On Demand Runners

A orb that lets users spin up runners on the demand durning a pipeline. Users are able to provision an EC2 instance on the fly, install the runner agent, and allow the runner to accept units of work.

CircleCI Orb Registry Page

Why On Demand Runners?

On Demand Runners grew from the need to not have "pet" servers that people had to manage. Having a server sitting in the cloud with the runner agent install just waiting for a job costs money. Furthermore, you have to make sure your server stays up to date and patched. Now multiple that responsibility by X, where X is the number of runners you need.

Why not have a model where you only have a server with runner agent installed when you need it? Hence On Demand Runners were born!

On Demand Runners will spin up a fresh EC2 instance for you when you have a pipeline that requires a runner. No longer do you need to pay for an idle server that you have to keep patched. On Demand Runners allow you to escape the "pet" server and lets you move to having "cattle".

How to utilize On Demand Runners!

On Demand Runners could not be more simple to use! On Demand Runners utilizes CircleCI's Orb system to make the end user experience as easy as possible. Here is a little example of how one could use On Demand Runners:

  version: 2.1

  orbs:
    on-demand-runners: crowley-namespace/on-demand-runners@0.0.1

  jobs:
    run-test-on-runner:
      machine: true
      resource_class: crowley-namespace/<< pipeline.id >>
      steps:
        - run: echo "Hello World! I am running on a On Demand Runner!"

  workflows:
    example-workflow:
      jobs:
        - on-demand-runners/deploy:
            resource_class: << pipeline.id >>
            context:
                - aws-creds
        - run-test-on-runner:
            requires:
              - on-demand-runners/deploy
        - on-demand-runners/destroy:
            resource_class: << pipeline.id >>
            context:
                - aws-creds
            requires:
              - "run-test-on-runner"

The first step in the pipeline calls the On Demand Runners' orb. Calling deploy will deploy an EC2 instance and install runner on that instance. After running the deploy job, you are now ready to utilize the runner which you just deployed. After doing whatever job(s) you need to do on the runner, you can call destroy. This job will deprovision the runner you spun up using the deploy job.

Once destroy is called the EC2 instance will be terminated and all runner related pieces(resource class and runner token), will be deleted as well. Thus leaving you with a clean slate!

To see what environment variables you need to set in order for On Demand Runners to work, please refer to the jobs section on the orb registry.

Tips and Tricks

Here are some tips and tricks to make working with On Demand Runners that much easier!

1. Utilize CircleCI's contexts to set the needed environment variables. It is recommended to use at least an AWS context to store command AWS credentials/secrets. In the both example, aws-creds stores all the needed credentials for spinning up an EC2 instance on AWS.

2. Utilize CircleCI's project environment variables to set the specific values for On Demand Runner like AWS_INSTANCE_TYPE, NUMBER_OF_RUNNERS, AWS_AMI_ID, etc. These values will changed from project to project to best suit your needs.

3. If you need more than one On Demand Runner, you can change NUMBER_OF_RUNNERS to be whatever you want. For example, you might have a need for 5 runners so setting NUMBER_OF_RUNNERS to 5 will spin up 5 EC2 instance configured under the same resource class.

4. Use a UNIQUE resource class name, like pipeline.id. This is because when you call destroy it will delete all runner tokens associated for the given resource class and then finally delete the resource class. This might cause issues with you utilize a resource class name that is in use already.

5. Upload the SSH key you will be utilizing to the CircleCI project with no hostname defined. This will allow Ansible to ssh into the EC2 instance it spun up. If you do not upload the SSH key to CircleCI you will run into a connection error. Please note, the SSH key you upload to CircleCI should match the one you set in AWS_SSH_KEY.