This scenario explains how to launch a simple, multi-tier web application using Kubernetes and Docker. The Guestbook example application stores notes from guests in Redis via JavaScript API calls. Redis contains a master (for storage), and a replicated set of redis ‘slaves‘.
The following core concepts will be covered during this scenario. These are the foundations of understanding Kubernetes.
Pods
Replication Controllers
Services
NodePorts
To start we need a running Kubernetes cluster. The details of this are explained in the Launch Kubernetes cluster scenario.
Start a single-node cluster using the helper script. The helper script will launch the API, Master, a Proxy and DNS discovery. The Web App uses DNS Discovery to find the Redis slave to store data.
launch.shHealth Check
Check everything is up using the following health Check:
kubectl cluster-infokubectl get nodes
If the node returns NotReady then it is still waiting. Wait a couple of seconds before retrying.
The first stage of launching the application is to start the Redis Master. A Kubernetes service deployment has, at least, two parts. A replication controller and a service.
The replication controller defines how many instances should be running, the Docker Image to use, and a name to identify the service. Additional options can be utilized for configuration and discovery. Use the editor above to view the YAML definition.
If Redis were to go down, the replication controller would restart it on an active node.
In this example, the YAML defines a redis server called redis-master using the official redis running port 6379.
The kubectl create command takes a YAML definition and instructs the master to start the controller.
kubectl create -f redis-master-controller.yaml
What‘s running?
The above command created a Replication Controller. The Replication
kubectl get rc
All containers described as Pods. A pod is a collection of containers that makes up a particular application, for example Redis. You can view this using kubectl
kubectl get pods
The second part is a service. A Kubernetes service is a named load balancer that proxies traffic to one or more containers. The proxy works even if the containers are on different nodes.
Services proxy communicate within the cluster and rarely expose ports to an outside interface.
When you launch a service it looks like you cannot connect using curl or netcat unless you start it as part of Kubernetes. The recommended approach is to have a LoadBalancer service to handle external communications.
The YAML defines the name of the replication controller, redis-master, and the ports which should be proxied.
kubectl create -f redis-master-service.yaml
List & Describe Services
kubectl get serviceskubectl describe services redis-masterIn this example we‘ll be running Redis Slaves which will have replicated data from the master. More details of Redis replication can be found at http://redis.io/topics/replication
As previously described, the controller defines how the service runs. In this example we need to determine how the service discovers the other pods. The YAML represents the GET_HOSTS_FROM property as DNS. You can change it to use Environment variables in the yaml but this introduces creation-order dependencies as the service needs to be running for the environment variable to be defined.
In this case, we‘ll be launching two instances of the pod using the image kubernetes/redis-slave:v2. It will link to redis-mastervia DNS.
kubectl create -f redis-slave-controller.yaml
List Replication Controllers
kubectl get rc
As before we need to make our slaves accessible to incoming requests. This is done by starting a service which knows how to communicate with redis-slave.
Because we have two replicated pods the service will also provide load balancing between the two nodes.
kubectl create -f redis-slave-service.yamlkubectl get servicesWith the data services started we can now deploy the web application. The pattern of deploying a web application is the same as the pods we‘ve deployed before.
The YAML defines a service called frontend that uses the image _gcr.io/googlesamples/gb-frontend:v3. The replication controller will ensure that three pods will always exist.
kubectl create -f frontend-controller.yaml
List controllers and pods
kubectl get rckubectl get podsThe PHP code uses HTTP and JSON to communicate with Redis. When setting a value requests go to redis-master while read data comes from the redis-slave nodes.
To make the frontend accessible we need to start a service to configure the proxy.
The YAML defines the service as a NodePort. NodePort allows you to set well-known ports that are shared across your entire cluster. This is like -p 80:80 in Docker.
In this case, we define our web app is running on port 80 but we‘ll expose the service on 30080.
kubectl create -f frontend-service.yamlkubectl get servicesWith all controllers and services defined Kubernetes will start launching them as Pods. A pod can have different states depending on what‘s happening. For example, if the Docker Image is still being downloaded then the Pod will have a pendingstate as it‘s not able to launch. Once ready the status will change to running.
You can view the status using the following command:
kubectl get pods
Find NodePort
If you didn‘t assign a well-known NodePort then Kubernetes will assign an available port randomly. You can find the assigned NodePort using kubectl.
kubectl describe service frontend | grep NodePortView UI
Once the Pod is in running state you will be able to view the UI via port 30080. Use the URL to view the pagehttps://2886795360-30080-ollie02.environments.katacoda.com
Under the covers the PHP service is discovering the Redis instances via DNS. You now have a working multi-tier application deployed on Kubernetes.
We‘ll discuss NodePort in future scenarios.
