k8s is an open source container cluster manager.
Google donated it under the Apache Commons license.
It's obejctive is to provide a platform for deploying, scaling and operations of application containers across a cluster of hosts.
Google uses it to run billions of containers a day on Google Cloud.
Components of Kubernetes
- Nodes
- Pods
- Labels
- Selectors
- Controllers
- Services
- Control Page
- API
We will end up with one Master/Controller which can have n Minions, which can have n pods which themselves can container n containers.
Minions (Nodes)
These are the physical/virtual container clients. They host the various containers within the cluster.
Each of them run ETCD (a key pair management and communication service) - it's how we can keep everything in sync and the Kubernetes Proxy.
Docker must be installed on each of these minions as well.
Pods
One or more containers. These containers are then guarenteed by the Master/Controller to be located on the same host machine in order to facilitate sharing of resources.
Pods are assigned unique IPs within each cluster.
Pods can container definitions of disk volumes or share, and then provide access from those to all the members (containers) within the pod.
Labels
Clients can attach "key-value pairs" to any object in the system (like Pods or Minions). The become the labels that identify them in the config and management of them.
Selectors
Label Selectors represent queries that are made against those labels. The resolve to the corresponding matching objects.
These two items are the primary way that grouping is done in Kubernetes and determine which components that a given operation applies to when indicated.
Controllers
These are used in the management of your cluster. Controllers are the mechanism by which your desired configuration state is enforced.
Controllers manage a set of pods and - depending on the desired configuration state - may engage other controllers to handle replication and scaling (Replication Controller) of X number of containers and pods across the cluster. it is also responsible for replacing any container in a pod that fails (based on the desired state of the cluster).
Other controllers that can be engaged include a DaemonSet Controller (enforces a 1 to 1 ratio of pods to minions) and a Job Controller (that runs pods to "completion" such as in batch jobs).
Each set of pods any controller manages is determined by the label selectors that are part of its definition.
Services
Services allow pods to work together eg multi-tiered application eg. web layer, db layer etc. are defined by the label selector.
Kubernetes can then provide service discovery and handle routing with the static IP for each pod as well as load balancing (round robin based) connections to that service among the pods that match the label selector indicated.
By default, although a service is only exposed inside a cluster, it can also be exposed outside a cluster as needed.
Yet Another Markup Language, although it moved to YAML Ain't Markup Language
It's a data serialisation format. It's designed to be easy to map to high level languages.
--- # Our Favourite Movies of All Time
- The Terminator
- Star Wars
It is highly dependent on the dashes and indentations.
Ensure that for your nodes etc and the control-master that you apt-get/yum install ntp
We use ntp for accurate reporting. Ensure for each node you use systemctl enable ntpd && sysmctl start ntpd.
To ensure that they can all communicate, edit the /etc/hosts file and paste in the IP addresses and names of the minions. That hosts will allow you to ping that minion name.
In the master/controller, vim /etc/yum.repos.d/virt7-docker-common-release.repo
[virt7-docker-common-release]
name=virt7-docker-common-release
baseurl=http://cbs.centos.org/repos/virt7-docker-common-release/x86_64/0s/
gpgcheck=0
Then hit yum update.
Ensure systemctl status [iptables|firewalld] are not running.
Now to ensure that we can share and communicate values, we will install `etcd` and `kubernetes`.
`yum install -y --enablerepo=virt7-docker-common-release kubernetes docker`
### ---- Install and Configure the Master Controller
For the master, we want to edit the Kubernetes config file.
