atmos-gitops

name	atmos-gitops
description	CI/CD: GitHub Actions, Spacelift, Atlantis, `atmos describe affected` for change detection
metadata	{"copyright":"Copyright Cloud Posse, LLC 2026","version":"1.0.0"}

Atmos GitOps and CI/CD Integrations

Overview

Atmos provides native CI/CD integration patterns for managing Terraform infrastructure through automated pipelines. The three primary integration paths are GitHub Actions, Spacelift, and Atlantis. Each approach leverages Atmos stack configurations and the atmos describe affected command to detect changes between commits or branches, enabling efficient PR-based plan and merge-based apply workflows.

All integrations follow a common GitOps pattern:

1. Detect which stacks and components changed using atmos describe affected
2. Run atmos terraform plan for each affected component/stack pair
3. Review plan output in the PR or CI/CD UI
4. Apply changes on merge (or via manual trigger / label)

Change Detection with atmos describe affected

The atmos describe affected command is the foundation of all CI/CD integrations. It compares two Git commits to produce a list of affected Atmos components and stacks.

How It Works

The command performs these steps:

1. Clone or reference the target branch/commit (the comparison baseline)
2. Deep-merge all stack configurations for both the current working branch and the target branch
3. Identify changes in component directories (file-level diffs)
4. Compare each section of the stack configuration to detect differences
5. Output a list of affected components and stacks

When component source directories have changed, all related stacks are marked affected and Atmos skips further configuration comparison for those stacks, streamlining the process.

Command Usage

# Compare current branch against the default branch (main)
atmos describe affected

# Compare against a specific branch
atmos describe affected --ref refs/heads/main

# Compare against a specific commit SHA
atmos describe affected --sha 6a9b2c1

# Use a pre-cloned repo for the target reference
atmos describe affected --repo-path /path/to/cloned/target

# Output as JSON for CI/CD consumption
atmos describe affected --format json

Output Format

The command outputs a JSON array of affected items. Each item contains:

[
  {
    "component": "vpc",
    "component_type": "terraform",
    "stack": "plat-ue2-dev",
    "stack_slug": "plat-ue2-dev-vpc",
    "affected": "stack.vars"
  },
  {
    "component": "eks/cluster",
    "component_type": "terraform",
    "stack": "plat-ue2-prod",
    "stack_slug": "plat-ue2-prod-eks-cluster",
    "affected": "component"
  }
]

Fields:

• component -- The Atmos component name
• component_type -- Always terraform for Terraform components
• stack -- The Atmos stack name
• stack_slug -- A slug combining stack and component (used for matrix grouping)
• affected -- What changed: component (source files), stack.vars, stack.env, stack.settings, etc.

Filtering and Grouping for CI/CD Matrices

The output can be filtered and grouped for GitHub Actions matrix strategies using jq expressions configured in atmos.yaml:

# atmos.yaml
integrations:
  github:
    gitops:
      matrix:
        sort-by: .stack_slug
        group-by: .stack_slug | split("-") | [.[0], .[2]] | join("-")

The group-by expression creates groups for parallel execution, which helps work around the GitHub Actions 256-job matrix limit.

GitHub Actions Integration

Cloud Posse provides a set of GitHub Actions designed for Atmos-native GitOps workflows. These actions are open source (Apache 2.0 license), require no subscriptions, and use GitHub OIDC for authentication (no hardcoded credentials).

Required Infrastructure

GitHub Actions that manage planfiles require:

• S3 bucket for storing Terraform planfiles
• DynamoDB table for planfile metadata (hash key: id, GSI: pr-createdAt-index)
• Two IAM roles: one for plan/apply operations, one for accessing the S3/DynamoDB state storage
• atmos.yaml configuration with the integrations.github.gitops section

atmos.yaml Configuration for GitHub Actions

# atmos.yaml
integrations:
  github:
    gitops:
      terraform-version: 1.5.2
      infracost-enabled: false
      artifact-storage:
        region: us-east-2
        bucket: cptest-core-ue2-auto-gitops
        table: cptest-core-ue2-auto-gitops-plan-storage
        role: arn:aws:iam::xxxxxxxxxxxx:role/cptest-core-ue2-auto-gitops-gha
      role:
        plan: arn:aws:iam::yyyyyyyyyyyy:role/cptest-core-gbl-identity-gitops
        apply: arn:aws:iam::yyyyyyyyyyyy:role/cptest-core-gbl-identity-gitops
      matrix:
        sort-by: .stack_slug
        group-by: .stack_slug | split("-") | [.[0], .[2]] | join("-")

