GloriousFlywheel

Create Your First Overlay

This guide walks you through creating a private overlay repository from scratch. By the end, you’ll have a fully automated CI/CD pipeline that deploys Attic infrastructure to your Kubernetes cluster.

What is an Overlay?

An overlay is a thin private repository that layers your organization-specific configuration on top of the public upstream modules. Think of it like CSS for infrastructure: upstream provides the structure, your overlay provides the values.

upstream (public)                    overlay (private)
  tofu/modules/gitlab-runner/          tofu/stacks/runners/beehive.tfvars
  tofu/modules/runner-dashboard/       tofu/stacks/runners/main.tf  (optional)
  tofu/stacks/attic/main.tf            config/organization.yaml
  tofu/stacks/gitlab-runners/main.tf   .gitlab-ci.yml
  app/                                 .env  (gitignored)

The overlay contains only your delta – organization-specific config, secrets references, and CI pipeline. Upstream handles all the module logic.

Why not just fork? Forks diverge. With an overlay, you get upstream improvements (bug fixes, new runner types, dashboard features) by simply pulling the latest upstream commit. Your private configuration is never at risk of merge conflicts because it lives in separate files.

Prerequisites

Before starting, you need:

• A Kubernetes cluster (1.28+) with kubectl access
• A GitLab group where your projects live
• A GitLab Personal Access Token (PAT) with api scope
• Nix with flakes enabled (see Getting Started)
• Git and a text editor

Step 1: Clone Upstream

Clone the public upstream repo as a sibling directory to where your overlay will live:

git clone https://github.com/Jesssullivan/GloriousFlywheel.git ~/git/attic-iac

This directory must exist alongside your overlay – the build system references it via relative path.

Step 2: Create Your Overlay Repository

Create a new private repository on GitLab for your overlay, then clone it:

# Create the repo on GitLab (or use the GitLab UI)
# Then clone it as a sibling to attic-iac:
git clone git@gitlab.com:your-group/your-overlay.git ~/git/your-overlay
cd ~/git/your-overlay

Your directory layout should look like:

~/git/
  attic-iac/         # public upstream (from GitHub)
  your-overlay/      # private overlay (your GitLab repo)

Step 3: Add the Bazel Overlay Files

The overlay system uses three files to merge upstream and private sources. Copy them from the templates below.

MODULE.bazel

"""Your Organization - Private overlay module."""

module(
    name = "your-overlay",
    version = "0.1.0",
)

# Upstream dependency
bazel_dep(name = "attic-iac", version = "0.1.0")

# Local dev: points to sibling upstream checkout
local_path_override(module_name = "attic-iac", path = "../../attic-iac")

# Core Bazel rules
bazel_dep(name = "bazel_skylib", version = "1.8.2")
bazel_dep(name = "rules_pkg", version = "1.1.0")

# Overlay extension: creates @attic_merged repo
overlay = use_extension("//build:extensions.bzl", "overlay")
overlay.configure(name = "attic_merged")
use_repo(overlay, "attic_merged")

Note: The local_path_override path is relative to this file. Adjust if your directory layout differs from ~/git/attic-iac and ~/git/your-overlay.

build/overlay.bzl

This file implements the symlink-merge repository rule. Copy it from the upstream repo:

mkdir -p build
cp ~/git/attic-iac/docs/infrastructure/templates/overlay.bzl build/overlay.bzl

Or copy from an existing overlay. The file is generic and works without modification.

Full overlay.bzl source

"""Overlay repository rule for merging upstream and private sources.

