Getting Started with Kubernetes on CentOS 7


Kubernetes is an open-source platform developed by Google for managing containerized applications across a cluster of servers. It builds upon a decade and a half of experience that Google has with running clusters of containers at scale, and provides developers with Google style infrastructure, leveraging on best-of-breed open-source projects, such as:

  • Docker: an application container technology.
  • Etcd: a distributed key-value datastore that manages cluster-wide information and provides service discovery.
  • Flannel: an overlay network fabric enabling container connectivity across multiple servers.

Kubernetes lets developers define their application infrastructure declaratively through YAML files and abstractions such as Pods, RCs and Services (more on this later) and ensures that the underlying cluster matches the user defined state at all times.

Some of its features include:

  • Automatic scheduling of system resources and auto placement of application containers across a cluster.
  • Scaling applications on the fly with a single command.
  • Rolling updates with zero downtime.
  • Self healing: auto rescheduling of an application if a server fails, auto restart of containers, health checks.

Skip ahead to Installation if you’re already familiar with Kubernetes.

Basic concepts

Kubernetes offers the following abstractions (logical units) to developers:

  • Pods.
  • Replication controllers.
  • Labels.
  • Services.

Pods

It’s the basic unit of Kubernetes workloads. A pod models an application-specific “logical host” in a containerized environment. In layman terms, it models a group of applications or services that used to run on the same server in the pre-container world. Containers inside a pod share the same network namespace and can share data volumes as well.

Replication controllers

Pods are great for grouping multiple containers into logical application units, but they don’t offer replication or rescheduling in case of server failure.

This is where a replication controller or RC comes handy. A RC ensures that a number of pods of a given service is always running across the cluster.

Labels

They are key-value metadata that can be attached to any Kubernetes resource (pods, RCs, services, nodes, …).

Services

Pods and replication controllers are great for deploying and distributing applications across a cluster, but pods have ephemeral IPs that change upon rescheduling or container restart.

A Kubernetes service provides a stable endpoint (fixed virtual IP + port binding to the host servers) for a group of pods managed by a replication controller.

Kubernetes cluster

In its simplest form, a Kubernetes cluster is composed by two types of nodes:

  • 1 Kubernetes master.
  • N Kubernetes nodes.

Kubernetes master

The Kubernetes master is the control unit of the entire cluster.

The main components of the master are:

  • Etcd: a globally available datastore that stores information about the cluster and the services and applications running on the cluster.
  • Kube API server: this is main management hub of the Kubernetes cluster and it exposes a RESTful interface.
  • Controller manager: handles the replication of applications managed by replication controllers.
  • Scheduler: tracks resource utilization across the cluster and assigns workloads accordingly.

Kubernetes node

The Kubernetes node are worker servers that are responsible for running pods.

The main components of a node are:

  • Docker: a daemon that runs application containers defined in pods.
  • Kubelet: a control unit for pods in a local system.
  • Kube-proxy: a network proxy that ensures correct routing for Kubernetes services.

Installation

In this guide, we will create a 3 node cluster using CentOS 7 servers:

  • 1 Kubernetes master (kube-master)
  • 2 Kubernetes nodes (kube-node1, kube-node2)

You can add as many extra nodes as you want later on following the same installation procedure for Kubernetes nodes.

All nodes

Configure hostnames and /etc/hosts:

# /etc/hostnamekube-master# or kube-node1, kube-node2# append to /etc/hostsreplace-with-master-server-ip kube-masterreplace-with-node1-ip kube-node1replace-with-node2-ip kube-node2

Disable firewalld:

systemctl disable firewalldsystemctl stop firewalld

Kubernetes master

Install Kubernetes master packages:

yum install etcd kubernetes-master

Configuration:

# /etc/etcd/etcd.conf# leave rest of the lines unchangedETCD_LISTEN_CLIENT_URLS="http://0.0.0.0:2379"ETCD_LISTEN_PEER_URLS="http://localhost:2380"ETCD_ADVERTISE_CLIENT_URLS="http://0.0.0.0:2379"# /etc/kubernetes/config# leave rest of the lines unchangedKUBE_MASTER="--master=http://kube-master:8080"# /etc/kubernetes/apiserver# leave rest of the lines unchangedKUBE_API_ADDRESS="--address=0.0.0.0"KUBE_ETCD_SERVERS="--etcd_servers=http://kube-master:2379"

