app.json

migetpacks supports app.json as an application manifest file. This file defines environment variables, process formation, deployment scripts, and buildpack configuration for your application.

Supported Fields

Field	Description
`env`	Environment variables for build and runtime
`formation`	Process configuration with replicas and sizing
`scripts`	Deployment lifecycle scripts
`buildpacks`	Buildpack (language) ordering
`addons`	Service dependencies (databases, caches, etc.)

Environment Variables

The env field defines environment variables that are exported during the build and included in the result JSON. Each variable can specify a fixed value, a generator, or be marked as required.

{
  "env": {
    "RAILS_ENV": {
      "value": "production"
    },
    "SECRET_KEY_BASE": {
      "generator": "secret"
    },
    "DATABASE_URL": {
      "required": true
    },
    "OPTIONAL_VAR": {
      "description": "An optional config variable"
    }
  }
}

Variable Properties

Property	Description
`value`	A fixed value for the variable. Exported during build and included in result JSON.
`generator`	Set to `"secret"` to auto-generate a random secret value. The generated value appears in the result JSON.
`required`	Set to `true` to indicate this variable must be provided at deploy time. Appears in result JSON with a `null` value.
`description`	Human-readable description (informational only, not used during build).

Formation

The formation field defines process types with their replica count and compute size. This information is passed through to the result JSON for use by deployment orchestrators.

{
  "formation": {
    "web": {
      "quantity": 2,
      "size": "standard-1x"
    },
    "worker": {
      "quantity": 1,
      "size": "standard-2x"
    },
    "clock": {
      "quantity": 1
    }
  }
}

Formation Properties

Property	Description
`quantity`	Number of replicas (instances) for this process type
`size`	Compute size identifier (e.g., `standard-1x`, `performance-m`)

Scripts

The scripts field defines lifecycle hooks that run at specific points during deployment. These are included in the result JSON for the deployment system to execute.

{
  "scripts": {
    "postdeploy": "bundle exec rails db:migrate",
    "pr-predestroy": "bundle exec rails db:drop"
  }
}

Supported Scripts

Script	When It Runs
`postdeploy`	After a successful deployment (e.g., database migrations)
`pr-predestroy`	Before a review app is destroyed (e.g., cleanup)

Buildpacks

The buildpacks field maps to migetpacks language support. Heroku-style buildpack URLs are automatically translated to migetpacks language identifiers.

{
  "buildpacks": [
    {"url": "heroku/nodejs"},
    {"url": "heroku/ruby"}
  ]
}

Buildpack URL Mapping

Buildpack URL	migetpacks Language
`heroku/nodejs`	`nodejs`
`heroku/ruby`	`ruby`
`heroku/python`	`python`
`heroku/go`	`go`
`heroku/java`	`java`
`heroku/php`	`php`
`heroku/scala`	`scala`
`heroku/clojure`	`clojure`

When multiple buildpacks are specified, the order defines the build sequence. The first buildpack becomes the primary language (determines the runtime base image), and subsequent buildpacks provide additional build stages.

The buildpacks field in app.json is equivalent to setting the BUILDPACKS environment variable. If both are provided, the BUILDPACKS env var takes precedence.

Addons

The addons field declares service dependencies for your application. Each addon can be specified as a simple string or as an object with additional configuration.

String Format

{
  "addons": [
    "heroku-postgresql:hobby-dev",
    "heroku-redis:premium-0"
  ]
}

Object Format

{
  "addons": [
    {
      "plan": "heroku-postgresql:standard-0",
      "as": "PRIMARY_DB"
    },
    {
      "plan": "mongolab:shared-single-small",
      "as": "MONGO",
      "options": {
        "version": "5.0"
      }
    }
  ]
}

Addon Properties

Property	Description
`plan`	The addon plan identifier (e.g., `heroku-postgresql:hobby-dev`)
`as`	Attachment alias — overrides the default config var prefix
`options`	Provider-specific configuration options

String-format addons are normalized to object format in the result JSON with as and options set to null.

Full Example

{
  "name": "my-rails-app",
  "env": {
    "RAILS_ENV": {
      "value": "production"
    },
    "SECRET_KEY_BASE": {
      "generator": "secret"
    },
    "DATABASE_URL": {
      "required": true
    },
    "REDIS_URL": {
      "required": true
    },
    "RAILS_LOG_TO_STDOUT": {
      "value": "enabled"
    }
  },
  "formation": {
    "web": {
      "quantity": 2,
      "size": "standard-1x"
    },
    "worker": {
      "quantity": 1,
      "size": "standard-2x"
    }
  },
  "scripts": {
    "postdeploy": "bundle exec rails db:migrate"
  },
  "buildpacks": [
    {"url": "heroku/nodejs"},
    {"url": "heroku/ruby"}
  ],
  "addons": [
    "heroku-postgresql:hobby-dev",
    {
      "plan": "heroku-redis:premium-0",
      "as": "CACHE"
    }
  ]
}

Result JSON Output

Values defined in app.json appear in the build result JSON output. Here is how each field maps:

{
  "status": "success",
  "images": [
    {"name": "registry.io/my-app:latest", "ports": ["5000/tcp"]}
  ],
  "processes": {
    "web": "bundle exec puma -C config/puma.rb",
    "release": "bundle exec rails db:migrate",
    "workers": [
      {"name": "sidekiq", "command": "bundle exec sidekiq"}
    ]
  },
  "env": {
    "RAILS_ENV": "production",
    "SECRET_KEY_BASE": "a1b2c3d4e5f6...",
    "DATABASE_URL": null,
    "REDIS_URL": null,
    "RAILS_LOG_TO_STDOUT": "enabled"
  },
  "formation": [
    {"name": "web", "quantity": 2, "size": "standard-1x"},
    {"name": "worker", "quantity": 1, "size": "standard-2x"}
  ],
  "scripts": [
    {"name": "postdeploy", "command": "bundle exec rails db:migrate"}
  ],
  "addons": [
    {"plan": "heroku-postgresql:hobby-dev", "as": null, "options": null},
    {"plan": "heroku-redis:premium-0", "as": "CACHE", "options": null}
  ],
  "language": "ruby",
  "timestamp": "2025-12-18T20:00:00Z"
}

Variables with "generator": "secret" will have their generated value populated in the env section of the result JSON. Variables with "required": true appear with a null value, indicating they must be supplied by the deployment system.

Getting Started

Guides

Configuration

Languages

Deploying

Reference

Supported Fields

Environment Variables

Variable Properties

Formation

Formation Properties

Scripts

Supported Scripts

Buildpacks

Buildpack URL Mapping

Addons

String Format

Object Format

Addon Properties

Full Example

Result JSON Output

​Supported Fields

​Environment Variables

​Variable Properties

​Formation

​Formation Properties

​Scripts

​Supported Scripts

​Buildpacks

​Buildpack URL Mapping

​Addons

​String Format

​Object Format

​Addon Properties

​Full Example

​Result JSON Output

Supported Fields

Environment Variables

Variable Properties

Formation

Formation Properties

Scripts

Supported Scripts

Buildpacks

Buildpack URL Mapping

Addons

String Format

Object Format

Addon Properties

Full Example

Result JSON Output