Run Queue

Terragrunt’s “Run Queue” is the mechanism it uses to manage the run order and concurrency when running OpenTofu/Terraform commands across multiple Terragrunt units. This is particularly relevant when using the run --all or run --graph commands.

How it Works: The Dependency Graph (DAG)

At its core, the Run Queue relies on a Directed Acyclic Graph (DAG) built from the dependencies defined between your Terragrunt units. These dependencies are typically established using dependency or dependencies blocks in your terragrunt.hcl files.

Terragrunt analyzes these dependencies to determine the correct order of operations:

1. Discovery: Terragrunt discovers configurations that might be relevant to a run based on the current working directory.
2. Constructing the Queue: Based on the command being run, Terragrunt creates an ordered queue.
   • For commands like plan or apply, dependencies are run before the units that depend on them.
   • For commands like destroy, dependent units are run before their dependencies.
3. Runs: Terragrunt dequeues the units in the queue and runs them, respecting the queue order. By default, it runs units concurrently up to a certain limit (controlled by the --parallelism flag), but it will always wait for a unit’s dependencies (or dependents for destroys) to complete successfully before running that unit.

Example DAG

Consider a setup where:

• Unit “dependent” depends on unit “dependency”.
• Unit “dependency” depends on unit “ancestor-dependency”.
• Unit “independent” has no dependencies nor dependents.

Represented textually, the file structure might look like:

root
├── ancestor-dependency
│   └── terragrunt.hcl
├── independent
│   └── terragrunt.hcl
└── subtree
    ├── dependency
    │   └── terragrunt.hcl
    └── dependent
        └── terragrunt.hcl

Assuming a current working directory of the root directory, Terragrunt would run units in the following order:

• run --all plan Order: Terragrunt would run independent and ancestor-dependency concurrently. Once ancestor-dependency finishes, dependency would run. Once dependency finishes, dependent would run.
• run --all destroy Order: Terragrunt would run dependent and independent concurrently. Once dependent finishes, dependency would run. Once dependency finishes, ancestor-dependency would run.

Controlling the Queue

Several flags allow you to customize how Terragrunt builds and executes the run queue. By default, Terragrunt will include all units that are in the current working directory.

Include by default

By default, when using the --all flag, Terragrunt will include all units that are in the current working directory, and any external dependencies.

Certain flags trigger “Exclude by default” behavior, meaning that Terragrunt will no longer automatically include all units in the current working directory, and will instead rely on discovering configurations based on the provided queue control flags.

Those flags will be discussed in the next section.

Filtering Units

You can control which units are included or excluded from the queue:

• --queue-include-dir: Specify glob patterns for directories to include. Can be used multiple times.

  e.g. terragrunt run --all plan --queue-include-dir "subtree/*"

  Include units within the subtree directory (along with their dependencies), i.e., subtree/dependent, subtree/dependency and ancestor-dependency.

  Note: Using the --queue-include-dir automatically triggers “Exclude by default” behavior, as mentioned above.

  Note: ancestor-dependency is still included by default here because it’s a dependency of dependency. Using --queue-strict-include would prevent that.

• --queue-exclude-dir: Specify glob patterns for directories to exclude. Can be specified multiple times.

  e.g. terragrunt run --all plan --queue-exclude-dir "independent"

  Exclude the independent unit. ancestor-dependency, subtree/dependency, and subtree/dependent would still be processed according to their dependencies.

  Note: Dependencies of excluded units will still be included unless they are also explicitly excluded. In this example, excluding subtree/dependency would not automatically exclude ancestor-dependency.

• --queue-excludes-file: Provide a file containing a list of directories to exclude.

  e.g. terragrunt run --all plan --queue-excludes-file ".tg-excludes"

  # .tg-excludes
  
  independent
  subtree/dependency
  

  Exclude independent and subtree/dependency from the run, only running subtree/dependent and ancestor-dependency.

  Tip: The default value for this flag is .terragrunt-excludes. Populate a file named this in your project root to exclude units from being run by default, without using the --queue-excludes-file flag.

• --queue-strict-include: Only include units matching --queue-include-dir.

  e.g. terragrunt run --all plan --queue-include-dir "subtree/dependency" --queue-strict-include

  Only include the subtree/dependency unit. Its dependency, ancestor-dependency, will be excluded because it does not match the include and strict mode is enabled.

