kubernetes replication controller

Question

i've a simple kubernetes cluster with a master and 3 minions. In this scenario, if i run a simple pod of a nginx or a mysql it works properly but, if i change type of KIND into yaml file and i try to run a replicated service, pods will start but i can't access to the service.

this is my yaml file for nginx with 3 replicas:

apiVersion: v1
kind: ReplicationController
metadata:
  name: nginx
spec:
  replicas: 3
  selector:
    app: nginx
  template:
    metadata:
      name: nginx
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx
        ports:
        - containerPort: 80

this is service yaml config file:

apiVersion: v1
kind: Service
metadata: 
  labels: 
    name: nginx
  name: nginx
spec: 
  ports:
    - port: 80
  selector: 
    name: nginx

i run it with:

# kubectl create -f nginx-rc.yaml
# kubectl create -f nginx-rc-service.yaml

if i run:

# kubectl get pod,svc,rc -o wide

i see:

NAME          READY     STATUS    RESTARTS   AGE       NODE
nginx-kgq1s   1/1       Running   0          1m        node01
nginx-pomx3   1/1       Running   0          1m        node02
nginx-xi54i   1/1       Running   0          1m        node03
NAME         LABELS                                    SELECTOR     IP(S)                           PORT(S)
kubernetes   component=apiserver,provider=kubernetes   <none>       10.254.0.1      443/TCP
nginx        name=nginx                                name=nginx   10.254.47.150   80/TCP
CONTROLLER   CONTAINER(S)   IMAGE(S)   SELECTOR    REPLICAS
nginx        nginx          nginx      app=nginx   3

i can see description for pod:

Name:                           nginx-kgq1s
Namespace:                      default
Image(s):                       nginx
Node:                           node01/node01
Labels:                         app=nginx
Status:                         Running
Reason:
Message:
IP:                             172.17.52.3
Replication Controllers:        nginx (3/3 replicas created)
Containers:
  nginx:
    Image:              nginx
    State:              Running
      Started:          Thu, 11 Feb 2016 16:28:08 +0100
    Ready:              True
    Restart Count:      0
Conditions:
  Type          Status
  Ready         True 
Events:
  FirstSeen                             LastSeen                        Count   From                            SubobjectPath                           Reason          Message
  Thu, 11 Feb 2016 16:27:47 +0100       Thu, 11 Feb 2016 16:27:47 +0100 1       {scheduler }                                                            scheduled       Successfully assigned nginx-kgq1s to node01
  Thu, 11 Feb 2016 16:27:57 +0100       Thu, 11 Feb 2016 16:27:57 +0100 1       {kubelet node01}        implicitly required container POD       pulled          Pod container image "gcr.io/google_containers/pause:0.8.0" already present on machine
  Thu, 11 Feb 2016 16:28:02 +0100       Thu, 11 Feb 2016 16:28:02 +0100 1       {kubelet node01}        implicitly required container POD       created         Created with docker id bed30a90c6eb
  Thu, 11 Feb 2016 16:28:02 +0100       Thu, 11 Feb 2016 16:28:02 +0100 1       {kubelet node01}        implicitly required container POD       started         Started with docker id bed30a90c6eb
  Thu, 11 Feb 2016 16:28:07 +0100       Thu, 11 Feb 2016 16:28:07 +0100 1       {kubelet node01}        spec.containers{nginx}                  created         Created with docker id 0a5c69cd0481
  Thu, 11 Feb 2016 16:28:08 +0100       Thu, 11 Feb 2016 16:28:08 +0100 1       {kubelet node01}        spec.containers{nginx}                  started         Started with docker id 0a5c69cd0481

this is what i see if i get description for rc:

Name:           nginx
Namespace:      default
Image(s):       nginx
Selector:       app=nginx
Labels:         app=nginx
Replicas:       3 current / 3 desired
Pods Status:    3 Running / 0 Waiting / 0 Succeeded / 0 Failed
Events:
  FirstSeen                             LastSeen                        Count   From                            SubobjectPath   Reason                  Message
  Thu, 11 Feb 2016 16:27:47 +0100       Thu, 11 Feb 2016 16:27:47 +0100 1       {replication-controller }                       successfulCreate        Created pod: nginx-kgq1s
  Thu, 11 Feb 2016 16:27:47 +0100       Thu, 11 Feb 2016 16:27:47 +0100 1       {replication-controller }                       successfulCreate        Created pod: nginx-pomx3
  Thu, 11 Feb 2016 16:27:47 +0100       Thu, 11 Feb 2016 16:27:47 +0100 1       {replication-controller }                       successfulCreate        Created pod: nginx-xi54i

and this is what i see if i get description of service:

Name:                   nginx
Namespace:              default
Labels:                 name=nginx
Selector:               name=nginx
Type:                   ClusterIP
IP:                     10.254.47.150
Port:                   <unnamed>       80/TCP
Endpoints:              <none>
Session Affinity:       None
No events.

as i can see, the problem may be that i don't have an ENDPOINT but i don't have any idea how i could solve.

Answer 1

To build on @jonskeet's answer, the reason the labels have to match is because the Pods can run on any node in your k8s cluster and Services need a way to locate them.

Therefore, the Service you're slapping in front of the Pod, needs to be able to filter through the cluster and, particularly, the set of Pods in the namespace it's in, and it leverages these matching k/v's in both selectors as its methodology to do so.

Answer 2

It looks to me like the selector for your service is wrong. It's looking for a label of name: nginx, but your pods actually have app: nginx.

Try changing your service file to:

apiVersion: v1
kind: Service
metadata: 
  labels: 
    name: nginx
  name: nginx
spec: 
  ports:
    - port: 80
  selector: 
    app: nginx

... or change your replication controller template to use name: nginx instead of app: nginx as the label. Basically, the labels have to match so that the service knows how to present a unified facade over your pods.