This example shows how to build a simple multi-tier web application using Kubernetes and Docker.
The example combines a web frontend, a redis master for storage and a replicated set of redis slaves.
This example assumes that you have forked the repository and turned up a Kubernetes cluster:
$ cd kubernetes
$ hack/dev-build-and-up.shUse the file examples/guestbook/redis-master.json which describes a single pod running a redis key-value server in a container.
{
"id": "redis-master-2",
"kind": "Pod",
"apiVersion": "v1beta1",
"desiredState": {
"manifest": {
"version": "v1beta1",
"id": "redis-master-2",
"containers": [{
"name": "master",
"image": "dockerfile/redis",
"ports": [{
"containerPort": 6379,
}]
}]
}
},
"labels": {
"name": "redis-master"
}
}Create the redis pod in your Kubernetes cluster using the kubecfg CLI:
$ cluster/kubecfg.sh -c examples/guestbook/redis-master.json create podsOnce that's up you can list the pods in the cluster, to verify that the master is running:
cluster/kubecfg.sh list podsYou'll see a single 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).
ID Image(s) Host Labels Status
---------- ---------- ---------- ---------- ----------
redis-master-2 dockerfile/redis kubernetes-minion-3.c.briandpe-api.internal name=redis-master Running
If you ssh to that machine, you can run docker ps to see the actual pod:
$ gcutil ssh --zone us-central1-b kubernetes-minion-3
$ sudo docker ps
me@kubernetes-minion-3:~$ sudo docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
417ab993cdf8 dockerfile/redis:latest redis-server /etc/re 8 minutes ago Up 8 minutes 0.0.0.0:6379->6379/tcp master--redis_-_master_-_2--6b944b49(Note that initial docker pull may take a few minutes, depending on network conditions.)
A Kubernetes 'service' is a named load balancer that proxies traffic to one or more containers. 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. Use the file examples/guestbook/redis-master-service.json
{
"id": "redismaster",
"kind": "Service",
"apiVersion": "v1beta1",
"port": 6379,
"containerPort": 6379,
"selector": {
"name": "redis-master"
}
}to create the service with the kubecfg cli:
$ cluster/kubecfg.sh -c examples/guestbook/redis-master-service.json create services
ID Labels Selector Port
---------- ---------- ---------- ----------
redismaster name=redis-master 6379This will cause all pods to see the redis master apparently running on :6379.
Once created, the service proxy on each minion is configured to set up a proxy on the specified port (in this case port 6379).
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.
Use the file examples/guestbook/redis-slave-controller.json
{
"id": "redisSlaveController",
"kind": "ReplicationController",
"apiVersion": "v1beta1",
"desiredState": {
"replicas": 2,
"replicaSelector": {"name": "redisslave"},
"podTemplate": {
"desiredState": {
"manifest": {
"version": "v1beta1",
"id": "redisSlaveController",
"containers": [{
"name": "slave",
"image": "brendanburns/redis-slave",
"ports": [{"containerPort": 6379}]
}]
}
},
"labels": {"name": "redisslave"}
}},
"labels": {"name": "redisslave"}
}to create the replication controller by running:
$ cluster/kubecfg.sh -c examples/guestbook/redis-slave-controller.json create replicationControllers
ID Image(s) Selector Replicas
---------- ---------- ---------- ----------
redisSlaveController brendanburns/redis-slave name=redisslave 2The 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 ${REDISMASTER_SERVICE_HOST:-$SERVICE_HOST} $REDISMASTER_SERVICE_PORTOnce that's up you can list the pods in the cluster, to verify that the master and slaves are running:
$ cluster/kubecfg.sh list pods
ID Image(s) Host Labels Status
---------- ---------- ---------- ---------- ----------
redis-master-2 dockerfile/redis kubernetes-minion-3.c.briandpe-api.internal name=redis-master Running
4d65822107fcfd52 brendanburns/redis-slave kubernetes-minion-3.c.briandpe-api.internal name=redisslave,replicationController=redisSlaveController Running
78629a0f5f3f164f brendanburns/redis-slave kubernetes-minion-4.c.briandpe-api.internal name=redisslave,replicationController=redisSlaveController RunningYou will see a single redis master pod and two redis slave pods.
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 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 kubecfg -l "label=value" list services command.
Now that you have created the service specification, create it in your cluster with the kubecfg CLI:
$ cluster/kubecfg.sh -c examples/guestbook/redis-slave-service.json create services
ID Labels Selector Port
---------- ---------- ---------- ----------
redisslave name=redisslave name=redisslave 6379 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.
The pod is described in the file examples/guestbook/frontend-controller.json:
{
"id": "frontendController",
"kind": "ReplicationController",
"apiVersion": "v1beta1",
"desiredState": {
"replicas": 3,
"replicaSelector": {"name": "frontend"},
"podTemplate": {
"desiredState": {
"manifest": {
"version": "v1beta1",
"id": "frontendController",
"containers": [{
"name": "php-redis",
"image": "brendanburns/php-redis",
"ports": [{"containerPort": 80, "hostPort": 8000}]
}]
}
},
"labels": {"name": "frontend"}
}},
"labels": {"name": "frontend"}
}Using this file, you can turn up your frontend with:
$ cluster/kubecfg.sh -c examples/guestbook/frontend-controller.json create replicationControllers
ID Image(s) Selector Replicas
---------- ---------- ---------- ----------
frontendController brendanburns/php-redis name=frontend 3Once 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/kubecfg.sh list pods
ID Image(s) Host Labels Status
---------- ---------- ---------- ---------- ----------
redis-master-2 dockerfile/redis kubernetes-minion-3.c.briandpe-api.internal name=redis-master Running
4d65822107fcfd52 brendanburns/redis-slave kubernetes-minion-3.c.briandpe-api.internal name=redisslave,replicationController=redisSlaveController Running
380704bb7b4d7c03 brendanburns/php-redis kubernetes-minion-3.c.briandpe-api.internal name=frontend,replicationController=frontendController Running
55104dc76695721d brendanburns/php-redis kubernetes-minion-2.c.briandpe-api.internal name=frontend,replicationController=frontendController Running
365a858149c6e2d1 brendanburns/php-redis kubernetes-minion-1.c.briandpe-api.internal name=frontend,replicationController=frontendController Running
78629a0f5f3f164f brendanburns/redis-slave kubernetes-minion-4.c.briandpe-api.internal name=redisslave,replicationController=redisSlaveController RunningYou 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('REDISMASTER_SERVICE_HOST') ?: getenv('SERVICE_HOST'),
'port' => getenv('REDISMASTER_SERVICE_PORT'),
]);
$client->set($_GET['key'], $_GET['value']);
print('{"message": "Updated"}');
} else {
$read_port = getenv('REDISMASTER_SERVICE_PORT');
if (isset($_ENV['REDISSLAVE_SERVICE_PORT'])) {
$read_port = getenv('REDISSLAVE_SERVICE_PORT');
}
$client = new Predis\Client([
'scheme' => 'tcp',
'host' => getenv('REDISMASTER_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, grab the external IP of that host from the Google Cloud Console or the gcutil tool, and visit http://<host-ip>:8000.
$ gcutil listinstancesYou may need to open the firewall for port 8000 using the console or the gcutil tool. The following command will allow traffic from any source to instances tagged kubernetes-minion:
$ gcutil addfirewall --allowed=tcp:8000 --target_tags=kubernetes-minion kubernetes-minion-8000If you are running Kubernetes locally, you can just visit http://localhost:8000 For details about limiting traffic to specific sources, see the gcutil documentation
To turn down a Kubernetes cluster:
$ cluster/kube-down.sh