Start Etcd:

systemctl start etcd

Install and configure Flannel overlay network fabric (this is needed so that containers running on different servers can see each other):

yum install flannel

Create a Flannel configuration file (flannel-config.json):

{  "Network": "10.20.0.0/16",  "SubnetLen": 24,  "Backend": {    "Type": "vxlan",    "VNI": 1  }  }

Set the Flannel configuration in the Etcd server:

etcdctl set coreos.com/network/config < flannel-config.json

Point Flannel to the Etcd server:

# /etc/sysconfig/flanneldFLANNEL_ETCD="http://kube-master:2379"

Enable services so that they start on boot:

systemctl enable etcdsystemctl enable kube-apiserversystemctl enable kube-controller-managersystemctl enable kube-schedulersystemctl enable flanneld

Reboot server.

Kubernetes node

Install Kubernetes node packages:

yum install docker kubernetes-node

The next two steps will configure Docker to use overlayfs for better performance. For more information visit this blog post:

Delete the current docker storage directory:

systemctl stop dockerrm -rf /var/lib/docker

Change configuration files:

# /etc/sysconfig/docker# leave rest of lines unchangedOPTIONS='--selinux-enabled=false'# /etc/sysconfig/docker# leave rest of lines unchangedDOCKER_STORAGE_OPTIONS=-s overlay

Configure kube-node1 to use our previously configured master:

# /etc/kubernetes/config# leave rest of lines unchangedKUBE_MASTER="--master=http://kube-master:8080"# /etc/kubernetes/kubelet# leave rest of the lines unchangedKUBELET_ADDRESS="--address=0.0.0.0"# comment this line, so that the actual hostname is used to register the node# KUBELET_HOSTNAME="--hostname_override=127.0.0.1"KUBELET_API_SERVER="--api_servers=http://kube-master:8080"

Install and configure Flannel overlay network fabric (again – this is needed so that containers running on different servers can see each other):

yum install flannel

Point Flannel to the Etcd server:

# /etc/sysconfig/flanneldFLANNEL_ETCD="http://kube-master:2379"

Enable services:

systemctl enable dockersystemctl enable flanneldsystemctl enable kubeletsystemctl enable kube-proxy

Reboot the server.

Test your Kubernetes server

After all of the servers have rebooted, check if your Kubernetes cluster is operational:

[root@kube-master ~]# kubectl get nodesNAME         LABELS                              STATUSkube-node1   kubernetes.io/hostname=kube-node1   Readykube-node2   kubernetes.io/hostname=kube-node2   Ready

Example: Deploying a Selenium grid using Kubernetes

Selenium is a framework for automating browsers for testing purposes. It’s a powerful tool of the arsenal of any web developer.

Selenium grid enables scalable and parallel remote execution of tests across a cluster of Selenium nodes that are connected to a central Selenium hub.

Since Selenium nodes are stateless themselves and the amount of nodes we run is flexible, depending on our testing workloads, this is a perfect candidate application to be deployed on a Kubernetes cluster.

In the next section, we’ll deploy a grid consisting of 5 application containers:

  • 1 central Selenium hub that will be the remote endpoint to which our tests will connect.
  • 2 Selenium nodes running Firefox.
  • 2 Selenium nodes running Chrome.

Deployment strategy

To automatically manage replication and self-healing, we’ll create a Kubernetes replication controller for each type of application container we listed above.

To provide developers who are running tests with a stable Selenium hub endpoint, we’ll create a Kubernetes service connected to the hub replication controller.

Selenium hub

Replication controller
# selenium-hub-rc.yamlapiVersion: v1kind: ReplicationControllermetadata:  name: selenium-hubspec:  replicas: 1  selector:    name: selenium-hub  template:    metadata:      labels:        name: selenium-hub    spec:      containers:        - name: selenium-hub          image: selenium/hub          ports:            - containerPort: 4444

Deployment:

