Hey there, Kubernetes fan.
When you’re working with Kubernetes, you’ll soon find that its declarative nature means that you’re going to apply a manifest to the cluster. A manifest describes an object that you want to exist in your cluster.
You write manifests for Kubernetes resources in JSON or YAML, and then use the Kubernetes API to apply to them to the cluster.
For me, the object I probably create most often is a Deployment, which has a rather large and complex spec.
In Kubernetes, a Deployment spec is a definition of a Deployment that you would like to exist in the cluster. It represents the state that the Deployment should have.
Writing these manifests manually is a bit of a slog. So sometimes it’s helpful to see what a real manifest looks like, so you can use it as starting point for your own.
And at the end of this article, I’ll show you some time-saving tools to use, so you don’t even need to write a manifest manually if you don’t want to.
So what’s the difference between a Kubernetes manifest and a spec?
A manifest is a JSON or YAML representation of an object that you want to exist in your Kubernetes cluster. Within the manifest, a field called spec describes the state that the object should have.
In this article I’ll show you a few real-world examples, so that you can see what a typical Deployment looks like. Hopefully this will give you a starting point for creating your own Kubernetes Deployment manifests.
Example Deployment YAMLs for Kubernetes
You can write manifests for Kubernetes in either JSON or YAML. I prefer YAML, because it’s less verbose and I don’t need to worry about missing brackets.
However the disavantage of YAML is that you need to pay special attention to your indenting!
How to convert these examples to JSON
If you prefer to write your Kubernetes manifests in JSON, you can convert these examples to JSON, using our YAML to JSON converter that runs in your web browser!
Or you can convert these examples locally with the
yq command line tool. Once you’ve installed it, you can convert a YAML file to JSON by piping it to
yq, like this:
cat deployment.yml | yq
And out will pop the JSON equivalent of your YAML file.
Deploying a simple application
This is a simple Pod spec, to begin. This Deployment creates replicas to run the Nginx web server. Each replica (a Pod) has just one container, which runs the Docker image
This will run the
nginx image from Docker Hub.
apiVersion: apps/v1 kind: Deployment metadata: name: nginx labels: app: nginx spec: selector: matchLabels: app: nginx template: metadata: labels: app: nginx spec: containers: - image: nginx name: nginx ports: - containerPort: 8080
Note how the labels in
selector.matchLabels match the labels in
matchLabels is a query that allows the Deployment to find and manage the Pods it creates.
This is a very basic example, and is fine for deploying an off-the-shelf container from Docker Hub. But what happens when you want to provide some runtime configuration to the application?
Configuring your application with environment variables
When you want to provide runtime configuration to your container in Kubernetes, you might use environment variables. Env vars are a well-known way of providing configuration to a container.
You can set env vars directly on a Deployment. But, you don’t have to hardcode their values, especially if you use env vars to set something environment-specific, like a database connection string, or a hostname.
Instead, you can populate the environment variables with values from a ConfigMap or Secret.
This example Deployment shows how to deploy an app which has environment variables taken from a ConfigMap and a Secret:
apiVersion: apps/v1 kind: Deployment metadata: name: postgres spec: selector: matchLabels: app: postgres template: metadata: labels: app: postgres spec: containers: - name: postgres image: postgres ports: - containerPort: 5432 env: - name: POSTGRES_DB value: mydatabase # an explicit env var value - name: POSTGRES_USER valueFrom: configMapKeyRef: # populate from a ConfigMap name: postgres-config # ... with this name key: my.username # ... and look for this key - name: POSTGRES_PASSWORD valueFrom: secretKeyRef: # populate from a Secret name: postgres-secret # ... with this name key: secret.password # ... and look for this key
Externalising configuration is a really common pattern – one of the 12 Factor App principles – and a great one to follow for most apps. It means that you can set and update the ConfigMap in the cluster separately, without updating the Deployment itself.