__Master Kube Config__
cd /etc/kubernetes vim config
In the file, the `KUBE_LOTOSTDERR` we want one, `KUBE_LOG_LEBEL` can be set as 0 for debug, `KUBE_ALLOW_PRIV` can be left as false - it is a way to allow any docker container to run in our kube cluster, but we can learn about this later. `KUBE_MASTER` we want to change to ensure that it binds to an interface that we can communicate with. It will default at localhost, so for this example we would do something like `--master=http://centos-master:8080` where centos-master is what we set up in `/etc/hosts`
We will then add `KUBE_ETCD_SERVERS="--etcd-servers=http://centos-master:2379"`
__Setting up etcd__
cd /etc/etcd vim etcd.conf
In the file, we want to change `ETCD_LISTEN_CLIENT_URLS="http://0.0.0.0:2379"` and the same value in the cluster section for `ETCD_ADVERTISE_CLIENT_URLS`
__Editing the Kubernetes APIServer___
cd /etc/kubernetes vim apiserver
Inside, we need to change to the following for the following fields:
KUBE_API_ADDRESS="--address=0.0.0.0" KUBE_API_PORT="--port=8080" KUBELET_PORT="--kubelet-port=10250"
// for the basic config, we can comment out the KUBE_ADMISSION_CONTROL
__systemctl enable___
Finally, we want to enable the following with the command:
systemctl enable etcd kube-apiserver kube-controller-manager kube-scheduler systemctl start etcd kube-apiserver kube-controller-manager kube-scheduler systemctl status etcd kube-apiserver kube-controller-manager kube-scheduler | grep"(running)" | wc -l
### ---- Install and Configure the Minions
At this stage, we should be able to see the master controller from the minions.
__Minion Kube Config__
KUBE_MASTER="--master=http://centos-master:8080"
// add the following KUBE_ETCD_SERVERS="--etcd-server=http://centos-master:2379"
__Kublet Config__
Edit the `/etc/kubernetes/kublet` file.
KUBELET_ADDRESS="--address=0.0.0.0"
KUBELET_PORT="--port=102050"
KUBELET_HOSTNAME="--hostname-override=centos-minion1"
KUBELET_API_SERVER="--api-servers=http://centos-master:8080"
// comment out pod_infra_container
Afterwards, ensure you run the following:
systemctl enable kube-proxy kubelet docker systemctl start kube-proxy kubelet docker systemctl status kube-proxy kubelet docker | grep "(running)" | wc-l
### ---- Exploring the Environment
We will use `kubectl` from the command line to control the cluster manager.
Main functions:
// list of registered nodes for the cluster kubectl get nodes
// to get help on it man kubectl-get
// how to get the ip addresses/info kubetrl describe nodes
// getting the json info kubectl get nodes -o jsonpath='{.items[*].status.addresses[?(@.type=="ExternalIP")].address}'
***
## Pods, Tags and Services
### ---- Create and Deploy Pod Definitions
Let's start running containers in pods in our cluster.
For configuration, you can use both `json` or `yaml` file format. From a definitions standpoint, it may be better for the current input configuration.
`Desired State` for our system is a key concept. It's the only responsibility of Kubernetes to match the defined `desired state`.
On the master node, `cd Builds` and inside that directory `vim nginx.yaml`
apiVersion: v1 kind: Pod metadata: name: nginx spec: containers: - name: nginx image: nginx:1.7.9 # this is not the latest - used for a reason ports: - containerPort: 80
If we now use `kubectl get pods`, we should get no pods that are running.
That will be because there are no containers running on any minions.
Run `kubectl create -f ./nginx.yaml` and we'll get a notification.
`kubectl get pods` should now return results and the minion should now have a container running!
__Describing a pod__
`kubectl describe pod nginx`
This will tell us a number of things including IDs assigned to the minion, labels if they are assigned, IP etc and info on the containers.
The events will also describe how the container has gone.
__Accessing the pod from master___
Can we ping that address? No. The reason is that we have no extenal route to that pod. What we can do is run a busy-box image. This will allow us to connect to or test our container.
// -t is not --tty for kubectl kubectl run busybox --image=busybox --restart=Never --tty -i --generator=run-pod/v1
// this will spin up the pod called busybox // we will then be in the command line for that pod // If we have this pod running within the minion // we will then have access to other pods on the // same environment wget -q0- http://172.1.0.2
To clean up the system, we can use `kubectl delete pod podName`
From the master, we can then see that the pod is no longer running.
