Architecture

The Kubernetes realm is the cluster, everything needed is contained within this cluster. Inside it, you will find two types of nodes: the Control Plane and the Worker Nodes.

The control plane is a centralized set of processes that manages the cluster resources, load balance, health, and more. A Kubernetes cluster usually has multiple controller nodes for availability and load balancing purposes. As a developer, you will most likely interact through the API gateway for interactions.

The worker node is any kind of host running a local Kubernetes agent Kubelet and a communication process Kube-Proxy. The former handles the operations commanded by the control plane on the local container runtime (e.g. docker), while the latter redirects connectivity to the right pods.

Namespaces

After some time, a Kubernetes cluster may become huge and heavily used. In order to keep things well organized, Kubernetes created the concept of Namespace. A namespace is basically a virtual cluster inside the actual cluster.

Most of the resources will be contained inside a namespace, thus unaware of resources from other namespaces. Only a few kinds of resources are completely agnostic of namespaces, and they define computational power or storage sources (i.e. Nodes and PersistentVolumes). However, access to those can be limited by namespace using Quotas.

Namespace-aware resources will always be contained in a namespace as Kubernetes creates and uses a namespace named default if nothing is specified.

There is no silver bullet on the way to use namespaces, as it widely depends on your organization and needs. However, we can note some usual namespaces usages:

1. Divide the cluster by team or project, to avoid naming conflict and help repartition of resources.

2. Divide the cluster by environment (i.e. dev, staging, prod), to keep a consistent architecture.

3. Deploy with more granularity (e.g. blue/green deployment), to quickly fall back on an untouched working environment in case of issue.

Further reading:

Namespace Documentation

Manage The Cluster Namespaces

Glossary

Kubernetes did a great work of remaining agnostic of any technology in their design. This means two things: handle multiple technologies under the hood and there is a whole new terminology to learn.

Fortunately, these concepts are pretty straightforward and can most of the time be compared to a unit element of classic system infrastructure. The table below will summarize the binding of the most basic concepts. The comparison might not be a hundred per cent accurate but rather here to help understand the need behind each concept.

Further reading:

Official Kubernetes Glossary

Official Concepts Documentation

Metadata and labels

The metadata entry is critical while creating any resource as it will enable Kubernetes and yourself to easily identify and select the resource.

In this entry, you will define a name and a namespace (defaults to default), thanks to which the control plane will automatically be able to tell if the file is a new addition to the cluster or the revision of a previously loaded file.

On top of those elements, you can define a labels section. It is composed of a set of key-value pairs to narrow down the context and content of your resource. Those labels can later be used in almost any CLI commands through Selectors. As those entries are not used in the core behavior of Kubernetes, you can use any name you want, even if Kubernetes defines some best practices recommendations.

Finally, you can also create an annotations section, which is almost identical to labels but not used by Kubernetes at all. Those can be used on the applicative side to trigger behaviors or simply add data to ease debugging.

# <metadata> narrows down selection and identify the resource
metadata:
  # The <name> entry is required and used to identify the resource
  name: my-resource
  namespace: my-namespace-or-default
  # <labels> is optional but often needed for resource selection
  labels:
    app: application-name
    category: back
  # <annotations> is optional and not needed for the configuration of Kubernetes
  annotations:
    version: 4.2

Further reading:

Naming and Identification

Labels and Selectors

Annotations

Data centric configuration files

Those files define key-value mappings that can be used later in other resources. Usually, those resources (i.e. Secrets and ConfigMap) are loaded before anything else, as it is more likely than not that your infrastructure files are dependent on them.

apiVersion: v1
# <kind> defines the resource described in this file
kind: ConfigMap
metadata:
  name: my-config
data:
  # <data> configures data to load
  configuration_key: "configuration_value"
  properties_entry: |
    # Any multiline content is accepted
    multiline_config=true

Infrastructure centric configuration files

Those files define the infrastructure to deploy on the cluster, potentially using content from the data files.