[root@kube-master ~]# kubectl create -f selenium-hub-rc.yamlreplicationcontrollers/selenium-hub[root@kube-master ~]# kubectl get rcCONTROLLER     CONTAINER(S)   IMAGE(S)       SELECTOR            REPLICASselenium-hub   selenium-hub   selenium/hub   name=selenium-hub   1[root@kube-master ~]# kubectl get podsNAME                 READY     STATUS    RESTARTS   AGEselenium-hub-pilc8   1/1       Running   0          50s[root@kube-master ~]# kubectl describe pod selenium-hub-pilc8Name:               selenium-hub-pilc8Namespace:          defaultImage(s):           selenium/hubNode:               kube-node2/45.63.16.92Labels:             name=selenium-hubStatus:             RunningReason:             Message:            IP:             10.20.101.2Replication Controllers:    selenium-hub (1/1 replicas created)Containers:  selenium-hub:    Image:      selenium/hub    State:      Running      Started:      Sat, 24 Oct 2015 16:01:39 +0000    Ready:      True    Restart Count:  0Conditions:  Type      Status  Ready     TrueEvents:  FirstSeen             LastSeen            Count   From            SubobjectPath               Reason      Message  Sat, 24 Oct 2015 16:01:02 +0000   Sat, 24 Oct 2015 16:01:02 +0000 1   {scheduler }                            scheduled   Successfully assigned selenium-hub-pilc8 to kube-node2  Sat, 24 Oct 2015 16:01:05 +0000   Sat, 24 Oct 2015 16:01:05 +0000 1   {kubelet kube-node2}    implicitly required container POD   pulled      Successfully pulled Pod container image "gcr.io/google_containers/pause:0.8.0"  Sat, 24 Oct 2015 16:01:05 +0000   Sat, 24 Oct 2015 16:01:05 +0000 1   {kubelet kube-node2}    implicitly required container POD   created     Created with docker id 6de00106b19c  Sat, 24 Oct 2015 16:01:05 +0000   Sat, 24 Oct 2015 16:01:05 +0000 1   {kubelet kube-node2}    implicitly required container POD   started     Started with docker id 6de00106b19c  Sat, 24 Oct 2015 16:01:39 +0000   Sat, 24 Oct 2015 16:01:39 +0000 1   {kubelet kube-node2}    spec.containers     pulled      Successfully pulled image "selenium/hub"  Sat, 24 Oct 2015 16:01:39 +0000   Sat, 24 Oct 2015 16:01:39 +0000 1   {kubelet kube-node2}    spec.containers     created     Created with docker id 7583cc09268c  Sat, 24 Oct 2015 16:01:39 +0000   Sat, 24 Oct 2015 16:01:39 +0000 1   {kubelet kube-node2}    spec.containers     started     Started with docker id 7583cc09268c

Here we can see that Kubernetes has placed my selenium-hub container on kube-node2.

Service
# selenium-hub-service.yamlapiVersion: v1kind: Servicemetadata:  name: selenium-hubspec:  type: NodePort  ports:  - port: 4444    protocol: TCP    nodePort: 30000  selector:    name: selenium-hub

Deployment:

[root@kube-master ~]# kubectl create -f selenium-hub-service.yamlYou have exposed your service on an external port on all nodes in yourcluster.  If you want to expose this service to the external internet, you mayneed to set up firewall rules for the service port(s) (tcp:30000) to serve traffic.See http://releases.k8s.io/HEAD/docs/user-guide/services-firewalls.md for more details.services/selenium-hub[root@kube-master ~]# kubectl get servicesNAME           LABELS                                    SELECTOR            IP(S)           PORT(S)kubernetes     component=apiserver,provider=kubernetes   <none>              10.254.0.1      443/TCPselenium-hub   <none>                                    name=selenium-hub   10.254.124.73   4444/TCP

After deploying the service, it’ll be reachable from:

  • Any Kubernetes node, via the virtual IP 10.254.124.73 and the port 4444.
  • External networks, via any Kubernetes nodes’ public IPs, on the port 30000.

Getting Started with Kubernetes on CentOS 7Getting Started with Kubernetes on CentOS 7
(using the public IP of another Kubernetes node)

Selenium nodes

Firefox node replication controller:

# selenium-node-firefox-rc.yamlapiVersion: v1kind: ReplicationControllermetadata:  name: selenium-node-firefoxspec:  replicas: 2  selector:    name: selenium-node-firefox  template:    metadata:      labels:        name: selenium-node-firefox    spec:      containers:        - name: selenium-node-firefox          image: selenium/node-firefox          ports:            - containerPort: 5900          env:            - name: HUB_PORT_4444_TCP_ADDR              value: "replace_with_service_ip"            - name: HUB_PORT_4444_TCP_PORT              value: "4444"

Deployment:

Replace replace_with_service_ip in selenium-node-firefox-rc.yaml with the actual Selenium hub service IP, in this case 10.254.124.73.