• --queue-include-external: Include external dependencies (those outside the current working directory) by default.

  e.g. terragrunt run --all plan --working-dir subtree --queue-include-external

  Include ancestor-dependency in addition to the subtree/dependent and subtree/dependency units.

  Note: The --queue-include-external flag is simply a convenience flag to avoid the interactive prompt to request inclusion of external dependencies. By default, Terragrunt will wait for user input to determine whether or not external dependencies should be included.

• --queue-exclude-external: Exclude external dependencies.

  e.g. terragrunt run --all plan --working-dir subtree --queue-exclude-external

  Exclude ancestor-dependency from the run. Only run subtree/dependent and subtree/dependency.

  Note: This flag is simply a convenience flag to avoid the interactive prompt to request exclusion of external dependencies. By default, Terragrunt will wait for user input to determine whether or not external dependencies should be excluded.

• --queue-include-units-reading: Include units that read a specific file (via includes or HCL functions like mark_as_read).

  e.g. terragrunt run --all plan --queue-include-units-reading "subtree/common.hcl"

  Example subtree/dependent/terragrunt.hcl:

  dependency "dep" {
    config_path  = "../dependency"
    skip_outputs = true
  }
  
  include "common" {
    path = find_in_parent_folders("common.hcl")
  }
  

  Example subtree/dependency/terragrunt.hcl:

  dependency "dep" {
    config_path  = "../../ancestor-dependency"
    skip_outputs = true
  }
  
  include "common" {
    path = find_in_parent_folders("common.hcl")
  }
  

  Example subtree/common.hcl:

  # Intentionally empty
  

  Include subtree/dependent and subtree/dependency (the units that read subtree/common.hcl) in the run.

  Tip: Sometimes, it can be impossible for Terragrunt to know that certain files are read by a unit (e.g. if the file is read in the OpenTofu/Terraform module, not in Terragrunt configuration). In cases like this, you can use the mark_as_read HCL function to explicitly tell Terragrunt that a unit reads a file.

Modifying Order and Error Handling

• --queue-construct-as (--as): Build the run queue as if a particular command was run. Useful for performing dry-runs of run using discovery commands, like find and list. e.g. terragrunt list --queue-construct-as destroy This lists the units in the order they’d be processed for run --all destroy:

  $ terragrunt list --as destroy -l
  Type  Path
  unit  independent
  unit  subtree/dependent
  unit  subtree/dependency
  unit  ancestor-dependency
  

  $ terragrunt list --as plan -l
  Type  Path
  unit  ancestor-dependency
  unit  independent
  unit  subtree/dependency
  unit  subtree/dependent
  

• --queue-ignore-dag-order: Execute units concurrently without respecting the dependency order.

  e.g. terragrunt run --all plan --queue-ignore-dag-order

  Run plan on ancestor-dependency, subtree/dependency, subtree/dependent, and independent all concurrently, without waiting for their defined dependencies. For instance, subtree/dependent’s plan would not wait for subtree/dependency’s plan to complete.

  Caution: This flag is useful for faster runs in stateless commands like validate or plan, but is dangerous for commands that modify state like apply or destroy. You might encounter failed applies if unit dependencies are not applied before dependents, and conversely, failed destroys if unit dependents are not destroyed before dependencies.

• --queue-ignore-errors: Continue processing the queue even if some units fail.

  e.g. terragrunt run --all plan --queue-ignore-errors

  If ancestor-dependency’s plan fails, Terragrunt will still attempt to run plan for subtree/dependency, then subtree/dependent, and also for independent.

  Caution: This flag is useful for identifying all errors at once, but can lead to inconsistent state if used with apply or destroy. You might encounter failed applies if unit dependencies are not applied successfully before dependents, and conversely, failed destroys if unit dependents are not destroyed successfully before dependencies.

Important Considerations

Caution: When using run --all plan with units that have dependencies (e.g. via dependency or dependencies blocks), the command will fail if those dependencies have never been deployed. This is because Terragrunt cannot resolve dependency outputs without existing state.

To work around this issue, use mock outputs in dependency blocks.

Caution: Do not set TF_PLUGIN_CACHE_DIR when using run --all (unless using OpenTofu >= 1.10).

This can cause concurrent access issues with the provider cache. Instead, use Terragrunt’s built-in Provider Cache Server.

Caution: When using run --all with apply or destroy, Terragrunt automatically adds the -auto-approve flag due to limitations with shared stdin making individual approvals impossible. Use --no-auto-approve to override this, but be aware you might need alternative approval workflows.