apiVersion: v1
# <kind> defines the resource described in this file
kind: Pod
metadata:
  name: my-web-server
spec:
  # <spec> is a domain specific description of the resource.
  # The specification entries will be very different from one kind to another

ConfigMap

ConfigMap is used to hold properties that can be used later in your resources.

apiVersion: v1
kind: ConfigMap
metadata:
  name: simple-web-config
  namespace: default
data:
  configuration_key: "Configuration value"

The configuration defined above can then be selected from another resource definition with the following snippet:

valueFrom:
  configMapKeyRef:
    name: simple-web-config
    key: configuration_key

Note: ConfigMaps are only available in the namespace in which they are defined.

Further reading:

ConfigMap Documentation

Secret

All sensitive data should be put in Secret files (e.g. API keys, passphrases, …). By default, the data is simply held as base64 encoded values without encryption. However, Kubernetes proposes ways of mitigating leakage risks by integrating a Role-Based Access Control or encrypting secrets.

The Secret file defines a type key at its root, which can be used to add validation on the keys declared in the data entry. By default, the type is set to Opaque which does not validate the entries at all.

apiVersion: v1
kind: Secret
metadata:
  name: simple-web-secrets
# Opaque <type> can hold generic secrets, so no validation will be done.
type: Opaque
data:
  # Secrets should be encoded in base64
  secret_configuration_key: "c2VjcmV0IHZhbHVl"

The secret defined above can then be selected from another resource definition with the following snippet:

valueFrom:
  secretKeyRef:
    name: simple-web-secrets
    key: secret_configuration_key

Note: Secrets are only available in the namespace in which they are defined.

Further reading:

Secrets Documentation

Available Secret Types

Pod

A Pod definition file is pretty straightforward but can become pretty big due to the quantity of configuration available. The name and image fields are the only mandatory ones, but you might commonly use:

• ports to define the ports to open on both the container and pod.

• env to define the environment variables to load on the container.

• args and entrypoint to customize the container startup sequence.

Pods are usually not created as standalone resources on Kubernetes, as the best practice indicates to use pod as part of higher level definition (e.g. Deployment). In those cases, the Pod file's content will simply be embedded in the other resource's file.

apiVersion: v1
kind: Pod
metadata:
  name: my-web-server
spec:
  # <containers> is a list of container definition to embed in the pod
  containers:
    - name: web
      image: nginx
      ports:
        - name: web
          containerPort: 80
          protocol: TCP
      env:
        - name: SOME_CONFIG
          # Create a line "value: <config_entry>" from the ConfigMap data
          valueFrom:
            configMapKeyRef:
              name: simple-web-config
              key: configuration_key
        - name: SOME_SECRET
          # Create a line "value: <config_entry>" from the Secret data
          valueFrom:
            secretKeyRef:
              name: simple-web-secrets
              key: secret_configuration_key

Note: Pods are only available in the namespace in which they are defined.

Further reading:

Pod Documentation

Advanced Pod Configuration

Fields available in Pod <spec> entry

Fields available in Pod <containers> entry

Deployment

The Deployment is generally used as the atomic working unit since it will automatically:

• Create a pod definition based on the template entry.

• Create a ReplicaSet on pods selected by the selector entry, with the value of replicas as a count of pods that should be running.

The following file requests 3 instances of an Nginx server running at all times. The file may look a bit heavy, but most of it is the Pod definition copied from above.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-web-server-deployment
  namespace: default
  labels:
    app: webserver
spec:
  # <selector> should retrieve the Pod defined below, and possibly more
  selector:
    matchLabels:
      app: webserver
      instance: nginx-ws-deployment
  # <replicas> asks for 3 pods running in parallel at all time
  replicas: 3
  # The content of <template> is a Pod definition file, without <apiVersion> nor <kind>
  template:
    metadata:
      name: my-web-server
      namespace: default
      labels:
        app: webserver
        instance: nginx-ws-deployment
    spec:
      containers:
        - name: web
          image: nginx
          ports:
            - name: web
              containerPort: 80
              protocol: TCP
          env:
            - name: SOME_CONFIG
              # Create a line "value: <config_entry>" from the ConfigMap data
              valueFrom:
                configMapKeyRef:
                  name: simple-web-config
                  key: configuration_key
            - name: SOME_SECRET
              # Create a line "value: <config_entry>" from the Secret data
              valueFrom:
                secretKeyRef:
                  name: simple-web-secrets
                  key: secret_configuration_key