Creates a repository that symlinks all files from an upstream Bazel module,
then overlays private files on top. Private files win on conflict.
"""

_EXCLUDE_DIRS = [".git", "bazel-", "node_modules", ".terraform", ".svelte-kit", "__pycache__"]
_EXCLUDE_FILES = ["MODULE.bazel", "MODULE.bazel.lock", "WORKSPACE", "WORKSPACE.bazel"]
_EXCLUDE_EXTENSIONS = [".tfstate", ".tfstate.backup"]

def _should_exclude(path):
    for d in _EXCLUDE_DIRS:
        if d in path.split("/"):
            return True
        for part in path.split("/"):
            if part.startswith("bazel-"):
                return True
    for f in _EXCLUDE_FILES:
        if path == f or path.endswith("/" + f):
            return True
    for ext in _EXCLUDE_EXTENSIONS:
        if path.endswith(ext):
            return True
    return False

def _find_files(ctx, root_path):
    result = ctx.execute(
        ["find", root_path, "-type", "f", "-not", "-path", "*/.git/*"],
        timeout = 30,
    )
    if result.return_code != 0:
        fail("Failed to list files in %s: %s" % (root_path, result.stderr))
    files = []
    for line in result.stdout.strip().split("\n"):
        if not line:
            continue
        rel = line
        if rel.startswith(root_path + "/"):
            rel = rel[len(root_path) + 1:]
        elif rel.startswith(root_path):
            rel = rel[len(root_path):]
        if rel and not _should_exclude(rel):
            files.append(rel)
    return files

def _ensure_parent_dirs(ctx, path):
    parent = "/".join(path.split("/")[:-1])
    if parent:
        ctx.execute(["mkdir", "-p", parent], timeout = 5)

def _overlay_repository_impl(ctx):
    upstream_path = str(ctx.path(ctx.attr.upstream_marker).dirname)
    overlay_path = str(ctx.path(ctx.attr.overlay_marker).dirname)
    ctx.watch_tree(upstream_path)
    ctx.watch_tree(overlay_path)

    upstream_files = _find_files(ctx, upstream_path)
    for f in upstream_files:
        _ensure_parent_dirs(ctx, f)
        ctx.symlink(ctx.path(upstream_path + "/" + f), f)

    overlay_files = _find_files(ctx, overlay_path)
    overlaid = []
    for f in overlay_files:
        _ensure_parent_dirs(ctx, f)
        target = ctx.path(f)
        if f in upstream_files:
            ctx.delete(target)
            overlaid.append(f)
        ctx.symlink(ctx.path(overlay_path + "/" + f), f)

    ctx.file("BUILD.bazel", """\
package(default_visibility = ["//visibility:public"])
filegroup(name = "all_files", srcs = glob(["**/*"], exclude = ["BUILD.bazel"]))
""")

overlay_repository = repository_rule(
    implementation = _overlay_repository_impl,
    attrs = {
        "upstream_marker": attr.label(mandatory = True),
        "overlay_marker": attr.label(mandatory = True),
    },
    local = True,
)

build/extensions.bzl

"""Module extension bridging MODULE.bazel tag declarations to overlay_repository."""

load("//build:overlay.bzl", "overlay_repository")

_configure = tag_class(
    attrs = {
        "name": attr.string(mandatory = True),
        "upstream_marker": attr.label(default = Label("@attic-iac//:MODULE.bazel")),
        "overlay_marker": attr.label(default = Label("//:MODULE.bazel")),
    },
)

def _overlay_extension_impl(module_ctx):
    for mod in module_ctx.modules:
        for cfg in mod.tags.configure:
            overlay_repository(
                name = cfg.name,
                upstream_marker = cfg.upstream_marker,
                overlay_marker = cfg.overlay_marker,
            )

overlay = module_extension(
    implementation = _overlay_extension_impl,
    tag_classes = {"configure": _configure},
)

BUILD.bazel

load("@rules_pkg//pkg:tar.bzl", "pkg_tar")

package(default_visibility = ["//visibility:public"])

# Your org-specific filegroups
filegroup(name = "org_config", srcs = ["config/organization.yaml"])
filegroup(name = "org_tfvars", srcs = glob(["tofu/stacks/**/*.tfvars"]))
filegroup(name = "gitlab_ci", srcs = glob([".gitlab-ci.yml", ".gitlab/ci/**/*.yml"]))

