Many services have a responsibility to inform the application when something has gone wrong—for example, that contact with the server has been lost and data wasn’t able to be saved. If your service contains its own data paths it can be difficult to test that it behaves correctly in the face of an IOError, but if you inject your paths as dependencies you can configure your test service with a bad url very easily.

This example comes from a different application requiring a connection to a remote server in order to submit usage statistics: