GuestBook example
This example shows how to build a simple, multi-tier web application using Kubernetes and Docker.
The example consists of:
- A web frontend
- A redis master (for storage and a replicated set of redis slaves)
The web front end interacts with the redis master via javascript redis API calls.
Step Zero: Prerequisites
This example assumes that you have a basic understanding of kubernetes services and that you have forked the repository and turned up a Kubernetes cluster: This example requires a kubernetes cluster.
See the companion Setup Kubernetes for some quick notes on how to get started.
If you are running from source, replace commands such as "kubectl" below with calls to cluster/kubectl.sh.
Step One: Fire up the redis master
Note: This redis-master is not highly available. Making it highly available would be a very interesting, but intricate exersize - redis doesn't actually support multi-master deployments at the time of this writing, so high availability would be a somewhat tricky thing implement, and might involve periodic serialization to disk, and so on.
Use (or just create) the file examples/guestbook/redis-master.json which describes a single pod running a redis key-value server in a container:
{
"id": "redis-master",
"kind": "Pod",
"apiVersion": "v1beta1",
"desiredState": {
"manifest": {
"version": "v1beta1",
"id": "redis-master",
"containers": [{
"name": "master",
"image": "dockerfile/redis",
"cpu": 100,
"ports": [{
"containerPort": 6379, # containerPort: Where traffic to redis ultimately is routed to.
}]
}]
}
},
"labels": {
"name": "redis-master" # This label needed for when we start our redis-master service.
}
}
Now, create the redis pod in your Kubernetes cluster by running:
kubectl create -f examples/guestbook/redis-master.json
Once that's up you can list the pods in the cluster, to verify that the master is running:
kubectl get pods
You'll see all kubernetes components, most importantly the redis master pod. It will also display the machine that the pod is running on once it gets placed (may take up to thirty seconds):
NAME IMAGE(S) HOST LABELS STATUS
redis-master dockerfile/redis kubernetes-minion-2.c.myproject.internal/130.211.156.189 name=redis-master Running
If you ssh to that machine, you can run docker ps to see the actual pod:
me@workstation$ gcloud compute ssh --zone us-central1-b kubernetes-minion-2
me@kubernetes-minion-2:~$ sudo docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
e3eed3e5e6d1 dockerfile/redis:latest "redis-server /etc/re 2 minutes ago Up 2 minutes k8s_master.9c0a9146_redis-master.etcd_6296f4bd-70fa-11e4-8469-0800279696e1_45331ebc
(Note that initial docker pull may take a few minutes, depending on network conditions. You can monitor the status of this by running journalctl -f -u docker to check when the image is being downloaded. Of course, you can also run journalctl -f -u kubelet to see what state the kubelet is in as well during this time.
Step Two: Fire up the master service
A Kubernetes 'service' is a named load balancer that proxies traffic to one or more containers. This is done using the labels metadata which we defined in the redis-master pod above. As mentioned, in redis there is only one master, but we nevertheless still want to create a service for it. Why? Because it gives us a deterministic way to route to the single master using an elastic IP.
The services in a Kubernetes cluster are discoverable inside other containers via environment variables.
Services find the containers to load balance based on pod labels.
The pod that you created in Step One has the label name=redis-master. The selector field of the service determines which pods will receive the traffic sent to the service, and the port and containerPort information defines what port the service proxy will run at.
Use the file examples/guestbook/redis-master-service.json:
{
"id": "redis-master",
"kind": "Service",
"apiVersion": "v1beta1",
"port": 6379,
"containerPort": 6379,
"selector": {
"name": "redis-master"
},
"labels": {
"name": "redis-master"
}
}
to create the service by running:
$ kubectl create -f examples/guestbook/redis-master-service.json
redis-master
$ cluster/kubectl.sh get services
NAME LABELS SELECTOR IP PORT
kubernetes <none> component=apiserver,provider=kubernetes 10.0.29.11 443
kubernetes-ro <none> component=apiserver,provider=kubernetes 10.0.141.25 80
redis-master name=redis-master name=redis-master 10.0.16.143 6379
This will cause all pods to see the redis master apparently running on :6379. The traffic flow from slaves to masters can be described in two steps, like so.
- A redis slave will connect to "port" on the redis master service
- Traffic will be forwarded from the service "port" (on the service node) to the containerPort on the pod which (a node the service listens to).
Thus, once created, the service proxy on each minion is configured to set up a proxy on the specified port (in this case port 6379).
Step Three: Fire up the replicated slave pods
Although the redis master is a single pod, the redis read slaves are a 'replicated' pod. In Kubernetes, a replication controller is responsible for managing multiple instances of a replicated pod. The replicationController will automatically launch new Pods if the number of replicas falls (this is quite easy - and fun - to test, just kill the docker processes for your pods at will and watch them come back online on a new node shortly thereafter).
