Post to Slack

A composite action that posts a message to a Slack channel via an incoming webhook. It runs inline in the calling job, so a notification adds a single step — no separate run or runner. The webhook URL is sensitive — store it in H2O Secure Store and pass it in as a secret input (secret: true), so the value is redacted from logs.

Action

id: notify-slack
name: Post Message to Slack

inputs:
  webhook_url:
    type: string
    required: true
    secret: true
    description: "Slack incoming webhook URL"
  message:
    type: string
    required: true
    description: "Message text to post"
  channel:
    type: string
    default: ""
    description: "Override channel (optional, uses webhook default if empty)"
  username:
    type: string
    default: "H2O Workflows"
    description: "Bot username displayed in Slack"

steps:
  - name: "Send Slack notification"
    timeout: "1m"
    env:
      WEBHOOK_URL: "${{ .inputs.webhook_url }}"
      MESSAGE: "${{ .inputs.message }}"
      CHANNEL: "${{ .inputs.channel }}"
      USERNAME: "${{ .inputs.username }}"
    run: |
      python3 -c "
      import os, json, urllib.request

      payload = {
          'text': os.environ['MESSAGE'],
          'username': os.environ['USERNAME'],
      }
      if os.environ.get('CHANNEL'):
          payload['channel'] = os.environ['CHANNEL']

      req = urllib.request.Request(
          os.environ['WEBHOOK_URL'],
          data=json.dumps(payload).encode(),
          headers={'Content-Type': 'application/json'},
      )
      resp = urllib.request.urlopen(req)
      print(f'Slack response: {resp.status} {resp.read().decode()}')
      "

Publish the action to make it callable from workflows.

Use it from your workflow

Reference your stored secret with ${{ .secrets.<name> }} and pass it to the action via with:

id: train-and-notify
name: Train and Notify

secrets:
  - name: workspaces/<workspace-id>/secrets/slack-webhook
    as: slack_webhook

jobs:
  train:
    steps:
      - name: Train model
        run: python train.py

      - uses: <workspace-id>:notify-slack@latest
        with:
          webhook_url: "${{ .secrets.slack_webhook }}"
          message: "Training finished successfully"

Alert on failure

Because the action runs inline as a step, it combines with an if condition to notify only when an earlier step failed:

      - uses: <workspace-id>:notify-slack@latest
        if: failure()
        with:
          webhook_url: "${{ .secrets.slack_webhook }}"
          message: "Training FAILED — check the run logs"