The Heroku add-on architecture

Add-ons are one way to extend the capabilities of a Heroku application. The user can add a new service such as caching or data storage to the app by provisioning an add-on and consuming the service offered. Heroku users have two ways to provision the add-ons:

  • The add-on marketplace
  • The Heroku toolbelt client

The entire process of add-on provisioning and consumption is shown in the following diagram:

The Heroku add-on architecture

The following steps describe how a new add-on is provisioned and consumed by the Heroku app:

  1. The Heroku user requests an add-on using either of the two ways indicated in the previous paragraph.
  2. Heroku sends a provisioning request for the current account to the service provider for the given service.
  3. The service provider validates the request and creates a private resource for the caller and returns a URL that specifies the exact location and the credential required to access the service.

    For example, the Amazon RDS service provider might return:

    MYSQL_URL=mysql://user:pass@mydbhost.com/database

  4. At this point, Heroku adds the returned URL to the environment of the requesting app, rebuilds the app again, and restarts the related dynos for the app.
  5. The app is ready to consume the services of the new add-on.
  6. The app extracts the resource URL from the environment and issues a read/write request to the service, passing the necessary parameters.
  7. The invoked resource within the service provider validates the authenticity of the requests and grants a privilege to execute the operation if the request is valid and the user is authentic.
  8. The resource creates a response and sends it back to the app, which in turn uses the result to create a page or store the response in the app's memory for later use and continue.

Note that the request for service from the app could either be from the user making a request in the web browser or a worker dyno asking for some resource while performing a long running background job.

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