kustomize is a tool designed to let users “customize raw, template-free YAML files for multiple purposes, leaving the original YAML untouched and usable as is” (wording taken directly from the kustomize GitHub repository). Users can run kustomize directly, or—starting with Kubernetes 1.14—use kubectl -k to access the functionality (although the standalone binary is newer than the functionality built into kubectl as of the Kubernetes 1.15 release). In this post, I’d like to provide an introduction to kustomize.

In its simplest form/usage, kustomize is simply a set of resources (these would be YAML files that define Kubernetes objects like Deployments, Services, etc.) plus a set of instructions on the changes to be made to these resources. Similar to the way make leverages a file named Makefile to define its function or the way Docker uses a Dockerfile to build a container, kustomize uses a file named kustomization.yaml to store the instructions on the changes the user wants made to a set of resources.

Here’s a simple kustomization.yaml file:

resources:
- deployment.yaml
- service.yaml
namePrefix: dev-
namespace: development
commonLabels: environment: development

This article won’t attempt to explain all the various fields that could be present in a kustomization.yaml file (that’s well handled here), but here’s a quick explanation of this particular example:

• The resources field specifies which things (resources) kustomize will modify. In this case, it will look for resources inside the deployment.yaml and service.yaml files in the same directory (full or relative paths can be specified as needed here).
• The namePrefix field instructs kustomize to prefix the name attribute of all resources defined in the resources field with the specified value (in this case, “dev-”). So, if the Deployment specified a name of “nginx-deployment”, then kustomize would change the value to “dev-nginx-deployment”.
• The namespace field instructs kustomize to add a namespace value to all resources. In this case, the Deployment and the Service are modified to be placed into the “development” namespace.
• Finally, the commonLabels field includes a set of labels that will be added to all resources. In this example, kustomize will label the resources with the label name “environment” and a value of “development”.

When a user runs kustomize build . in the directory with the kustomization.yaml and the referenced resources (the files deployment.yaml and service.yaml), the output is the customized text with the changes found in the kustomization.yaml file. Users can redirect the output if they want to capture the changes:

kustomize build . > custom-config.yaml

The output is deterministic (given the same inputs, the output will always be the same), so it may not be necessary to capture the output in a file. Instead, users could pipe the output into another command:

kustomize build . | kubectl apply -f -

Users can also invoke kustomize functionality with kubectl -k (as of Kubernetes 1.14). However, be aware that the standalone kustomize binary is more recent than the functionality bundled into kubectl (as of the Kubernetes 1.15 release).

Readers may be thinking, “Why go through this trouble instead of just editing the files directly?” That’s a fair question. In this example, users could modify the deployment.yaml and service.yaml files directly, but what if the files were a fork of someone else’s content? Modifying the files directly makes it difficult, if not impossible, to rebase the fork when changes are made to the origin/source. However, using kustomize allows users to centralize those changes in the kustomization.yaml file, leaving the original files untouched and thereby facilitating the ability to rebase the source files if needed.

The benefits of kustomize become more apparent in more complex kustomize use cases. In the example shown above, the kustomization.yaml and the resources are in the same directory. However, kustomize supports use cases where there is a “base configuration” and multiple “variants”, also known as overlays. Say a user wanted to take this simple Nginx Deployment and Service I’ve been using as an example and create development, staging, and production versions (or variants) of those files. Using overlays with shared base resources would accomplish this.

To help illustrate the idea of overlays with base resources, let’s assume the following directory structure:

- base - deployment.yaml - service.yaml - kustomization.yaml
- overlays - dev - kustomization.yaml - staging - kustomization.yaml - prod - kustomization.yaml

In the base/kustomization.yaml file, users would simply declare the resources that should be included by kustomize using the resources field.

In each of the overlays/{dev,staging,prod}/kustomization.yaml files, users would reference the base configuration in the resources field, and then specify the particular changes for that environment. For example, the overlays/dev/kustomization.yaml file might look like the example shown earlier:

resources:
- ../../base
namePrefix: dev-
namespace: development
commonLabels: environment: development

However, the overlays/prod/kustomization.yaml file could look very different:

resources:
- ../../base
namePrefix: prod-
namespace: production
commonLabels: environment: production sre-team: blue

When a user runs kustomize build . in the overlays/dev directory, kustomize will generate a development variant. However, when a user runs kustomize build . in the overlays/prod directory, a production variant is generated. All without any changes to the original (base) files, and all in a declarative and deterministic way. Users can commit the base configuration and the overlay directories into source control, knowing that repeatable configurations can be generated from the files in source control.

There’s a lot more to kustomize that what I’ve touched upon in this post, but hopefully this gives enough of an introduction to get folks started.

Additional Resources

There are quite a few good articles and posts written about kustomize; here are a few that I found helpful:

Change base YAML config for different environments prod/test using Kustomize

Kustomize – The right way to do templating in Kubernetes

Declarative Management of Kubernetes Objects Using Kustomize

Customizing Upstream Helm Charts with Kustomize

If anyone has questions or suggestions for improving this post, I’m always open to reader feedback. Feel free to contact me via Twitter, or hit me up on the Kubernetes Slack instance. Have fun customizing your manifests with kustomize!