Core GitHub Actions

1. setup-atmos -- Installs Atmos in the workflow runner
2. atmos-terraform-plan -- Runs atmos terraform plan, stores planfiles in S3, posts Job Summaries
3. atmos-terraform-apply -- Retrieves planfiles from S3, runs atmos terraform apply
4. affected-stacks -- Runs atmos describe affected and outputs a matrix for parallel jobs
5. atmos-terraform-drift-detection -- Scheduled workflow to detect drift and create GitHub Issues
6. atmos-terraform-drift-remediation -- Applies fixes for drifted components via IssueOps

GitOps Workflow Pattern

The standard PR-based workflow:

1. Developer opens a PR with infrastructure changes
2. The plan workflow triggers: a. affected-stacks action identifies changed components/stacks b. Output feeds into a matrix strategy c. atmos-terraform-plan runs for each affected component/stack pair d. Plan summaries appear as GitHub Job Summaries
3. Team reviews plan output in the PR
4. On merge to main, the apply workflow triggers: a. atmos-terraform-apply retrieves stored planfiles from S3 b. Applies each affected component/stack pair c. Apply summaries appear as Job Summaries

Drift Detection and Remediation

Drift detection runs on a schedule (cron):

1. The drift detection workflow gathers all components and stacks
2. Runs atmos terraform plan for each component/stack pair
3. If drift is detected, creates a GitHub Issue with plan details
4. To remediate, add the apply label to the Issue
5. The drift remediation workflow detects the label, runs atmos terraform apply, and closes the Issue

The max-opened-issues input (default: 10) limits how many drift Issues are created per run.

256 Matrix Limitation

GitHub Actions supports at most 256 matrix jobs per workflow. For large environments, use a two-level workflow pattern:

• Top-level workflow groups affected stacks using the group-by jq expression
• Each group is dispatched to a reusable workflow
• The reusable workflow runs the actual plan/apply matrix for its group

Spacelift Integration

Atmos natively supports Spacelift through stack configuration settings and a Terraform module that reads YAML stack configs to provision Spacelift resources.

Stack Configuration for Spacelift

Configure Spacelift behavior in the settings.spacelift section of component configurations:

components:
  terraform:
    my-component:
      settings:
        spacelift:
          workspace_enabled: true
          administrative: false
          autodeploy: true
          before_init: []
          component_root: components/terraform/my-component
          description: "My component description"
          stack_destructor_enabled: false
          worker_pool_name: null
          policies_enabled: []
          administrative_trigger_policy_enabled: false
          policies_by_id_enabled:
            - my-custom-policy
          terraform_workflow_tool: TERRAFORM  # or OPEN_TOFU

Key settings:

• workspace_enabled -- Whether this component/stack is managed in Spacelift
• administrative -- Whether this is an admin stack that manages other stacks
• autodeploy -- Automatically apply on merge without manual confirmation
• component_root -- Path to the Terraform component directory
• policies_by_id_enabled -- List of Spacelift policy IDs to attach
• terraform_workflow_tool -- Use TERRAFORM or OPEN_TOFU

Spacelift Stack Dependencies

Define component dependencies using settings.depends_on:

components:
  terraform:
    my-component:
      settings:
        depends_on:
          1:
            component: "vpc"
          2:
            component: "eks/cluster"
            tenant: "plat"
            environment: "ue2"
            stage: "prod"

Each dependency specifies a component (required) and optional context variables (namespace, tenant, environment, stage) to reference components in other stacks.