We can also forward temporarily our ports to a remote copy. We can do this with port forward.
kubectl get pods kubectl create -f nginx.yaml kubectl get pods # will show the pods
// to forward ports // & means to run in the background // this will return a port above 34000 - otherwise we can specify kubectl port-forward nginx :80 & // from still in the master wget -q0- http://localhost:34853
### ---- Tags, Labels and Selectors
Create `nginx-pod-label.yaml`
apiVersion: v1 kind: Pod metadata: name: nginx labels: app: nginx spec: containers: - name: nginx image: nginx:1.7.9 # this is not the latest - used for a reason ports: - containerPort: 80
kubectl create -f nginx-pod-label.yaml kubectl get pods # will be running kubectl get -l app=nginx
If we copy that .yaml file and rename `nginx` to `nginx2`, we can get info just by searching for that name.
If we also do `kubectl decribe pod -l app=nginx2`, we'd just get the info for that name.
### ---- Deployment State
Create `nginx-deployment-prod.yaml`
This will make a number of changes. This will go from a simple pod definition to a deployment production set.
apiVersion: extensions/v1beta1 # this should now be in v1.3 kind: Deployment metadata: name: nginx-deployment-prod spec: replicas: 1 template: # this will be for the pod replicas metadata: labels: app: nginx-deployment-prod spec: containers: - name: nginx image: nginx:1.7.9 # this is not the latest - used for a reason ports: - containerPort: 80
kubectl create -f -nginx-deployent-prod.yaml kubectl get pods // this will now return the name + the id concatentated to the end! kubectl get deployments // this will now give us the nginx-deployment-prod with details
This seems like we're making it more complex than we need to be... but...
Create nginx-deployment-dev.yaml and just change everything to dev.
Again, create the .yaml kubectl. It will now show two deployments.
If we create nginx-deployment-dev-update.yaml and just change a few things. If we run kubectl apply -f nginx-deployment-dev-update.yaml and it will update by apply the contents of that to the name deployment cluster.
If we run kubectl describe deployments ..., we can get the details with things like the StrategyType etc.
Until now, we have been creating one or more pods either directly or using a file. However, there has only been one container in each of the pods that we've been working with.
We can use the replication controller to do more than one container in a pod. It will allow us to deploy 1 to n pods of a particular container.
kubectl get pods # we will initially have nothing
vim nginx-multi-label.yaml
nginx-multi-label.yaml
apiVersion: v1
kind: ReplicationController
metadata:
name: nginx-www
spec:
replicas: 3
select:
app: nginx
template:
metadata:
name: nginx
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx
ports:
- containerPort: 80
Ensure that if you're going to do this, start the kubelet and kube-proxy on the other nodes.
kubectl get nodes
# if everything is on, all three minions should report that they are ready
kubectl create -f nginx-multi-label.yaml
# replicationcontroller "nginx-www" created
kubectl get pods
# will show the pods
kubectrl describe replicationcontroller
# tells us 3 running and 3 pods
kubectl describe pods
# it will show all of our pods being happy!
kubectl get services
# gives us just the one service
If we delete a pod, we will still end up having a service! When we create a definition, that defines the expected state of the entire environment!
If there is an app error etc. the service will ensure that it gets back to the expected state.
Starting to put things together.
kubectl get replicationcontrollers
kubectl get pods
kubectl get nodes
Time to use the multicontainer configuration.
nginx-multi-label.yaml
apiVersion: v1
kind: ReplicationController
metadata:
name: nginx-www
spec:
replicas: 3
select:
app: nginx
template:
metadata:
name: nginx
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx
ports:
- containerPort: 80
kubectl create -f nginx-multi-label.yaml
Defining an nginx service
apiVersion: v1
kind: Service
metadata:
data: nginx-service
spec:
ports:
port: 8080
targetPort: 80
protocol: TCP
selector:
app: nginx
kubectl create -f nginx-service.yaml
Now, if we run kubectl get services, we will now have two services.
If we run kubectl describe service nginx-service, we can see info about and that it is a ClusterIP and that the endpoints are assigned to the minions with Kubernetes managing this.
So how do we connect?
kubectl run busybox --generator=run-pod/v1 --image=busybox --restart=Never --tty -i
Once it is running, we should be able to do wget -q0- http://10.254.197.123:8080
Now this idea is referring to a cluster of this IP. So now we've tied everything together to have a clustered referred to with just one IP address. We no longer even have to know the other IPs.