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:
- A pod that runs the container.
- A service that exposes the pod internally and gives it a predictable name.
- A route that will expose the service to the internet by redirecting
<myservice>.rahtiapp.fito the service object.
In practice, you should not deploy applications the way described in this tutorial. Instead, it is meant for learning the core concepts of Kubernetes.
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.
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://rahti.csc.fi:8443".
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 my-project-with-unique-name
The name of the project needs to be unique across the Rahti 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, 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
oc new-project my-project-with-unique-name --description='csc_project: #######'
Switch between projects using the command
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:
apiVersion: v1 kind: Pod metadata: name: mypod labels: app: serveapp pool: servepod spec: containers: - name: serve-cont image: "docker-registry.default.svc:5000/openshift/httpd"
This pod will run one container image specified in the field
The name of the pod is provided in
metadata.name. The pod can be referred to
oc get pods mypod
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
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 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:
apiVersion: v1 kind: Pod metadata: name: mypod labels: app: serveapp pool: servepod spec: containers: - name: serve-cont image: "docker-registry.default.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.
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:
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
protocol defaults it to TCP, and omitting
targetPort defaults to the
Let us ensure that the service actually works by launching a remote shell in the
container running in the pod
mypod and pinging the service:
$ oc rsh mypod sh-4.2$ ping serve PING serve.my-project-with-unique-name.svc.cluster.local (172.30.39.82) 56(84) bytes of data.
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.
apiVersion: 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: <myservice>.rahtiapp.fi to: kind: Service name: serve weight: 100
This route redirects traffic from the internet to the service in the cluster
This particular route also allows traffic only from the subnet
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
If the whitelist entry is malformed, OpenShift will discard the whitelist and allow all traffic.
By default, the hostname is
- + project name
.rahtiapp.fi unless otherwise specified in
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.
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
kind are missing.
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: "docker-registry.default.svc:5000/openshift/httpd"
The ReplicationControllers are functionally close to ReplicaSets, discussed
in the chapter "Kubernetes and OpenShift
A ReplicationController can be transformed into a ReplicaSet by
spec.selector.matchLabels and setting
kind: ReplicaSet. The motivation to understand the ReplicationController
object is that DeploymentConfig
objects generate ReplicationControllers.
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
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.
Once we are satisfied with the application, let us not keep it running in the
cluster but remove it with the command
oc delete all --selector app=serveapp
This will delete all objects with the label
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 edited Mon Jun 13 2022