OpenTofu Support

To make OpenTofu the default for all Spacelift stacks:

settings:
  spacelift:
    terraform_workflow_tool: OPEN_TOFU

Or configure per-component to override the global setting.

Atlantis Integration

Atmos supports Atlantis for Terraform Pull Request Automation through three commands:

1. atmos atlantis generate repo-config -- Generate atlantis.yaml repo-level configuration
2. atmos terraform generate backends -- Generate backend configuration for all components
3. atmos terraform generate varfiles -- Generate deep-merged varfiles for all stacks

Atlantis Configuration in atmos.yaml

integrations:
  atlantis:
    path: "atlantis.yaml"
    config_templates:
      config-1:
        version: 3
        automerge: true
        delete_source_branch_on_merge: true
        parallel_plan: true
        parallel_apply: true
        allowed_regexp_prefixes:
          - dev/
          - staging/
          - prod/
    project_templates:
      project-1:
        name: "{tenant}-{environment}-{stage}-{component}"
        workspace: "{workspace}"
        dir: "{component-path}"
        terraform_version: v1.8
        delete_source_branch_on_merge: true
        autoplan:
          enabled: true
          when_modified:
            - "**/*.tf"
            - "varfiles/$PROJECT_NAME.tfvars"
          apply_requirements:
            - "approved"
    workflow_templates:
      workflow-1:
        plan:
          steps:
            - run: terraform init -input=false
            - run: terraform workspace select $WORKSPACE
            - run: terraform plan -input=false -refresh -out $PLANFILE -var-file varfiles/$PROJECT_NAME.tfvars
        apply:
          steps:
            - run: terraform apply $PLANFILE

Generating the Atlantis Configuration

atmos atlantis generate repo-config --config-template config-1 --project-template project-1

This generates an atlantis.yaml file with a project entry for each Atmos component in every stack, using the specified config and project templates.

Stack-Level Atlantis Configuration

Atlantis settings can also be overridden in stack manifests using settings.atlantis, which deep-merges with the integrations.atlantis section in atmos.yaml.

Environment Promotion Patterns

Atmos supports environment promotion through its stack inheritance model:

1. Catalog-based defaults -- Define common configuration in stacks/catalog/
2. Environment overrides -- Override values per environment in stack manifests
3. PR-based promotion -- Use Git branches and PRs to promote changes through environments
4. Sequential apply -- Use CI/CD dependencies to apply to dev first, then staging, then prod

Example promotion workflow:

• Changes merged to main auto-apply to dev
• Manual approval (or separate PR) promotes to staging
• Another manual approval promotes to prod

Best Practices for CI/CD with Atmos

1. Use atmos describe affected to minimize CI/CD runtime by only planning/applying changed stacks
2. Store planfiles securely -- Use encrypted S3 buckets with DynamoDB locking for GitHub Actions
3. Use GitHub OIDC -- Never hardcode AWS credentials in workflows
4. Group matrix jobs -- Use the group-by setting to work around the 256-job GitHub Actions limit
5. Limit drift detection Issues -- Set max-opened-issues to keep drift manageable
6. Use separate IAM roles for plan and apply operations (least privilege)
7. Pin Atmos and Terraform versions in CI/CD to ensure reproducibility
8. Run atmos validate stacks early in CI/CD pipelines to catch configuration errors
9. Use Job Summaries instead of PR comments to avoid noisy pull requests
10. Test infrastructure changes in lower environments before promoting to production

Compatibility Notes

GitHub Actions versions must match the Atmos version:

• Atmos >= 1.63.0: Use v2+ of plan, apply, drift-remediation; v1+ of drift-detection and affected-stacks
• Atmos < 1.63.0: Use older action versions (v1 plan/apply, v0 drift-detection, v2 affected-stacks)

The integrations.github.gitops configuration in atmos.yaml is supported in Atmos >= 1.63.0. Earlier versions pass the settings directly as GitHub Action inputs.

Additional Resources

• For complete GitHub Actions inputs/outputs, see references/github-actions.md
• For Spacelift stack configuration details, see references/spacelift.md