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Introduction

In this tutorial, the Kubernetes core concepts pods, services, routes and ReplicationControllers and their YAML representations are discussed. In order to illustrate these Kubernetes API objects, an Apache HTTP server application is constructed by writing plain text YAML representations of these objects.

The objects minimally required defined in the cluster to have the server running:

  1. A pod that runs the container.
  2. A service that exposes the pod internally and gives it a predictable name.
  3. A route that will expose the service to the internet by redirecting traffic from <myservice>.2.rahtiapp.fi to the service object.

Info

In practice, you should not deploy applications the way described in this tutorial. Instead, it is meant for learning the core concepts of Kubernetes.

Network

Preparation

Make sure you have the oc command line installed, and that you are logged in. Please check the command line tool installation if you need help on that.

Projects

The command oc projects shows the projects you have access to:

$ oc projects
You have access to the following projects and can switch between them with 'oc
project <projectname>':

    someone-elses-public-project
  * my-project-with-unique-name

Using project "my-project-with-unique-name" on server "https://api.2.rahti.csc.fi:6443".

Info

The listing may include projects that other users have created to host public Docker images. While it is possible to switch to these projects, you only have read-only access to the Docker images hosted in them.

If there is no single suitable project, a new one can be created with the command oc new-project:

oc new-project my-project-with-unique-name

The name of the project needs to be unique across the Rahti 2 container cloud, and moreover, the name may only contain letters, digits and hyphen symbols, and it is not case sensitive. In essence, the name needs to be usable as part of a DNS name.

If you are a member of multiple CSC projects with access to Rahti 2, the description of the project must contain csc_project: #######, where ####### is the project that should be billed (see Projects and quota). The description can be included in the new-project command:

oc new-project my-project-with-unique-name --description='csc_project: #######'

Switch between projects using the command oc project:

oc project another-project

Pods and the command line interface

Pods are objects that run one or more containers. The containers in a pod share an IP address, and they can communicate via localhost or shared memory. Consequently, they need to be executed in a single physical node.

In our case, the pod will run a container image with a web server installed in it:

pod.yaml:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
  labels:
    app: serveapp
    pool: servepod
spec:
  containers:
  - name: serve-cont
    image: "image-registry.openshift-image-registry.svc:5000/openshift/httpd"

This pod will run one container image specified in the field spec.containers.image.

The name of the pod is provided in metadata.name. The pod can be referred to using oc:

oc get pods mypod

The field metadata.labels.pool is an arbitrary key-value pair that enables the pods to be grouped and referred by e.g. services.

The Kubernetes API objects are represented in the YAML format. Short introduction to YAML.

Pods and other Kubernetes/OpenShift API objects are created with the oc command line utility:

oc create -f pod.yaml

The pod should now appear in the "Overview" page in OpenShift's web console when the project is viewed.

Pods can be deleted using the command oc delete:

oc delete pod mypod

Consequently, the pod should disappear from the OpenShift web console, but let us keep this one running for now.

Resource requests and limits

Typically, you allocate resources to containers using requests and limits, but in these examples, we refrain from doing that for the sake of brevity. If no values are provided, default values will be used instead. The same pod as above with memory and CPU resources of 200 MB to 1 GB and 0,2 CPU to 1 CPU would read as:

pod.yaml:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
  labels:
    app: serveapp
    pool: servepod
spec:
  containers:
  - name: serve-cont
    image: "image-registry.openshift-image-registry.svc:5000/openshift/httpd"
    resources:
      requests:
        memory: "200M"
        cpu: "200m"
      limits:
        memory: "1G"
        cpu: "1"

Read more about requests and limits in the Kubernetes documentation.

Service

The IP addresses of pods are not consistent and may change if, for example, a pod is killed and recreated. Thus, in order to reliably access a pod, its IP address must be tracked and stored. Service objects do just that, and as a result, they provide a consistent network identity to pods:

service.yaml:

apiVersion: v1
kind: Service
metadata:
  name: serve
  labels:
    app: serveapp
spec:
  ports:
  - name: 8081-tcp
    port: 8081
    protocol: TCP
    targetPort: 8080
  selector:
    pool: servepod

This service will redirect TCP traffic internally from port 8081 in the project to the port 8080 of the pods having their labels listed in spec.selector. In this case, traffic is redirected to the pods with the label pool: servepod. If there are multiple pods matching spec.selector, then traffic is split between the pods. By default, splitting is done in a round-robin manner.

The only required field in the spec.ports field is port. Omitting protocol defaults it to TCP, and omitting targetPort defaults to the value of port.