[root@kube-master ~]# kubectl create -f selenium-node-firefox-rc.yamlreplicationcontrollers/selenium-node-firefox[root@kube-master ~]# kubectl get rcCONTROLLER              CONTAINER(S)            IMAGE(S)                SELECTOR                     REPLICASselenium-hub            selenium-hub            selenium/hub            name=selenium-hub            1selenium-node-firefox   selenium-node-firefox   selenium/node-firefox   name=selenium-node-firefox   2[root@kube-master ~]# kubectl get podsNAME                          READY     STATUS    RESTARTS   AGEselenium-hub-pilc8            1/1       Running   1          1hselenium-node-firefox-lc6qt   1/1       Running   0          2mselenium-node-firefox-y9qjp   1/1       Running   0          2m[root@kube-master ~]# kubectl describe pod selenium-node-firefox-lc6qtName:               selenium-node-firefox-lc6qtNamespace:          defaultImage(s):           selenium/node-firefoxNode:               kube-node2/45.63.16.92Labels:             name=selenium-node-firefoxStatus:             RunningReason:             Message:            IP:             10.20.101.3Replication Controllers:    selenium-node-firefox (2/2 replicas created)Containers:  selenium-node-firefox:    Image:      selenium/node-firefox    State:      Running      Started:      Sat, 24 Oct 2015 17:08:37 +0000    Ready:      True    Restart Count:  0Conditions:  Type      Status  Ready     TrueEvents:  FirstSeen             LastSeen            Count   From            SubobjectPath               Reason      Message  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {scheduler }                            scheduled   Successfully assigned selenium-node-firefox-lc6qt to kube-node2  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {kubelet kube-node2}    implicitly required container POD   pulled      Pod container image "gcr.io/google_containers/pause:0.8.0" already present on machine  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {kubelet kube-node2}    implicitly required container POD   created     Created with docker id cdcb027c6548  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {kubelet kube-node2}    implicitly required container POD   started     Started with docker id cdcb027c6548  Sat, 24 Oct 2015 17:08:36 +0000   Sat, 24 Oct 2015 17:08:36 +0000 1   {kubelet kube-node2}    spec.containers pulled      Successfully pulled image "selenium/node-firefox"  Sat, 24 Oct 2015 17:08:36 +0000   Sat, 24 Oct 2015 17:08:36 +0000 1   {kubelet kube-node2}    spec.containers created     Created with docker id 8931b7f7a818  Sat, 24 Oct 2015 17:08:37 +0000   Sat, 24 Oct 2015 17:08:37 +0000 1   {kubelet kube-node2}    spec.containers started     Started with docker id 8931b7f7a818[root@kube-master ~]# kubectl describe pod selenium-node-firefox-y9qjpName:               selenium-node-firefox-y9qjpNamespace:          defaultImage(s):           selenium/node-firefoxNode:               kube-node1/185.92.221.67Labels:             name=selenium-node-firefoxStatus:             RunningReason:             Message:            IP:             10.20.92.3Replication Controllers:    selenium-node-firefox (2/2 replicas created)Containers:  selenium-node-firefox:    Image:      selenium/node-firefox    State:      Running      Started:      Sat, 24 Oct 2015 17:08:13 +0000    Ready:      True    Restart Count:  0Conditions:  Type      Status  Ready     TrueEvents:  FirstSeen             LastSeen            Count   From            SubobjectPath               Reason      Message  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {scheduler }                            scheduled   Successfully assigned selenium-node-firefox-y9qjp to kube-node1  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {kubelet kube-node1}    implicitly required container POD   pulled      Pod container image "gcr.io/google_containers/pause:0.8.0" already present on machine  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {kubelet kube-node1}    implicitly required container POD   created     Created with docker id ea272dd36bd5  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {kubelet kube-node1}    implicitly required container POD   started     Started with docker id ea272dd36bd5  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {kubelet kube-node1}    spec.containers created     Created with docker id 6edbd6b9861d  Sat, 24 Oct 2015 17:08:13 +0000   Sat, 24 Oct 2015 17:08:13 +0000 1   {kubelet kube-node1}    spec.containers started     Started with docker id 6edbd6b9861d

As we can see, Kubernetes has created 2 replicas of selenium-firefox-node and it has distributed them across the cluster. Pod selenium-node-firefox-lc6qt is on kube-node2, while pod selenium-node-firefox-y9qjp is on kube-node1.

