Services are accessible from your application through their relationships. An app gains access to a service if a relationship is created for it, and communication is done through that service's protocol.
For example, an app with a relationship to a MySQL service will be able to connect to the MySQL database service on port 3306 using the service's internal hostname.
Service details can be read by querying the base64-encoded PLATFORM_RELATIONSHIPS environmental variable, or by using a config reader helper module for the language your app is written in.
For more information on this feature, please see the following page in the Platform.sh documentation: