Terragrunt and Atlantis solve adjacent but distinct problems in Terraform-based infrastructure management. Terragrunt wraps the Terraform CLI to eliminate code duplication across environments and modules through a hierarchical HCL configuration system. Atlantis operates as a webhook-driven server that watches pull requests and automatically runs terraform plan when infrastructure code changes, posting the output directly in the PR for team review before anyone types terraform apply.
The configuration management approach differs fundamentally. Terragrunt uses terragrunt.hcl files that inherit settings from parent directories, allowing teams to define common backend configurations, provider settings, and variable values once and override them per environment. Without Terragrunt, teams typically duplicate backend blocks and variable definitions across every module directory. Atlantis does not address configuration duplication at all; it assumes Terraform modules are already organized and focuses on automating when and how those modules execute.
The collaboration model reveals the clearest distinction. Atlantis turns infrastructure changes into a code review workflow where terraform plan runs automatically on every PR, team members review the planned changes alongside the code diff, and terraform apply executes only after approval. This prevents direct apply from developer workstations and creates an audit trail in version control. Terragrunt can be used in this same PR workflow when combined with CI/CD tools, but does not provide the PR automation natively.
Dependency management is a core Terragrunt capability that has no equivalent in Atlantis. Terragrunt resolves cross-module dependencies through dependency blocks and the terragrunt_output function, ensuring modules execute in the correct order and can reference outputs from other modules. Atlantis processes each module independently based on which files changed in the PR, without awareness of inter-module relationships unless explicitly configured through server-side project definitions.
The 1.0 release of Terragrunt introduced stacks for grouping related infrastructure components into deployable units, filters for selective execution across monorepos, and run reports for tracking changes across multi-module operations. These features address the orchestration complexity that grows as infrastructure codebases scale beyond a handful of modules. Atlantis has remained focused on its core PR automation workflow, with customization through server-side configuration and custom workflow definitions.
Locking and concurrency control take different approaches. Atlantis provides built-in workspace locking that prevents concurrent applies to the same infrastructure, avoiding state conflicts when multiple team members work on overlapping resources. Terragrunt relies on the backend state locking provided by Terraform itself through DynamoDB or similar mechanisms, without adding an additional coordination layer for human workflow management.