We repeat the same process for our Selenium Chrome nodes.

Chrome node replication controller:

# selenium-node-chrome-rc.yamlapiVersion: v1kind: ReplicationControllermetadata:  name: selenium-node-chrome  labels:    app: selenium-node-chromespec:  replicas: 2  selector:    app: selenium-node-chrome  template:    metadata:      labels:        app: selenium-node-chrome    spec:      containers:      - name: selenium-node-chrome        image: selenium/node-chrome        ports:          - containerPort: 5900        env:          - name: HUB_PORT_4444_TCP_ADDR            value: "replace_with_service_ip"          - name: HUB_PORT_4444_TCP_PORT            value: "4444"

Deployment:

[root@kube-master ~]# kubectl create -f selenium-node-chrome-rc.yamlreplicationcontrollers/selenium-node-chrome[root@kube-master ~]# kubectl get rcCONTROLLER              CONTAINER(S)            IMAGE(S)                SELECTOR                     REPLICASselenium-hub            selenium-hub            selenium/hub            name=selenium-hub            1selenium-node-chrome    selenium-node-chrome    selenium/node-chrome    app=selenium-node-chrome     2selenium-node-firefox   selenium-node-firefox   selenium/node-firefox   name=selenium-node-firefox   2[root@kube-master ~]# kubectl get podsNAME                          READY     STATUS    RESTARTS   AGEselenium-hub-pilc8            1/1       Running   1          1hselenium-node-chrome-9u1ld    1/1       Running   0          1mselenium-node-chrome-mgi52    1/1       Running   0          1mselenium-node-firefox-lc6qt   1/1       Running   0          11mselenium-node-firefox-y9qjp   1/1       Running   0          11m

Wrapping up

In this guide, we’ve set up a small Kubernetes cluster of 3 servers (1 master controller + 2 workers).

Using pods, RCs and a service, we’ve successfully deployed a Selenium Grid consisting of a central hub and 4 nodes, enabling developers to run 4 concurrent Selenium tests at a time on the cluster.

Kubernetes automatically scheduled the containers across the entire cluster.

Getting Started with Kubernetes on CentOS 7

Self-healing

Kubernetes automatically reschedules pods to healthy servers if one or more of our servers go down.
In my example, kube-node2 is currently running the Selenium hub pod and 1 Selenium Firefox node pod.

[root@kube-node2 ~]# docker psCONTAINER ID        IMAGE                                  COMMAND                CREATED             STATUS              PORTS               NAMES5617399f146c        selenium/node-firefox                  "/opt/bin/entry_poin   5 minutes ago       Up 5 minutes                            k8s_selenium-node-firefox.46e635d8_selenium-node-firefox-zmj1r_default_31c89517-7a75-11e5-8648-5600001611e0_baae8e00   185230a3b431        gcr.io/google_containers/pause:0.8.0   "/pause"               5 minutes ago       Up 5 minutes                            k8s_POD.3805e8b7_selenium-node-firefox-zmj1r_default_31c89517-7a75-11e5-8648-5600001611e0_40f809df                     fdd5834c249d        selenium/hub                           "/opt/bin/entry_poin   About an hour ago   Up About an hour                        k8s_selenium-hub.cb8bf0ed_selenium-hub-pilc8_default_6c98c1ff-7a68-11e5-8648-5600001611e0_5765e2c9                     00e4ccb0bda8        gcr.io/google_containers/pause:0.8.0   "/pause"               About an hour ago   Up About an hour                        k8s_POD.3b3ee8b9_selenium-hub-pilc8_default_6c98c1ff-7a68-11e5-8648-5600001611e0_8398ac33  

We’ll simulate server failure by shutting down kube-node2. After a couple of minutes, you should see that the containers which were running on kube-node2 have been rescheduled to kube-node1, ensuring minimal disruption of service.

