Atlantis

Atlantis is an open source self-hosted application that automates Terraform pull request workflows. By running Terraform plan and apply operations in response to pull request comments, Atlantis brings infrastructure changes into the same collaborative review process used for application code, enabling teams to review, discuss, and approve infrastructure changes before they are applied.

The core workflow in Atlantis is straightforward. When a developer opens a pull request that modifies Terraform code, Atlantis automatically runs terraform plan and posts the output as a comment on the pull request. This gives all reviewers immediate visibility into what infrastructure changes will actually be made, rather than requiring them to mentally trace through the code and understand the implications. When the team is satisfied with the plan, an authorized user types atlantis apply in a comment to execute the apply.

The integration with Git repositories is central to how Atlantis works. It supports GitHub, GitLab, Bitbucket, and Azure DevOps, and it receives webhook notifications when pull request events occur. When Atlantis receives a notification, it clones the repository, navigates to the appropriate Terraform root directory, and executes the requested command in a clean environment.

The atlantis.yaml configuration file allows teams to customize how Atlantis handles each project within a repository. For monorepos containing multiple Terraform projects in subdirectories, the configuration can define each project's path, workspace, workflow, and approval requirements separately. This flexibility makes Atlantis practical for large, complex infrastructure repositories with many independent Terraform root modules.

Custom workflows in Atlantis allow teams to override the default plan and apply steps with custom command sequences. This is useful for incorporating additional steps such as running terragrunt commands, generating provider credentials, running tflint or checkov validations, or notifying external systems. Workflows can be defined globally or per-project.

Server-side configuration allows Atlantis administrators to define policies about which repositories can be managed, which users have approval permissions, and what environment variables are available during Terraform execution. The locking mechanism ensures that only one plan or apply can be in progress at a time for a given Terraform workspace, preventing concurrent modifications that could cause state conflicts.

Atlantis is deployed as a container or binary on the team's own infrastructure. This self-hosted model means that cloud credentials, Terraform state, and infrastructure code never leave the organization's network, which is important for organizations with strict security requirements. The deployment can be secured with TLS and webhook secret validation to prevent unauthorized access.

As an open source tool with a simple, focused design, Atlantis has become the standard for teams that want to incorporate Terraform into GitOps workflows without adopting a commercial platform.