Taking Advantage of Sophisticated Configuration

Remote configuration

Configuration files are powerful, but they are not a panacea. Consider distributed systems where the code is possibly running in the cloud inside containers. There may be multiple instances of the program running at the same time. Updating a configuration file inside multiple containers at the same time is not a simple thing. In many cases, it requires a new deployment, which requires running a lot of tests and possibly even some temporary down time. In practice, there is almost no difference between changing the code and changing a configuration file.

Remote configuration is an approach for managing a configuration in a distributed system. It addresses many of the problems associated with using configuration files. The concept is that the configuration data is stored in a central highly available store. You could roll your own using a database or a shared file system, but it is probably in your best interest to use an existing distributed configuration store. A distributed configuration store has an API or client libraries you can use from anywhere to get and set configuration information with secure access.

The benefits of a single store are:

• It is centrally maintained
• It provides a solid foundation to solve hard problems like synchronization, rolling updates, and versioning.

Because the configuration store is a critical component of the system, you have to make sure it is sufficiently available and redundant.

Configuration defaults

Arguably the best configuration is no configuration. If you’re lucky, most of your configuration will have reasonable defaults, and in most cases, your users will not have to specify any configuration.

This often takes a lot of thinking and understanding of your users. Some applications have different types of users and there is no one default that works for everyone.

Combining multiple configuration sources

Programs often employ multiple forms of configuration for the same program. It can be done in an overlapping manner, meaning the same configuration options are available as a command-line argument, an environment variable, or a configuration file. There is a hierarchy where the default configuration is at the bottom. If the user doesn’t provide any other configuration, the default will be used. Then comes the configuration file, but which one? This file can be overruled by environment variables, which can be overruled again by command-line arguments that take precedence over all other forms of configuration.

The other approach is to provide some configuration via a configuration file and some other configuration via environment variables or command-line flags. It is very common to have a default path for the configuration file. We can also override it using an environment variable or command-line flag. For example, kubectl - which is the Kubernetes CLI, has a default config file in ~/.kube/config, but you can override it using the KUBECONFIG environment variable or the --kubeconfig command-line flag.

I have two local clusters: k3d-k3s-default and minikube.

When I check the list of nodes I get the nodes from the k3d-k3s-default as this is the current context in my ~/.kube/config file:

$ kubectl config current-context
k3d-k3s-default

$ kubectl nodes
NAME                       STATUS   ROLES    AGE   VERSION
k3d-k3s-default-agent-0    Ready    <none>   15d   v1.18.3+k3s1
k3d-k3s-default-agent-2    Ready    <none>   15d   v1.18.3+k3s1
k3d-k3s-default-server-0   Ready    master   15d   v1.18.3+k3s1
k3d-k3s-default-agent-1    Ready    <none>   15d   v1.18.3+k3s1

However, I get override it by passing the minikube context as a command-line argument, and I get the single node of the minikube cluster:

$ kubectl get nodes --context minikube
NAME       STATUS   ROLES    AGE   VERSION
minikube   Ready    master   78d   v1.18.3

You can also change the current context in the ~/.kube/config or even provide an alternative configuration file, but this is not a Kubernetes course.

You can explore kubectl in the terminal below.