Let us ensure that the service is defined by launching a remote shell in the container running in the pod mypod and querying the internal DNS service:

$ oc rsh mypod
sh-4.2$ nslookup serve
Server:     172.30.0.10
Address:    172.30.0.10#53

Name:   serve.test-sidecar.svc.cluster.local
Address: 172.30.103.178

Route

The route object is an OpenShift extension to Kubernetes that routes HTTP traffic from the internet (or whichever network the OpenShift cluster is connected to) to services in the OpenShift cluster.

route.yaml:

apiVersion: route.openshift.io/v1
kind: Route
metadata:
  labels:
    app: serveapp
  name: myservice
  annotations:
    haproxy.router.openshift.io/ip_whitelist: 192.168.1.0/24 10.0.0.1
spec:
  host: <myroute>.2.rahtiapp.fi
  to:
    kind: Service
    name: serve
    weight: 100

This route redirects traffic from the internet to the service in the cluster whose metadata.name equals spec.to.name.

  • You need to replace <myroute> with any value that is a valid DNS name (It is recommended is to use the project name).

In the particular route also allows traffic only from the subnet 192.168.1.0/24 and the IP 10.0.0.1. Security-wise, it is highly encouraged to utilize IP whitelisting for services that are not meant to be visible to the entire internet.

  • In order to be able to connect, you need to add (or replace) your own IP or a subnet mask that contains your IP. A second option is to delete the annotation altogether, this will allow anyone in the world to connect.

You can now go to your browser and type the address you set: <myservice>.2.rahtiapp.fi. It should return you the Apache test page:

Apache test page

Warning

If the whitelist entry is malformed, OpenShift will discard the whitelist and allow all traffic.

By default, the hostname is metadata.name + - + project name + .2.rahtiapp.fi unless otherwise specified in spec.host.

So far we have set up a pod, a service and a route. If the physical server where the pod lives gets shut down, you have to manually restart the pod using oc create -f pod.yaml. The ReplicationController and ReplicaSet objects are a mechanism that will, roughly speaking, do that for the user.

ReplicationController

A ReplicationController ensures that there are spec.replicas number of pods whose labels match the spec.selector running in the cluster. If there are too many, ReplicationController shuts down the extra ones, and if there are too few, it starts up pods according to the spec.template field. Actually, the template field is exactly the pod described in pod.yaml, except the fields apiVersion and kind are missing.

ReplicationController.yaml:

apiVersion: v1
kind: ReplicationController
metadata:
  labels:
    app: serveapp
  name: blogtest-replicator
spec:
  replicas: 1
  selector:
    app: serveapp
    pool: servepod
  template:
    metadata:
      name: mypod
      labels:
        app: serveapp
        pool: servepod
    spec:
      containers:
      - name: serve-cont
        image: "image-registry.openshift-image-registry.svc:5000/openshift/httpd"

The ReplicationControllers are functionally close to ReplicaSets, discussed in the chapter "Kubernetes and OpenShift concepts". A ReplicationController can be transformed into a ReplicaSet by changing spec.selector to spec.selector.matchLabels and setting kind: ReplicaSet. The motivation to understand the ReplicationController object is that DeploymentConfig objects generate ReplicationControllers.

Info

A central Kubernetes' concept coined reconciliation loop manifests in the ReplicationControllers. The reconciliation loop is a mechanism that measures the actual state of the system, constructs the current state based to the measurement of the system and performs such actions that the state of the system would equal to the desired state.

In such a terminology, ReplicationControllers are objects that describe the desired state of the cluster. Another such object is the service object encountered earlier. There, an another reconciliation loop compares the endpoints of the service to the actual pods that are ready and adjusts accordingly. As a result, the endpoints of the service always point to pods that are ready and only those pods whose labels contain all fields in the selector of the service object. In fact, every incidence of spec in a YAML representation of a Kubernetes object describes a specification for a reconciliation loop. The loops for pods just happen to be tied to the worker nodes of Kubernetes and are thus susceptible to deletion if, or when, the worker nodes are deprovisioned.

Cleaning up

Once we are satisfied with the application, let us not keep it running in the cluster but remove it with the command oc delete:

oc delete all --selector app=serveapp

This will delete all objects with the label app: serveapp.

Conclusion

In this tutorial, a static web page server was set up using YAML files representing the Kubernetes objects. The created objects can be further modified in the OpenShift web console where:

  • Routes can be modified to be secure ones encrypted by TLS.
  • Autoscalers, persistent storage, resource limits and health checks can be added to ReplicationControllers.
  • New routes can be added to services.
Last update: December 11, 2023