K
Q

Kubernetes multi-tier container application

10/25/2015

I have two Docker containers running Flask and redis each that communicate well when linked using Docker container linking.

I am trying to deploy the same on Kubernetes using services and pods but, it's not working. I am learning Kubernetes so I must be doing something wrong here.

Below are the Docker commands that work well:

$ docker run -d --name=redis -v /opt/redis:/redis -p 6379 redis_image redis-server
$ docker run -d -p 5000:5000 --link redis:redis --name flask flask_image

The kubernetes pod and services files are as below:

pod-redis.yaml

apiVersion: v1
kind: Pod
metadata:
  name: redis
  labels:
    name: redis
    app: redis
spec:
  containers:
  - name: redis
    image: dharmit/redis
    command:
    - "redis-server"
  volumeMounts:
  - mountPath: /redis
    name: redis-store
  volumes:
  - name: redis-store
    hostPath:
      path: /opt/redis

service-redis.yaml

apiVersion: v1
kind: Service
metadata:
  name: redis
  labels:
    name: redis
spec:
  ports:
  - port: 6379
    targetPort: 6379
  selector:
    app: redis

pod-flask.yaml

apiVersion: v1
kind: Pod
metadata:
  name: flask
  labels:
    name: flask
    app: flask
spec:
  containers:
  - name: flask
    image: dharmit/flask 
    ports:
    - containerPort: 5000

service-flask.yaml

apiVersion: v1
kind: Service
metadata:
  name: flask
  labels:
    name: flask
spec:
  ports: 
  - port: 5000
  selector:
    app: flask

When I do kubectl create -f /path/to/dir/ all services and pods start up fine and get listed by kubectl commands. But when I try to access the port 5000, Flask app complains that it cannot communicate with redis container. Below are the service related outputs:

flask service

Name:           flask
Namespace:      default
Labels:         name=flask
Selector:       app=flask
Type:           ClusterIP
IP:         10.254.155.179
Port:           <unnamed>   5000/TCP
Endpoints:      172.17.0.2:5000
Session Affinity:   None
No events.

redis service

Name:           redis
Namespace:      default
Labels:         name=redis
Selector:       app=redis
Type:           ClusterIP
IP:         10.254.153.217
Port:           <unnamed>   6379/TCP
Endpoints:      172.17.0.1:6379
Session Affinity:   None
No events.

And the output of curl command:

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/lib/python2.7/site-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/app/app/views.py", line 9, in index
    if r.get("count") == None:
  File "/usr/lib/python2.7/site-packages/redis/client.py", line 863, in get
    return self.execute_command('GET', name)
  File "/usr/lib/python2.7/site-packages/redis/client.py", line 570, in execute_command
    connection.send_command(*args)
  File "/usr/lib/python2.7/site-packages/redis/connection.py", line 556, in send_command
    self.send_packed_command(self.pack_command(*args))
  File "/usr/lib/python2.7/site-packages/redis/connection.py", line 532, in send_packed_command
    self.connect()
  File "/usr/lib/python2.7/site-packages/redis/connection.py", line 436, in connect
    raise ConnectionError(self._error_message(e))
ConnectionError: Error -2 connecting to redis:6379. Name or service not known.

What am I doing wrong here?

-- Dharmit
docker
kubernetes

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2 Answers

10/25/2015

You are trying to connect to redis:6379. But who is hostname redis? Probably not the pod that you just launched.

In order to use hostnames with pods and services, check if you can deploy sky-dns also in your cluster. For your case I presume that you only need to use the hostname of the redis service.

Edit

You don't want to connect to the pods directly, you want to use the service ip address for that.

So, you can use for connectivity the ip address of you service.

Or you can have hostnames for your services in order to connect to your pods.

For this the easiest way is to use kube-skydns. Read the documentation on how to deploy it and how to use it.

-- cristi
Source: StackOverflow
10/25/2015

You need to add containerPort in your pod-redis.yaml

- name: redis
  image: dharmit/redis
  command:
    - "redis-server"
  ports: 
    - containerPort: 6379
      hostPort: 6379
-- Mir Shahriar Sabuj
Source: StackOverflow