Then, once your app is deployed, you might be concerned about how you’re going to upgrade it when you want to roll out a new version.
Configuring zero-downtime upgrades for your app
Zero-downtime upgrades are a really cool feature of Kubernetes. The rolling update strategy allows you to do this.
In a rolling update, Kubernetes will upgrade your Pods to a new container image version, whilst ensuring that at least one instance (replica) of your application is available at all times, so your users don’t experience downtime.
To take advantage of this, you configure a rolling update strategy on the Deployment. You do this by setting
RollingUpdate. And you also set some parameters for the update, like whether Kubernetes can increase pods to higher than usual (this is called surging), to allow the upgrade to take place.
apiVersion: apps/v1 kind: Deployment metadata: name: nginx-rolling labels: app: nginx-rolling spec: replicas: 1 selector: matchLabels: app: nginx-rolling strategy: type: RollingUpdate # Upgrade this application with a rolling strategy rollingUpdate: maxSurge: 1 # maximum number of pods that can be scheduled # above the desired number of pods (replicas) maxUnavailable: 0 # the maximum number of pods that can be unavailable # during the update template: metadata: labels: app: nginx-rolling spec: containers: - image: nginx name: nginx ports: - containerPort: 8080
Putting multiple Deployments in the same file
If you want to put more than one Kubernetes manifest (like a Deployment, Service, etc) in a single file, you can separate each entry with three hyphens (
---). This is sometimes used if you want to keep all of your resources together; perhaps you want to track changes to a single file in Git.
Here’s an example of what I mean - I’m creating Deployments for Nginx and Postgresql, in the same YAML file. I separate the two manifests with the three hyphens:
apiVersion: apps/v1 kind: Deployment metadata: name: nginx labels: app: nginx spec: selector: matchLabels: app: nginx template: metadata: labels: app: nginx spec: containers: - image: nginx name: nginx ports: - containerPort: 8080 --- apiVersion: apps/v1 kind: Deployment metadata: name: postgres labels: app: postgres spec: selector: matchLabels: app: postgres template: metadata: labels: app: postgres spec: containers: - image: postgres name: postgres ports: - containerPort: 5432
This is a neat trick, and means you can distribute just one YAML file containing all of your application’s manifests, if you want.
You don’t need to memorise all this YAML
These examples have shown how to create Deployments which use some Kubernetes features, like configuration management and upgrades.
But, here some good news: you don’t need to memorise all this YAML, or write Kubernetes manifests manually. There are tools that can help you get there more quickly, like these:
kubectl, the Kubernetes CLI tool, can create a basic Deployment and apply it to the cluster. Just use
kubectl create deployment .... If you just want the YAML, use the
--dry-runoptions. Check the Help (
-h) for the options and examples.
Eclipse JKube - this plugin for Maven generates Kubernetes manifests for your Java applications. It creates Deployments, Services, and more.
Kubernetes YAML Generator - this web app from Octopus helps you generate a Deployment YAML in your browser. It covers almost every option imaginable!
You can always consult the official spec, using kubectl
Whenever you want to see the whole spec, and see all of the options that you can configure, you can get the full Deployment object specification with the
kubectl explain deployment and you’ll see the top-level structure of a Deployment:
$ kubectl explain deployment KIND: Deployment VERSION: apps/v1 DESCRIPTION: Deployment enables declarative updates for Pods and ReplicaSets. FIELDS: apiVersion <string> APIVersion defines the versioned schema of this representation of an object. Servers should convert recognized schemas to the latest internal value, and may reject unrecognized values. More info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#resources ...continues...
You can fetch further layers in the spec, like this:
kubectl explain deployment.spec. You can keep using this technique to drill right down into the spec.
Or, you can add the
--recursive option which will dump out an outline of the whole spec, e.g.
kubectl explain deployment.spec --recursive.
This is a really handy tip that I use all the time!
And, with that, I wish you happy Deployment spec-writing. ʕ ·͡ᴥ· ʔ
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