[root@kube-node1 ~]# docker psCONTAINER ID        IMAGE                                  COMMAND                CREATED             STATUS              PORTS               NAMES5bad5f582698        selenium/hub                           "/opt/bin/entry_poin   19 minutes ago      Up 19 minutes                           k8s_selenium-hub.cb8bf0ed_selenium-hub-hycf2_default_fe9057cf-7a76-11e5-8648-5600001611e0_ccaad50a                     dd1565a94919        selenium/node-firefox                  "/opt/bin/entry_poin   20 minutes ago      Up 20 minutes                           k8s_selenium-node-firefox.46e635d8_selenium-node-firefox-g28z5_default_fe932673-7a76-11e5-8648-5600001611e0_fc79f977   2be1a316aa47        gcr.io/google_containers/pause:0.8.0   "/pause"               20 minutes ago      Up 20 minutes                           k8s_POD.3805e8b7_selenium-node-firefox-g28z5_default_fe932673-7a76-11e5-8648-5600001611e0_dc204ad2                     da75a0242a9e        gcr.io/google_containers/pause:0.8.0   "/pause"               20 minutes ago      Up 20 minutes                           k8s_POD.3b3ee8b9_selenium-hub-hycf2_default_fe9057cf-7a76-11e5-8648-5600001611e0_1b10c0e7                              c611b68330de        selenium/node-firefox                  "/opt/bin/entry_poin   33 minutes ago      Up 33 minutes                           k8s_selenium-node-firefox.46e635d8_selenium-node-firefox-8ylo2_default_31c8a8f3-7a75-11e5-8648-5600001611e0_922af821   828031da6b3c        gcr.io/google_containers/pause:0.8.0   "/pause"               33 minutes ago      Up 33 minutes                           k8s_POD.3805e8b7_selenium-node-firefox-8ylo2_default_31c8a8f3-7a75-11e5-8648-5600001611e0_289cd555                     caf4e725512e        selenium/node-chrome                   "/opt/bin/entry_poin   46 minutes ago      Up 46 minutes                           k8s_selenium-node-chrome.362a34ee_selenium-node-chrome-mgi52_default_392a2647-7a73-11e5-8648-5600001611e0_3c6e855a     409a20770787        selenium/node-chrome                   "/opt/bin/entry_poin   46 minutes ago      Up 46 minutes                           k8s_selenium-node-chrome.362a34ee_selenium-node-chrome-9u1ld_default_392a15a4-7a73-11e5-8648-5600001611e0_ac3f0191     7e2d942422a5        gcr.io/google_containers/pause:0.8.0   "/pause"               47 minutes ago      Up 47 minutes                           k8s_POD.3805e8b7_selenium-node-chrome-9u1ld_default_392a15a4-7a73-11e5-8648-5600001611e0_f5858b73                      a3a65ea99a99        gcr.io/google_containers/pause:0.8.0   "/pause"               47 minutes ago      Up 47 minutes                           k8s_POD.3805e8b7_selenium-node-chrome-mgi52_default_392a2647-7a73-11e5-8648-5600001611e0_20a70ab6

Scaling your Selenium Grid

Scaling your Selenium Grid is super easy with Kubernetes. Imagine that instead of 2 Firefox nodes, I would like to run 4. The upscaling can be done with a single command:

[root@kube-master ~]# kubectl scale rc selenium-node-firefox --replicas=4scaled[root@kube-master ~]# kubectl get rcCONTROLLER              CONTAINER(S)            IMAGE(S)                SELECTOR                     REPLICASselenium-hub            selenium-hub            selenium/hub            name=selenium-hub            1selenium-node-chrome    selenium-node-chrome    selenium/node-chrome    app=selenium-node-chrome     2selenium-node-firefox   selenium-node-firefox   selenium/node-firefox   name=selenium-node-firefox   4[root@kube-master ~]# kubectl get podsNAME                          READY     STATUS    RESTARTS   AGEselenium-hub-pilc8            1/1       Running   1          1hselenium-node-chrome-9u1ld    1/1       Running   0          14mselenium-node-chrome-mgi52    1/1       Running   0          14mselenium-node-firefox-8ylo2   1/1       Running   0          40sselenium-node-firefox-lc6qt   1/1       Running   0          24mselenium-node-firefox-y9qjp   1/1       Running   0          24mselenium-node-firefox-zmj1r   1/1       Running   0          40s

Getting Started with Kubernetes on CentOS 7

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