Containers in pod won't talk to each other in Kubernetes - django

Question

I have three containers in a pod: nginx, redis, custom django app. It seems like none of them talk to each other with kubernetes. In docker compose they do but I can't use docker compose in production.
The django container gets this error:
[2022-06-20 21:45:49,420: ERROR/MainProcess] consumer: Cannot connect to redis://redis:6379/0: Error 111 connecting to redis:6379. Connection refused..
Trying again in 32.00 seconds... (16/100)
and the nginx container starts but never shows any traffic. Trying to connect to localhost:8000 gets no reply.
Any idea whats wrong with my yml file?
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
creationTimestamp: null
name: djangonetwork
spec:
ingress:
- from:
- podSelector:
matchLabels:
io.kompose.network/djangonetwork: "true"
podSelector:
matchLabels:
io.kompose.network/djangonetwork: "true"
---
apiVersion: v1
data:
DB_HOST: db
DB_NAME: django_db
DB_PASSWORD: password
DB_PORT: "5432"
DB_USER: user
kind: ConfigMap
metadata:
creationTimestamp: null
labels:
io.kompose.service: web
name: envs--django
---
apiVersion: apps/v1
kind: Deployment
metadata:
labels:
io.kompose.service: web
name: web
spec:
replicas: 1
selector:
matchLabels:
io.kompose.service: web
strategy:
type: Recreate
template:
metadata:
labels:
io.kompose.network/djangonetwork: "true"
io.kompose.service: web
spec:
containers:
- image: nginx:alpine
name: nginxcontainer
ports:
- containerPort: 8000
- image: redis:alpine
name: rediscontainer
ports:
- containerPort: 6379
resources: {}
- env:
- name: DB_HOST
valueFrom:
configMapKeyRef:
key: DB_HOST
name: envs--django
- name: DB_NAME
valueFrom:
configMapKeyRef:
key: DB_NAME
name: envs--django
- name: DB_PASSWORD
valueFrom:
configMapKeyRef:
key: DB_PASSWORD
name: envs--django
- name: DB_PORT
valueFrom:
configMapKeyRef:
key: DB_PORT
name: envs--django
- name: DB_USER
valueFrom:
configMapKeyRef:
key: DB_USER
name: envs--django
image: localhost:5000/integration/web:latest
name: djangocontainer
ports:
- containerPort: 8000
resources: {}
restartPolicy: Always
status: {}
---
apiVersion: v1
kind: Service
metadata:
labels:
io.kompose.service: web
name: web
spec:
ports:
- name: "8000"
port: 8000
targetPort: 8000
selector:
io.kompose.service: web

Answer 1

You've put all three containers into a single Pod. That's usually not the preferred approach: it means you can't restart one of the containers without restarting all of them (any update to your application code requires discarding your Redis cache) and you can't individually scale the component parts (if you need five replicas of your application, do you also need five reverse proxies and can you usefully use five Redises?).
Instead, a preferred approach is to split these into three separate Deployments (or possibly use a StatefulSet for Redis with persistence). Each has a corresponding Service, and then those Service names can be used as DNS names.
A very minimal example for Redis could look like:
apiVersion: apps/v1
kind: Deployment
metadata:
name: redis
spec:
replicas: 1
template:
metadata:
labels:
service: web
component: redis
spec:
containers:
- name: redis
image: redis
ports:
- name: redis
containerPort: 6379
---
apiVersion: v1
kind: Service
metadata:
name: redis # <-- this name will be a DNS name
spec:
selector: # matches the template: { metadata: { labels: } }
service: web
component: redis
ports:
- name: redis
port: 6379
targetPort: redis # matches a containerPorts: [{ name: }]
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: app
spec:
...
env:
- name: REDIS_HOST
value: redis # matches the Service
If all three parts are in the same Pod, then the Service can't really distinguish which part it's talking to. In principle, between these containers, they share a network namespace and need to talk to each other as localhost; the containers: [{ name: }] have no practical effect.