Note: Deployments are only available in the namespace in which they are defined.

Further reading:

Deployment Documentation

Service

A pod might be deleted and recreated at any time. When it occurs the pod's IP address will change, which could result in a loss of connection if you are directly contacting it. To solve this issue, a Service provides a stable contact point to a set of Pods, while remaining agnostic of their state and configuration. Usually, Pods are chosen to be part of a Service through a selector entry, thus based on its labels. A Pod is selected if and only if all the labels in the selector are worn by the pod.

There are three types of services that are acting quite differently, among which you can select using the type entry.

The ClusterIP service is bound to an internal IP from the cluster, hence only internally reachable. This is the type of service created by default and is suitable for binding different applications inside the same cluster.

A NodePort service will bind a port (by default in range 30000 to 32767) on the nodes hosting the selected pods. This enables you to contact the service directly through the node IP. That also means that your service will be as accessible as the virtual or physical machines hosting those pods.

Note: Using NodePort can pose security risks, as it enables a direct connection from outside the cluster.

A LoadBalancer service will automatically create a load balancer instance from the cloud service provider on which the cluster is running. This load balancer is created outside the cluster but will automatically be bound to the nodes hosting the selected pods.

This is an easy way to expose your service but can end up being costly as each service will be managed by a single load balancer.

If you are setting up your own Ingress as we will do here, you may want to use a ClusterIp service, as other services are made for specific use cases.

apiVersion: v1
kind: Service
metadata:
  name: simple-web-service-clusterip
spec:
  # ClusterIP is the default service <type>
  type: ClusterIP
  # Select all pods declaring a <label> entry "app: webserver"
  selector:
    app: webserver
  ports:
    - name: http
      protocol: TCP
      # <port> is the port to bind on the service side
      port: 80
      # <targetPort> is the port to bind on the Pod side
      targetPort: 80

Note: Services are defined in a namespace but can be contacted from other namespaces.

Further reading:

Service Documentation

In Depth Service Comparison

Create an External Load Balancer

Ingress

Ingress enables you to publish internal services without necessarily using a load balancer from cloud service providers. You usually need only one ingress per namespace, where you can bind as many routing rules and backends as you want. A backend will typically be an internally routed ClusterIP service.

Please note that Kubernetes does not handle ingress resources by itself and relies on third-party implementations. As a result, you will have to choose and install an Ingress Controller before using any ingress resource. On the other hand, it makes the ingress resource customizable depending on the needs of your cluster.

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: simple-web-ingress
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /
spec:
  rules:
    # Using <host> redirects all request matching the given DNS name to this rule
    - host: "*.minikube.internal"
      http:
        paths:
          - path: /welcome
            pathType: Prefix
            backend:
              service:
                name: simple-web-service-clusterip
                port:
                  number: 80
    # All other requests will be redirected through this rule
    - http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: simple-web-service-clusterip
                port:
                  number: 80

Note: Ingresses are defined in the namespace but may contact services from other namespaces and are publicly accessible outside the cluster.

Further reading:

Ingress Documentation

Available Ingress Controllers

Enable Ingress on Minikube

Nginx Ingress Annotations

Create and manage resources

This section showcases the basic CLI commands to manipulate resources. As said before, while it is possible to manually manage resources, a better practice is to use files.