Use the file examples/guestbook/redis-slave-controller.json, which looks like this:
{
"id": "redis-slave-controller",
"kind": "ReplicationController",
"apiVersion": "v1beta1",
"desiredState": {
"replicas": 2,
"replicaSelector": {"name": "redisslave"},
"podTemplate": {
"desiredState": {
"manifest": {
"version": "v1beta1",
"id": "redis-slave-controller",
"containers": [{
"name": "slave",
"image": "brendanburns/redis-slave",
"cpu": 200,
"ports": [{"containerPort": 6379, "hostPort": 6380}]
}]
}
},
"labels": {
"name": "redisslave",
"uses": "redis-master"
}
}},
"labels": {"name": "redisslave"}
}
to create the replication controller by running:
$ kubectl create -f examples/guestbook/redis-slave-controller.json
redis-slave-controller
# kubectl.sh get replicationcontrollers
NAME IMAGE(S) SELECTOR REPLICAS
redis-slave-controller brendanburns/redis-slave name=redisslave 2
The redis slave configures itself by looking for the Kubernetes service environment variables in the container environment. In particular, the redis slave is started with the following command:
redis-server --slaveof ${REDIS_MASTER_SERVICE_HOST:-$SERVICE_HOST} $REDIS_MASTER_SERVICE_PORT
You might be curious about where the REDIS_MASTER_SERVICE_HOST is coming from. It is provided to this container when it is launched via the kubernetes services, which create environment variables (there is a well defined syntax for how service names get transformed to environment variable names in the documentation linked above).
Once that's up you can list the pods in the cluster, to verify that the master and slaves are running:
$ kubectl get pods
NAME IMAGE(S) HOST LABELS STATUS
redis-master dockerfile/redis kubernetes-minion-2.c.myproject.internal/130.211.156.189 name=redis-master Running
ee68394b-7fca-11e4-a220-42010af0a5f1 brendanburns/redis-slave kubernetes-minion-3.c.myproject.internal/130.211.179.212 name=redisslave,uses=redis-master Running
ee694768-7fca-11e4-a220-42010af0a5f1 brendanburns/redis-slave kubernetes-minion-4.c.myproject.internal/130.211.168.210 name=redisslave,uses=redis-master Running
You will see a single redis master pod and two redis slave pods.
Step Four: Create the redis slave service
Just like the master, we want to have a service to proxy connections to the read slaves. In this case, in addition to discovery, the slave service provides transparent load balancing to web app clients.
The service specification for the slaves is in examples/guestbook/redis-slave-service.json:
{
"id": "redisslave",
"kind": "Service",
"apiVersion": "v1beta1",
"port": 6379,
"containerPort": 6379,
"labels": {
"name": "redisslave"
},
"selector": {
"name": "redisslave"
}
}
This time the selector for the service is name=redisslave, because that identifies the pods running redis slaves. It may also be helpful to set labels on your service itself as we've done here to make it easy to locate them with the cluster/kubectl.sh get services -l "label=value" command.
Now that you have created the service specification, create it in your cluster by running:
$ cluster/kubectl.sh create -f examples/guestbook/redis-slave-service.json
redisslave
$ cluster/kubectl.sh get services
NAME LABELS SELECTOR IP PORT
kubernetes <none> component=apiserver,provider=kubernetes 10.0.29.11 443
kubernetes-ro <none> component=apiserver,provider=kubernetes 10.0.141.25 80
redis-master name=redis-master name=redis-master 10.0.16.143 6379
redisslave name=redisslave name=redisslave 10.0.217.148 6379
Step Five: Create the frontend pod
This is a simple PHP server that is configured to talk to either the slave or master services depending on whether the request is a read or a write. It exposes a simple AJAX interface, and serves an angular-based UX. Like the redis read slaves it is a replicated service instantiated by a replication controller.
It can now leverage writes to the load balancing redis-slaves, which can be highly replicated.
