Kubernetes Patterns, 2nd Edition

Chapter 19. EnvVar Configuration

In this EnvVar Configuration pattern, we look into the simplest way to configure applications. For small sets of configuration values, the easiest way to externalize configuration is by putting them into universally supported environment variables. We’ll see different ways of declaring environment variables in Kubernetes but also the limitations of using environment variables for complex configurations.

Problem

Every nontrivial application needs some configuration for accessing data sources, external services, or production-level tuning. And we knew well before the twelve-factor app manifesto that it is a bad thing to hardcode configurations within the application. Instead, the configuration should be externalized so that we can change it even after the application has been built. That provides even more value for containerized applications that enable and promote sharing of immutable application artifacts. But how can this be done best in a containerized world?

Solution

The twelve-factor app manifesto recommends using environment variables for storing application configurations. This approach is simple and works for any environment and platform. Every operating system knows how to define environment variables and how to propagate them to applications, and every programming language also allows easy access to these environment variables. It is fair to claim that environment variables are universally applicable. When using environment variables, ...