---
title: Kustomize (optional)
sidebar_position: 40
---

[Kustomize](https://kustomize.io/) allows you to manage Kubernetes manifest files using declarative "kustomization" files. It provides the ability to express "base" manifests for your Kubernetes resources and then apply changes using composition, customization and easily making cross-cutting changes across many resources.

For example, take a look at the following manifest file for the `checkout` Deployment:

```file
manifests/base-application/checkout/deployment.yaml
```

This file has already been applied in the previous [Getting Started](../getting-started) lab, but let's say we wanted to scale this component horizontally by updating the `replicas` field using Kustomize. Rather than manually updating this YAML file, we'll use Kustomize to update the `spec/replicas` field from 1 to 3.

To do so, we'll apply the following kustomization.

* The first tab shows the kustomization we're applying
* The second tab shows a preview of what the updated `Deployment/checkout` file looks like after the kustomization is applied
* Finally, the third tab shows just the diff of what has changed

```kustomization
modules/introduction/kustomize/deployment.yaml
Deployment/checkout
```

You can generate the final Kubernetes YAML that applies this kustomization with the `kustomize` CLI:

```bash
$ kustomize build ~/environment/eks-workshop/modules/introduction/kustomize
```

This will generate a lot of YAML files, which represents the final manifests you can apply directly to Kubernetes. Let's demonstrate this by piping the output from `kustomize` directly to `kubectl`:

```bash
$ kustomize build ~/environment/eks-workshop/modules/introduction/kustomize | kubectl apply -f -
namespace/checkout unchanged
serviceaccount/checkout unchanged
configmap/checkout unchanged
service/checkout unchanged
service/checkout-redis unchanged
deployment.apps/checkout configured
deployment.apps/checkout-redis unchanged
```

You'll notice that a number of different `checkout`-related resources are "unchanged", with the `deployment.apps/checkout` being "configured". This is intentional — we only want to apply changes to the `checkout` deployment. This happens because running the previous command actually applied two files: the Kustomize `deployment.yaml` that we saw above, as well as the following `kustomization.yaml` file which matches all files in the `~/environment/eks-workshop/base-application/checkout` folder. The `patches` field specifies the specific file to be patched:

```file
manifests/modules/introduction/kustomize/kustomization.yaml
```

To check that the number of replicas has been updated, run the following command:

```bash
$ kubectl get pod -n checkout -l app.kubernetes.io/component=service
NAME                        READY   STATUS    RESTARTS   AGE
checkout-585c9b45c7-c456l   1/1     Running   0          2m12s
checkout-585c9b45c7-b2rrz   1/1     Running   0          2m12s
checkout-585c9b45c7-xmx2t   1/1     Running   0          40m
```

Although we have used the `kustomize` CLI directly in this section, Kustomize is also integrated directly with the `kubectl` CLI, and can be applied with `kubectl apply -k <kustomization_directory>`. This approach is used through this workshop to make it easier to apply changes to manifest files, while clearly surfacing the changes to be applied.

Let's try that:

```bash
$ kubectl apply -k ~/environment/eks-workshop/modules/introduction/kustomize
```

To reset the application manifests back to their initial state, you can simply apply the original set of manifests:

```bash timeout=300 wait=30
$ kubectl apply -k ~/environment/eks-workshop/base-application
```

Now that you understand how Kustomize works, proceed to the [Fundamentals module](/docs/fundamentals).

To learn more about Kustomize, you can refer to the official Kubernetes [documentation](https://kubernetes.io/docs/tasks/manage-kubernetes-objects/kustomization/).