style = true` in `environments/$ENV/tofu/s3.tf`.
+
+9. Once it works, commit `environments/$ENV/tofu/s3.tf` and `environments/$ENV/activate`.
+
+OpenTofu is now configured to use the cloud's S3-compatible storage to store
+state for this environment.
+
+Repeat for each environment needing remote state.
+
+For more configuration options, see the OpenTofu [s3 backend docs](https://opentofu.org/docs/language/settings/backends/s3/).
+
+### Per-checkout configuration
+
+The ec2 credentials will automatically be loaded when activating the environment.
+For a new checkout simply initialise OpenTofu as normal as described in step 7 above.

docs/production.md

@@ -316,6 +316,9 @@ The value chosen should be the highest value demonstrated during testing. Note
 that any time spent blocked due to this parallelism limit does not count
 against the (un-overridable) internal OpenTofu timeout of 30 minutes
 
+Consider configuring [OpenTofu remote state](./opentofu-remote-state.md) for any
+environments which should be unique, e.g. production and staging.
+
 ## Configure appliance
 
 ### Production configuration to consider

environments/site/tofu/example-backends/gitlab.tf

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+variable "gitlab_username" {
+  type        = string
+  description = <<-EOF
+        Username of actual GitLab user, for personal access token only.
+        Default uses bot account name, for project access token.
+    EOF
+  default     = null
+}
+
+variable "gitlab_access_token" {
+  type        = string
+  description = <<-EOF
+        GitLab Project or Personal access token.
+        Must have Maintainer role (for Project token) and API scope
+    EOF
+}
+
+variable "gitlab_project_id" {
+  type        = string
+  description = "GitLab project ID - click 3-dot menu at the top right of project page"
+  #default = # add here
+}
+
+locals {
+  gitlab_username      = coalesce(var.gitlab_username, "project_${var.gitlab_project_id}_bot")
+  gitlab_state_name    = basename(var.environment_root)
+  gitlab_state_address = "https://gitlab.com/api/v4/projects/${var.gitlab_project_id}/terraform/state/${local.gitlab_state_name}"
+}
+
+# tflint-ignore: terraform_required_version
+terraform {
+  backend "http" {
+    address        = local.gitlab_state_address
+    lock_address   = "${local.gitlab_state_address}/lock"
+    unlock_address = "${local.gitlab_state_address}/lock"
+    username       = local.gitlab_username
+    password       = var.gitlab_access_token
+    lock_method    = "POST"
+    unlock_method  = "DELETE"
+    retry_wait_min = 5
+  }
+}

environments/site/tofu/example-backends/s3.tf

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+variable "s3_backend_endpoint" {
+  type        = string
+  description = "radosgw address without protocol or path e.g. leafcloud.store"
+  #default = # add here
+}
+
+# tflint-ignore: terraform_required_version
+terraform {
+  backend "s3" {
+    endpoint = var.s3_backend_endpoint
+    bucket   = "${var.cluster_name}-${basename(var.environment_root)}-tfstate"
+    key      = "environment.tfstate"
+
+    # Reginon is required but not used in radosgw:
+    region                 = "dummy"
+    skip_region_validation = true
+
+    # Normally STS is not configured in radosgw:
+    skip_credentials_validation = true
+
+    # Enable path-style S3 URLs (https://<HOST>/<BUCKET> instead of https://<BUCKET>.<HOST>)
+    # may or may not be required depending on radosgw configuration
+    use_path_style = true
+  }
+}