megascaler

WIP Autoscaler for self-hosted runners.

This program will communticate with GitHub to receive job requests and will delegate running the job to a worker program, while the worker program has no access to the runner / PAT tokens if it is spawned in an isolated environment.

Works with both the official https://github.com/actions/runner and the unofficial https://github.com/ChristopherHX/github-act-runner.

Known problems

you should avoid using multiple labels
- if you really want to use them always provide an entry for any possible configuration
- your jobs with more labels may wait longer for execution, due to other jobs taking away your runner
- might need to create runners with more labels than requested to minimize queue time
webhooks cannot always be delivered, need to add github api polling for reliability if waiting long

Goals

listen for workflow_job webhooks
create an inmemory runner with a PAT token
track job assignment, so you can control in which environment the job executes
cache runner registration token, valid for ca. 1 hour
cache runner registration jwt, very short lived you can still register ca. > 1000 runners without extra rate limit cost
use both actions/runner and nektos/act worker
provide a script to allocate a target enviroment for a job
provide a script to execute the worker process on the target enviroment, by forwarding stdin

Extras for actions/runner

${{ github.megascaler.jobid }} is assigned to the rest api job id
${{ github.megascaler.job_url }} is assigned to the job html url
${{ github.megascaler.run_url }} is assigned to the workflow run html url
${{ github.megascaler.labels }} is assigned to an array of string with all labels from runs-on of the current instance
${{ github.megascaler.variables }} is assigned to the otherwise hidden system variables of Actions
${{ github.megascaler.jobDisplayName }} is assigned to the otherwise hidden job display name

Non representative Statistics

68 of 81 jobs have received the wrong runner
average time between job start and webhook in_progress is between 1s and 60s

Getting started

create a config.yml

worker:                                             # list of worker to spawn if the label list has an exact match
- labels:
  - github-act-runner                               # labels to spawn the specfic worker, if this matches what you added to the `runs-on` key this program will spawn a worker
  args:
  - /workspaces/megascaler/runner/github-act-runner # Path to https://github.com/ChristopherHX/github-act-runner binary
  - worker                                          # Execute in worker mode
- labels:
  - actions-runner                                  # labels to spawn the specfic worker, if this matches what you added to the `runs-on` key this program will spawn a worker
  args:                                             # create a `/workspaces/megascaler/runner/.runner` textfile with content `{"workFolder": "_work"}`
  - pwsh                                            # Path to https://github.com/powershell/powershell binary or add it to your `PATH` env variable
  - actions-runner-worker.ps1                       # Get the file from https://github.com/ChristopherHX/github-act-runner/blob/main/compat/actions-runner-worker.ps1
  - /workspaces/megascaler/runner/bin/Runner.Worker # Path to https://github.com/actions/runner binary
address: 0.0.0.0:9403                               # webhook listener to receive `workflow_job` events from GitHub
# secret: mysecret                                  # validate the webhook signature to harden the webhook endpoint
pat: ghp_XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX       # GitHub Classic PAT with full `repo` scope

You can also pipe the worker via the args list via ssh or other programs, it will connect to GitHub via http and communicates over stdin/stdout.

run

go build
./megascaler