# Aliases to upstream targets
alias(name = "validate_modules", actual = "@attic-iac//tofu/modules:all_validate")
alias(name = "app", actual = "@attic-iac//app:build")

# Deployment bundle
pkg_tar(
    name = "deployment_bundle",
    srcs = [":org_config", ":org_tfvars", ":gitlab_ci"],
    extension = "tar.gz",
    strip_prefix = ".",
)

Step 4: Add Organization Configuration

config/organization.yaml

mkdir -p config

Create config/organization.yaml with your cluster details:

organization:
  name: your-org
  full_name: "Your Organization"
  group_path: your-gitlab-group

gitlab:
  url: https://gitlab.com
  project_id: "YOUR_OVERLAY_PROJECT_ID"
  agent_group: your-group/kubernetes/agents

clusters:
  - name: dev
    role: development
    domain: dev.example.com
    context: your-group/kubernetes/agents:dev

namespaces:
  attic:
    dev: attic-cache-dev
  runners:
    all: gitlab-runners

links:
  upstream_repo: "https://github.com/Jesssullivan/GloriousFlywheel"
  source_repo: "https://gitlab.com/your-group/your-overlay"

.env (gitignored)

cp ~/git/attic-iac/.env.example .env

Set the TF_HTTP_ credentials (see .env.example) to your GitLab PAT. This is used for the OpenTofu HTTP state backend.

.gitignore

.env
*.tfstate
*.tfstate.backup
.terraform/
.svelte-kit/
kubeconfig-*

Step 5: Add Stack Configuration (tfvars)

Each stack needs a tfvars file with your environment-specific values. Create them under tofu/stacks/:

Cache Platform (attic stack)

mkdir -p tofu/stacks/attic

Create tofu/stacks/attic/dev.tfvars (provisions CNPG, PostgreSQL, MinIO, Attic API, GC, DNS):

# Cluster access
cluster_context = "your-group/kubernetes/agents:dev"  # CI uses GitLab Agent path
namespace       = "attic-cache-dev"

# PostgreSQL
pg_instances    = 1
pg_storage      = "10Gi"
pg_storage_class = "default"  # Use your cluster's StorageClass

# MinIO
minio_storage   = "50Gi"

# Ingress
api_host = "nix-cache.dev.example.com"

GitLab Runners

mkdir -p tofu/stacks/gitlab-runners

Create tofu/stacks/gitlab-runners/dev.tfvars:

cluster_context = "your-group/kubernetes/agents:dev"
namespace       = "gitlab-runners"
gitlab_url      = "https://gitlab.com"

# Your GitLab group ID (Settings > General > Group ID)
gitlab_group_id = 12345678

# Runner types to deploy (disable what you don't need)
deploy_docker_runner = true
deploy_dind_runner   = true
deploy_rocky8_runner = false
deploy_rocky9_runner = false
deploy_nix_runner    = true

# Attic cache (for Nix runner)
attic_server   = "https://nix-cache.dev.example.com"
attic_cache    = "main"
nix_store_size = "20Gi"

# HPA
hpa_enabled = true

Runner Dashboard

mkdir -p tofu/stacks/runner-dashboard

Create tofu/stacks/runner-dashboard/dev.tfvars:

cluster_context  = "your-group/kubernetes/agents:dev"
namespace        = "runner-dashboard"
ingress_host     = "dashboard.dev.example.com"
runners_namespace = "gitlab-runners"

Step 6: Self-Contained Stacks (Optional)

Some stacks need overlay-specific logic beyond what tfvars provide. For example, enrolling specific GitLab projects with dedicated runner registrations requires HCL resources that reference project IDs unique to your organization.

For these cases, create a self-contained main.tf in the overlay stack directory instead of using the upstream stack .tf files:

# Example: self-contained runner stack for project-level registrations
mkdir -p tofu/stacks/your-org-runners

See the Bates runner enrollment docs for a worked example of project-level runner registration in a self-contained overlay stack.

