Resource group

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By default, pipelines in GitLab CI/CD run concurrently. Concurrency is an important factor to improve the feedback loop in merge requests, however, there are some situations that you may want to limit the concurrency on deployment jobs to run them one by one. Use resource groups to strategically control the concurrency of the jobs for optimizing your continuous deployments workflow with safety.

Add a resource group

You can add only one resource to a resource group.

Provided that you have the following pipeline configuration (.gitlab-ci.yml file in your repository): 

build:
  stage: build
  script: echo "Your build script"

deploy:
  stage: deploy
  script: echo "Your deployment script"
  environment: production

Every time you push a new commit to a branch, it runs a new pipeline that has two jobs build and deploy. But if you push multiple commits in a short interval, multiple pipelines start running simultaneously, for example:

  • The first pipeline runs the jobs build -> deploy
  • The second pipeline runs the jobs build -> deploy

In this case, the deploy jobs across different pipelines could run concurrently to the production environment. Running multiple deployment scripts to the same infrastructure could harm/confuse the instance and leave it in a corrupted state in the worst case.

To ensure that a deploy job runs once at a time, you can specify resource_group keyword to the concurrency sensitive job: 

deploy:
  ...
  resource_group: production

With this configuration, the safety on the deployments is assured while you can still run build jobs concurrently for maximizing the pipeline efficiency.

Prerequisites

Process modes

You can choose a process mode to strategically control the job concurrency for your deployment preferences. The following modes are supported:

  • Unordered: This is the default process mode that limits the concurrency on running jobs. It’s the easiest option to use when you don’t care about the execution order of the jobs. It starts processing the jobs whenever a job is ready to run.

  • Oldest first: This process mode limits the concurrency of the jobs. When a resource is free, it picks the first job from the list of upcoming jobs (created, scheduled, or waiting_for_resource state) that are sorted by pipeline ID in ascending order.

    This mode is efficient when you want to ensure that the jobs are executed from the oldest pipeline. It is less efficient compared to the unordered mode in terms of the pipeline efficiency, but safer for continuous deployments.

  • Newest first: This process mode limits the concurrency of the jobs. When a resource is free, it picks the first job from the list of upcoming jobs (created, scheduled or waiting_for_resource state) that are sorted by pipeline ID in descending order.

    This mode is efficient when you want to ensure that the jobs are executed from the newest pipeline and prevent all of the old deploy jobs with the prevent outdated deployment jobs feature. This is the most efficient option in terms of the pipeline efficiency, but you must ensure that each deployment job is idempotent.

Change the process mode

To change the process mode of a resource group, you must use the API and send a request to edit an existing resource group by specifying the process_mode:

  • unordered
  • oldest_first
  • newest_first

An example of difference between the process modes

Consider the following .gitlab-ci.yml, where we have two jobs build and deploy each running in their own stage, and the deploy job has a resource group set to production: 

build:
  stage: build
  script: echo "Your build script"

deploy:
  stage: deploy
  script: echo "Your deployment script"
  environment: production
  resource_group: production

If three commits are pushed to the project in a short interval, that means that three pipelines run almost at the same time:

  • The first pipeline runs the jobs build -> deploy. Let’s call this deployment job deploy-1.
  • The second pipeline runs the jobs build -> deploy. Let’s call this deployment job deploy-2.
  • The third pipeline runs the jobs build -> deploy. Let’s call this deployment job deploy-3.

Depending on the process mode of the resource group:

  • If the process mode is set to unordered:
    • deploy-1, deploy-2, and deploy-3 do not run concurrently.
    • There is no guarantee on the job execution order, for example, deploy-1 could run before or after deploy-3 runs.
  • If the process mode is oldest_first:
    • deploy-1, deploy-2, and deploy-3 do not run concurrently.
    • deploy-1 runs first, deploy-2 runs second, and deploy-3 runs last.
  • If the process mode is newest_first:
    • deploy-1, deploy-2, and deploy-3 do not run concurrently.
    • deploy-3 runs first, deploy-2 runs second and deploy-1 runs last.

Pipeline-level concurrency control with cross-project/parent-child pipelines

You can define resource_group for downstream pipelines that are sensitive to concurrent executions. The trigger keyword can trigger downstream pipelines and the resource_group keyword can co-exist with it. resource_group is efficient to control the concurrency of deployment pipelines, while other jobs can continue to run concurrently.

The following example has two pipeline configurations in a project. When a pipeline starts running, non-sensitive jobs are executed first and aren’t affected by concurrent executions in other pipelines. However, GitLab ensures that there are no other deployment pipelines running before triggering a deployment (child) pipeline. If other deployment pipelines are running, GitLab waits until those pipelines finish before running another one. 

