A backing service is any service the app consumes over the network as part of its normal operation. Examples include datastores (such as MySQL or CouchDB), messaging/queueing systems (such as RabbitMQ or Beanstalkd), SMTP services for outbound email (such as Postfix), and caching systems (such as Memcached).

Some backing services like the database are usually managed locally (same server as the app); others are third party services, like SMTP or API-accessible consumer services (OpenAI, Twitter, etc…)

The code for a twelve-factor app makes no distinction between local and third party services. To the app, both are attached resources, accessed via a URL or other locator/credentials stored in the config. A deploy of the twelve-factor app should be able to swap out a local MySQL database with one managed by a third party without any changes to the app’s code.

Each distinct backing service is a resource. Resources can be attached to and detached from deploys at will.

References

• https://12factor.net/backing-services