Step 7: Set Up CI Pipeline

Create .gitlab-ci.yml in your overlay root:

# Your Organization - Overlay CI Pipeline
workflow:
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

stages:
  - validate
  - plan
  - deploy

variables:
  UPSTREAM_REPO_URL: "https://github.com/Jesssullivan/GloriousFlywheel.git"
  UPSTREAM_REF: "main"
  TF_IN_AUTOMATION: "true"
  TF_INPUT: "false"

default:
  tags: [kubernetes]  # Target SaaS shared runners

# Clone upstream and symlink modules into overlay workspace
.setup_upstream: &setup_upstream
  - git clone --depth=1 --branch=$UPSTREAM_REF $UPSTREAM_REPO_URL /tmp/upstream
  # Symlink upstream modules into overlay
  - ln -sf /tmp/upstream/tofu/modules tofu/modules

# Base template for OpenTofu jobs
.tofu_base:
  image:
    name: ghcr.io/opentofu/opentofu:latest
    entrypoint: [""]
  variables:
    # CI job token authenticates to the HTTP state backend via TF_HTTP_* env vars
    TF_HTTP_USERNAME: gitlab-ci-token
  before_script:
    - *setup_upstream
    - |
      cd tofu/stacks/${STACK}
      tofu init \
        -backend-config="address=https://gitlab.com/api/v4/projects/${CI_PROJECT_ID}/terraform/state/${STATE_NAME}" \
        -backend-config="lock_address=https://gitlab.com/api/v4/projects/${CI_PROJECT_ID}/terraform/state/${STATE_NAME}/lock" \
        -backend-config="unlock_address=https://gitlab.com/api/v4/projects/${CI_PROJECT_ID}/terraform/state/${STATE_NAME}/lock"

# Attic Cache
plan:attic:
  extends: .tofu_base
  stage: plan
  variables:
    STACK: attic
    STATE_NAME: attic-dev
  script:
    - tofu plan -var-file=dev.tfvars -out=plan.tfplan
  artifacts:
    paths: [tofu/stacks/attic/plan.tfplan]
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

deploy:attic:
  extends: .tofu_base
  stage: deploy
  variables:
    STACK: attic
    STATE_NAME: attic-dev
  script:
    - tofu apply plan.tfplan
  needs: [plan:attic]
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

# Runners
plan:runners:
  extends: .tofu_base
  stage: plan
  variables:
    STACK: gitlab-runners
    STATE_NAME: runners-dev
  script:
    - tofu plan -var-file=dev.tfvars -var="gitlab_token=${GITLAB_TOKEN}" -out=plan.tfplan
  artifacts:
    paths: [tofu/stacks/gitlab-runners/plan.tfplan]
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

deploy:runners:
  extends: .tofu_base
  stage: deploy
  variables:
    STACK: gitlab-runners
    STATE_NAME: runners-dev
  script:
    - tofu apply plan.tfplan
  needs: [plan:runners]
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

Adapt the template to include as many stacks as you deploy.

Required CI/CD Variables

Set these in Settings > CI/CD > Variables on your GitLab overlay project:

Variable	Scope	Description
GITLAB_TOKEN	All	PAT with create_runner scope (for automated runner token lifecycle)
TF_VAR_gitlab_oauth_client_id	All	OAuth app client ID (dashboard auth)
TF_VAR_gitlab_oauth_client_secret	All	OAuth app client secret (dashboard auth)
TF_VAR_attic_token	All	Attic cache auth token (Nix runners)

Step 8: Deploy

First deployment (local)

For the initial deployment, run from your workstation:

cd ~/git/your-overlay
eval "$(grep '^TF_HTTP' .env)"  # Load state backend credentials