The pod is described in the file examples/guestbook/frontend-controller.json:
{
"id": "frontend-controller",
"kind": "ReplicationController",
"apiVersion": "v1beta1",
"desiredState": {
"replicas": 3,
"replicaSelector": {"name": "frontend"},
"podTemplate": {
"desiredState": {
"manifest": {
"version": "v1beta1",
"id": "frontend-controller",
"containers": [{
"name": "php-redis",
"image": "kubernetes/example-guestbook-php-redis",
"cpu": 100,
"memory": 50000000,
"ports": [{"containerPort": 80, "hostPort": 8000}]
}]
}
},
"labels": {
"name": "frontend",
"uses": "redisslave,redis-master"
}
}},
"labels": {"name": "frontend"}
}
Using this file, you can turn up your frontend with:
$ cluster/kubectl.sh create -f examples/guestbook/frontend-controller.json
frontend-controller
$ cluster/kubectl.sh get replicationcontrollers
NAME IMAGE(S) SELECTOR REPLICAS
redis-slave-controller brendanburns/redis-slave name=redisslave 2
frontend-controller kubernetes/example-guestbook-php-redis name=frontend 3
Once that's up (it may take ten to thirty seconds to create the pods) you can list the pods in the cluster, to verify that the master, slaves and frontends are running:
$ cluster/kubectl.sh get pods
NAME IMAGE(S) HOST LABELS STATUS
redis-master dockerfile/redis kubernetes-minion-2.c.myproject.internal/130.211.156.189 name=redis-master Running
ee68394b-7fca-11e4-a220-42010af0a5f1 brendanburns/redis-slave kubernetes-minion-3.c.myproject.internal/130.211.179.212 name=redisslave,uses=redis-master Running
ee694768-7fca-11e4-a220-42010af0a5f1 brendanburns/redis-slave kubernetes-minion-4.c.myproject.internal/130.211.168.210 name=redisslave,uses=redis-master Running
9fbad0d6-7fcb-11e4-a220-42010af0a5f1 kubernetes/example-guestbook-php-redis kubernetes-minion-1.c.myproject.internal/130.211.185.78 name=frontend,uses=redisslave,redis-master Running
9fbbf70e-7fcb-11e4-a220-42010af0a5f1 kubernetes/example-guestbook-php-redis kubernetes-minion-2.c.myproject.internal/130.211.156.189 name=frontend,uses=redisslave,redis-master Running
9fbdbeca-7fcb-11e4-a220-42010af0a5f1 kubernetes/example-guestbook-php-redis kubernetes-minion-4.c.myproject.internal/130.211.168.210 name=frontend,uses=redisslave,redis-master Running
You will see a single redis master pod, two redis slaves, and three frontend pods.
The code for the PHP service looks like this:
<?
set_include_path('.:/usr/share/php:/usr/share/pear:/vendor/predis');
error_reporting(E_ALL);
ini_set('display_errors', 1);
require 'predis/autoload.php';
if (isset($_GET['cmd']) === true) {
header('Content-Type: application/json');
if ($_GET['cmd'] == 'set') {
$client = new Predis\Client([
'scheme' => 'tcp',
'host' => getenv('REDIS_MASTER_SERVICE_HOST') ?: getenv('SERVICE_HOST'),
'port' => getenv('REDIS_MASTER_SERVICE_PORT'),
]);
$client->set($_GET['key'], $_GET['value']);
print('{"message": "Updated"}');
} else {
$read_port = getenv('REDIS_MASTER_SERVICE_PORT');
if (isset($_ENV['REDISSLAVE_SERVICE_PORT'])) {
$read_port = getenv('REDISSLAVE_SERVICE_PORT');
}
$client = new Predis\Client([
'scheme' => 'tcp',
'host' => getenv('REDIS_MASTER_SERVICE_HOST') ?: getenv('SERVICE_HOST'),
'port' => $read_port,
]);
$value = $client->get($_GET['key']);
print('{"data": "' . $value . '"}');
}
} else {
phpinfo();
} ?>
To play with the service itself, find the name of a frontend,
A few Google Container Engine specifics for playing around with the services.
- In GCE, you can grab the external IP of that host from the Google Cloud Console or the
gcloudtool, and visithttp://<host-ip>:8000.
$ gcloud compute instances list
In GCE, you also may need to open the firewall for port 8000 using the console or the gcloud tool. The following command will allow traffic from any source to instances tagged kubernetes-minion:
$ gcloud compute firewall-rules create --allow=tcp:8000 --target-tags=kubernetes-minion kubernetes-minion-8000
For GCE details about limiting traffic to specific sources, see the GCE firewall documentation.
In other environments, you can get the service IP from looking at the output of kubectl get pods,services, and modify your firewall using standard tools and services (firewalld, iptables, selinux) which you are already familar with.
And of course, finally, if you are running Kubernetes locally, you can just visit http://localhost:8000.
Step Six: Cleanup
To turn down a Kubernetes cluster, if you ran this from source, you can use
$ cluster/kube-down.sh
If you are in a live kubernetes cluster, you can just kill the pods, using a script such as this (obviously, read through it and make sure you understand it before running it blindly, as it will kill several pods automatically for you).
### First, kill services and controllers.
kubectl stop rc examples/guestbook/redis-slave-controller.json
kubectl stop rc examples/guestbook/frontend-controller.json
kubectl delete -f examples/guestbook/redis-master-service.json
kubectl delete -f examples/guestbook/redis-slave-service.json
kubectl delete pod redis-master # This is the only pod that requires manual removal.