Application Configuration

A limitation and its solution

Before we discuss upgrading production systems, we should cover application configuration. Most Elixir developers use config/config.exs to configure their applications and dependencies, but remember that Mix loads config/config.exs when building the release. That means environment variables or configuration files we read in config.exs will be available while we build our release, but not in production.

To work around this limitation, many projects like Ecto and Phoenix started to support a special value called {:system, "env"} to allow some dynamic configuration. The problem with this solution is that it works only for certain keys and only for certain applications. Fortunately, Ecto v2.1 and Phoenix v1.3 are moving to a more standardized approach. They’ll both move runtime configuration to inside the init callback. Take the following example.

In earlier Ecto versions, if we wanted to dynamically configure the database URL Ecto connects to, we’d do so in our config/config.exs, like this: