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

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Supported Fields

FieldDescription
envEnvironment variables for build and runtime
formationProcess configuration with replicas and sizing
scriptsDeployment lifecycle scripts
buildpacksBuildpack (language) ordering

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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"
    }
  }
}

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Variable Properties

PropertyDescription
valueA fixed value for the variable. Exported during build and included in result JSON.
generatorSet to "secret" to auto-generate a random secret value. The generated value appears in the result JSON.
requiredSet to true to indicate this variable must be provided at deploy time. Appears in result JSON with a null value.
descriptionHuman-readable description (informational only, not used during build).

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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
    }
  }
}

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Formation Properties

PropertyDescription
quantityNumber of replicas (instances) for this process type
sizeCompute size identifier (e.g., standard-1x, performance-m)

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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"
  }
}

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Supported Scripts

ScriptWhen It Runs
postdeployAfter a successful deployment (e.g., database migrations)
pr-predestroyBefore a review app is destroyed (e.g., cleanup)

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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"}
  ]
}

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Buildpack URL Mapping

Buildpack URLmigetpacks Language
heroku/nodejsnodejs
heroku/rubyruby
heroku/pythonpython
heroku/gogo
heroku/javajava
heroku/phpphp
heroku/scalascala
heroku/clojureclojure
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.

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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"}
  ]
}

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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"}
  ],
  "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.