# <kind> is the type of resource to create (e.g. deployment, secret, namespace, quota, ...)
$ kubectl create <kind> <name>
$ kubectl edit   <kind> <name>
$ kubectl delete <kind> <name>

# All those commands can be used through a description file.
$ kubectl create -f <resource>.yaml
$ kubectl edit   -f <resource>.yaml
$ kubectl delete -f <resource>.yaml

To ease resources manipulations through files, you can reduce the interactions to the CLI to the two following commands:

# Create and update any resource
$ kubectl apply   -f <resource>.yaml
# Delete any resource
$ kubectl delete  -f <resource>.yaml

Further reading:

Managing Resources

Monitor and Debug

# Fetch everything
$ kubectl get all
NAME                                            READY   STATUS    RESTARTS   AGE
pod/my-web-server-deployment-58c4fd887f-5vm2b   1/1     Running   0          128m
pod/my-web-server-deployment-58c4fd887f-gq6lr   1/1     Running   0          128m
pod/my-web-server-deployment-58c4fd887f-gs6qb   1/1     Running   0          128m

NAME                                   TYPE           CLUSTER-IP       EXTERNAL-IP   PORT(S)                      AGE
service/simple-web-service-clusterip   ClusterIP      10.96.96.241     <none>        80/TCP,443/TCP               60m
service/simple-web-service-lb          LoadBalancer   10.108.182.232   <pending>     80:31095/TCP,443:31940/TCP   60m
service/simple-web-service-np          NodePort       10.101.77.203    <none>        80:31899/TCP,443:31522/TCP   60m

NAME                                       READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/my-web-server-deployment   3/3     3            3           136m

NAME                                                  DESIRED   CURRENT   READY   AGE
replicaset.apps/my-web-server-deployment-58c4fd887f   3         3         3       128m

# We can ask for more details
$ kubectl get deployment -o wide
NAME                       READY   UP-TO-DATE   AVAILABLE   AGE    CONTAINERS   IMAGES  SELECTOR
my-web-server-deployment   3/3     3            3           121m   web          nginx   app=webserver

# Some resources are not visible using "all" but available
$ kubectl get configmap
NAME                DATA   AGE
kube-root-ca.crt    1      38d
simple-web-config   3      3h17m

# Let's describe the ingress for the sake of example
$ kubectl describe ingress/simple-web-ingress
Name:             simple-web-ingress
Namespace:        default
Address:          192.168.64.2
Default backend:  default-http-backend:80 (<error: endpoints "default-http-backend" not found>)
Rules:
  Host                 Path  Backends
  ----                 ----  --------
  *.minikube.internal
                       /welcome   simple-web-service-clusterip:80 (172.17.0.4:80,172.17.0.5:80,172.17.0.6:80 + 1 more...)
  *
                       /   simple-web-service-clusterip:80 (172.17.0.4:80,172.17.0.5:80,172.17.0.6:80 + 1 more...)
Annotations:           nginx.ingress.kubernetes.io/rewrite-target: /
Events:
  Type    Reason  Age                 From                      Message
  ----    ------  ----                ----                      -------
  Normal  UPDATE  7m6s (x6 over 23h)  nginx-ingress-controller  Ingress default/simple-web-ingress

$ kubectl logs deployments/my-web-server-deployment
Found 3 pods, using pod/my-web-server-deployment-755b499f77-4n5vn
# [logs]

# As for logs, when called on any resource enclosing Pods,
# Kubernetes will randomly chose one to  execute the action
$ kubectl exec -it deployment/my-web-server-deployment -- /bin/bash
root@my-web-server-deployment-56c4554cf9-qwtm6:/# ls
# [...]

Resources' repository

The resources definitions used in this article are available in the following GitHub repository.

CLI equivalents - Docker and Kubernetes

Managing containers with Docker and pods with Kubernetes is very similar, as you can see on the following table describing equivalent operations between both technologies.

Thanks to Sarra Habchi, Dimitri Delabroye, and Alexis Geffroy for the reviews