# Deploy in order: attic → runners → dashboard
for stack in attic gitlab-runners runner-dashboard; do
  cd tofu/stacks/$stack
  tofu init -backend-config="address=..." -backend-config="..."
  tofu plan -var-file=dev.tfvars -var="cluster_context=your-context" \
    -var="k8s_config_path=$HOME/git/your-overlay/kubeconfig"
  tofu apply
  cd ~/git/your-overlay
done

Subsequent deployments (CI)

After the initial deployment, push to your overlay’s main branch. The CI pipeline handles plan and apply automatically:

git add -A && git commit -m "feat: initial overlay deployment"
git push

Step 9: Verify

# Pods running?
kubectl get pods -n attic-cache-dev
kubectl get pods -n gitlab-runners
kubectl get pods -n runner-dashboard

# Runners registered?
# Check: GitLab > Your Group > Settings > CI/CD > Runners

Step 10: Enroll Projects

Once runners are deployed, project teams can start using them immediately by adding tags: to their .gitlab-ci.yml:

default:
  tags: [docker]  # Route all jobs to the docker runner

build:container:
  tags: [dind]    # Docker-in-Docker for container builds

build:nix:
  tags: [nix]     # Nix builds with Attic cache

See Self-Service Enrollment and Project Onboarding for complete guides.

Keeping Up with Upstream

To pull upstream improvements:

cd ~/git/attic-iac
git pull origin main

That’s it. The overlay’s local_path_override automatically picks up changes when you run bazel build or tofu plan locally. In CI, the pipeline clones the latest upstream on every run.

For pinned deployments (recommended for production), change UPSTREAM_REF in your .gitlab-ci.yml to a specific tag or commit SHA:

variables:
  UPSTREAM_REF: "v1.2.0"  # or: "da8a495"

Troubleshooting

local_path_override path not found

The path in MODULE.bazel is relative to the overlay repo root. If your directories aren’t siblings:

~/git/attic-iac/      # upstream
~/git/your-overlay/   # overlay -- MODULE.bazel says path = "../../attic-iac"

Adjust the path value to match your layout. Two ../ segments are needed because local_path_override resolves relative to the Bazel workspace root.

OpenTofu state backend errors

GitLab’s HTTP state backend occasionally returns 500. If a state becomes corrupted:

1. Try a different state name (append -v2)
2. Re-initialize with the new state name
3. Import existing resources if needed

Runners not visible in project

Runners registered at a GitLab group only propagate downward in the hierarchy. If your project is in a sibling subgroup, you need project-level registrations. See the enrollment docs for details.

CI pipeline can’t clone upstream

The CI pipeline clones upstream from GitHub over HTTPS. If your GitLab runners are on a restricted network (no GitHub access), you have two options:

1. Use SaaS shared runners for CI (set tags: [kubernetes] on pipeline jobs)
2. Mirror the upstream repo to your GitLab instance and change UPSTREAM_REPO_URL

Directory Structure Reference

A complete overlay repository looks like this:

your-overlay/
  MODULE.bazel              # Declares upstream dependency
  BUILD.bazel               # Build targets and aliases
  .gitlab-ci.yml            # CI/CD pipeline
  .gitignore                # Exclude .env, tfstate, etc.
  .env                      # Secrets (gitignored)
  build/
    overlay.bzl             # Symlink-merge repository rule
    extensions.bzl          # Module extension bridge
  config/
    organization.yaml       # Organization identity and cluster config
  tofu/
    stacks/
      attic/
        dev.tfvars          # Attic cache config per environment
      gitlab-runners/
        dev.tfvars          # Runner config per environment
        main.tf             # (optional) Self-contained stack for project enrollments
      runner-dashboard/
        dev.tfvars          # Dashboard config per environment
  docs/                     # (optional) Org-specific documentation

Related Documentation

• Overlay System – deep dive on the symlink-merge mechanics
• Bzlmod Topology – how Bazel modules connect
• Multi-Repo Layout – repository hosting and CI flow
• Quick Start – direct deployment guide
• Customization Guide – full organization.yaml reference

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