app.json

app.json is a manifest format for describing web apps. It declares cron tasks, healthchecks, and other information required to run an app on Dokku. This document describes the schema in detail.

Info

[!IMPORTANT] While the app.json format used by Dokku is based on the one supported by Heroku, not all Heroku functionality is supported by Dokku.

Cron

{
  "cron": [
    {
      "command": "echo 'hello'",
      "schedule": "0 1 * * *"
    }
  ]
}

(list, optional) A list of cron resources. Keys are the names of the process types. The values are an object containing one or more of the following properties:

• command: (string, required)
• schedule: (string, required)

Formation

{
  "formation": {
    "web": {
      "max_parallel": 1,
      "quantity": 1
    }
  }
}

(object, optional) A key-value object for process type configuration. Keys are the names of the process types. The values are an object containing one or more of the following properties:

• autoscaling (map of string to object, optional) autoscaling rules. See the autoscaling section for more details
• max_parallel: (int, optional) number of instances to deploy in parallel at a given time
• quantity: (int, optional) number of processes to maintain. Default 1 for web processes, 0 for all others.

Autoscaling

{
  "formation": {
    "web": {
      "autoscaling": {
        "cooldown_period_seconds": 300,
        "max_quantity": 10,
        "min_quantity": 1,
        "polling_interval_seconds": 30,
        "triggers": {
          "http": {
            "metadata": {
              "url": "https://example.com/health"
            }
          }
        }
      }
    }
  }
}

(object, optional) A key-value object for autoscaling configuration. Keys are the names of the process types. The values are an object containing one or more of the following properties:

• cooldown_period_seconds: (int, optional)
• max_quantity: (int, optional)
• min_quantity: (int, optional)
• polling_interval_seconds: (int, optional)
• triggers: (object, optional)

An autoscaling trigger consists of the following properties:

• name: (string, optional)
• type: (string, optional)
• metadata: (object, optional)

Healthchecks

{
  "healthchecks": {
    "web": [
      {
        "type":        "startup",
        "name":        "web check",
        "description": "Checking if the app responds to the /health/ready endpoint",
        "path":        "/health/ready",
        "attempts": 3
      }
    ]
  }
}

(object, optional) A key-value object specifying healthchecks to run for a given process type.

• attempts: (int, optional)
• command: (list of strings, optional)
• content: (string, optional)
• httpHeaders: (list of header objects, optional)
• initialDelay: (int, optional)
• listening: (boolean, optional)
• name: (string, optional)
• path: (string, optional)
• port: (int, optional)
• scheme: (string, optional)
• timeout: (int, optional)
• type: (string, optional)
• uptime: (int, optional)
• wait: (int, optional)
• warn: (boolean, optional)
• onFailure: (object, optional)

Scripts

{
  "scripts": {
    "dokku": {
      "predeploy": "touch /app/predeploy.test",
      "postdeploy": "curl https://some.external.api.service.com/deployment?state=success"
    },
    "postdeploy": "curl https://some.external.api.service.com/created?state=success"
  }
}

(object, optional) A key-value object specifying scripts or shell commands to execute at different stages in the build/release process.

• dokku.predeploy: (string, optional)
  • When to use: This should be used if your app does not support arbitrary build commands and you need to make changes to the built image.
  • Are changes committed to the image at this phase: Yes
  • Example use-cases
    • Bundling assets in a slightly different way
    • Installing a custom package from source or copying a binary into place
• dokku.postdeploy: (string, optional)
  • When to use: This should be used in conjunction with external systems to signal the completion of your deploy.
  • Are changes committed to the image at this phase: No
  • Example use-cases
    • Notifying slack that your app is deployed
    • Coordinating traffic routing with a central load balancer
• postdeploy: (string, optional)
  • When to use: This should be used when you wish to run a command once, after the app is created and not on subsequent deploys to the app.
  • Are changes committed to the image at this phase: No
  • Example use-cases
    • Setting up OAuth clients and DNS
    • Loading seed/test data into the app’s test database