# .gitlab-ci.yml (parent pipeline)

build:
  stage: build
  script: echo "Building..."

test:
  stage: test
  script: echo "Testing..."

deploy:
  stage: deploy
  trigger:
    include: deploy.gitlab-ci.yml
    strategy: depend
  resource_group: AWS-production

# deploy.gitlab-ci.yml (child pipeline)

stages:
  - provision
  - deploy

provision:
  stage: provision
  script: echo "Provisioning..."

deployment:
  stage: deploy
  script: echo "Deploying..."
  environment: production

You must define strategy: depend with the trigger keyword. This ensures that the lock isn’t released until the downstream pipeline finishes.

Troubleshooting

Avoid dead locks in pipeline configurations

Because oldest_first process mode enforces the jobs to be executed in a pipeline order, there is a case that it doesn’t work well with the other CI features.

For example, when you run a child pipeline that requires the same resource group with the parent pipeline, a dead lock could happen. Here is an example of a bad setup: 

# BAD
test:
  stage: test
  trigger:
    include: child-pipeline-requires-production-resource-group.yml
    strategy: depend

deploy:
  stage: deploy
  script: echo
  resource_group: production
  environment: production

In a parent pipeline, it runs the test job that subsequently runs a child pipeline, and the strategy: depend option makes the test job wait until the child pipeline has finished. The parent pipeline runs the deploy job in the next stage, that requires a resource from the production resource group. If the process mode is oldest_first, it executes the jobs from the oldest pipelines, meaning the deploy job is executed next.

However, a child pipeline also requires a resource from the production resource group. Because the child pipeline is newer than the parent pipeline, the child pipeline waits until the deploy job is finished, something that never happens.

In this case, you should specify the resource_group keyword in the parent pipeline configuration instead: 

# GOOD
test:
  stage: test
  trigger:
    include: child-pipeline.yml
    strategy: depend
  resource_group: production # Specify the resource group in the parent pipeline

deploy:
  stage: deploy
  script: echo
  resource_group: production
  environment: production

Jobs get stuck in “Waiting for resource”

Sometimes, a job hangs with the message Waiting for resource: <resource_group>. To resolve, first check that the resource group is working correctly:

  1. Go to the job details page.

  2. If the resource is assigned to a job, select View job currently using resource and check the job status.

    • If the status is running or pending, the feature is working correctly. Wait until the job finishes and releases the resource.
    • If the status is created and the process mode is either Oldest first or Newest first, the feature is working correctly. Visit the pipeline page of the job and check which upstream stage or job is blocking the execution.
    • If none of the above conditions are met, the feature might not be working correctly. Report the issue to GitLab.

  3. If View job currently using resource is not available, the resource is not assigned to a job. Instead, check the resource’s upcoming jobs.

    1. Get the resource’s upcoming jobs with the REST API.
    2. Verify that the resource group’s process mode is Oldest first.
    3. Find the first job in the list of upcoming jobs, and get the job details with GraphQL.
    4. If the first job’s pipeline is an older pipeline, try to cancel the pipeline or the job itself.
    5. Optional. Repeat this process if the next upcoming job is still in an older pipeline that should no longer run.
    6. If the problem persists, report the issue to GitLab.

Race conditions in complex or busy pipelines

If you can’t resolve your issue with the solutions above, you might be encountering a known race condition issue. The race condition happens in complex or busy pipelines. For example, you might encounter the race condition if you have:

  • A pipeline with multiple child pipelines.
  • A single project with multiple pipelines running simultaneously.

If you think you are running into this problem, report the issue to GitLab and leave a comment on issue 436988 with a link to your new issue. To confirm the problem, GitLab might ask for additional details such as your full pipeline configuration.

As a temporary workaround, you can:

  • Start a new pipeline.

  • Re-run a finished job that has the same resource group as the stuck job.

    For example, if you have a setup_job and a deploy_job with the same resource group, the setup_job might finish while the deploy_job is stuck at “waiting for resource”. Re-run the setup_job to restart the whole process and allow deploy_job to finish.

Get job details through GraphQL

You can get job information from the GraphQL API. You should use the GraphQL API if you use pipeline-level concurrency control with cross-project/parent-child pipelines because the trigger jobs are not accessible from the UI.

To get job information from the GraphQL API:

  1. Go to the pipeline details page.
  2. Select the Jobs tab and find the ID of the stuck job.
  3. Go to the interactive GraphQL explorer.

  4. Run the following query: 

    {
  project(fullPath: "<fullpath-to-your-project>") {
    name
    job(id: "gid://gitlab/Ci::Build/<job-id>") {
      name
      status
      detailedStatus {
        action {
          path
          buttonTitle
        }
      }
    }
  }
}

    The job.detailedStatus.action.path field contains the job ID using the resource.

  5. Run the following query and check job.status field according to the criteria above. You can also visit the pipeline page from pipeline.path field. 

    {
  project(fullPath: "<fullpath-to-your-project>") {
    name
    job(id: "gid://gitlab/Ci::Build/<job-id-currently-using-the-resource>") {
      name
      status
      pipeline {
        path
      }
    }
  }
}

Report an issue

Open a new issue with the following information:

  • The ID of the affected job.
  • The job status.
  • How often the problem occurs.

  • Steps to reproduce the problem.

    You can also contact support for further assistance, or to get in touch with the development team.