Pod and Network

Normally, we want for one container to access the program running on another container. Two scenarios are possible: containers run on the same pod or containers run on different pods.

Containers on the same Pod

        flowchart
    subgraph pod[Pod]
    flask
    postgres
    end

    subgraph network[Network]
    pod
    end

    subgraph host[Host]
    browser
    network
    end

    browser[Web browser] --> flask[Flask];
    flask .-> postgres[PostgreSQL];

    classDef classPod fill:lightblue,stroke:blue
    class pod classPod;

    classDef classNetwork stroke:black,stroke-dasharray: 5 5
    class network classNetwork;
    

Diagram of containers running on the same pod.

Containers in the same pod share the IP addresses, MAC addresses and port mappings. This allows the container to be reached by another container using localhost or the name of the container.

Example

Given the following Kubernetes manifest file

play.yml

apiVersion: v1
kind: ConfigMap
metadata:
  name: client-entrypoint
data:
  entrypoint.sh: |
    valkey-cli -h server set counter 0
    while [ -z "" ]
    do
      valkey-cli -h server incr counter
      sleep 5s
    done
---
apiVersion: v1
kind: Pod
metadata:
  labels:
    app: valkey
  name: valkey0
spec:
  containers:
    - name: server
      image: docker.io/valkey/valkey:8.0.2-alpine
    - name: client
      image: docker.io/valkey/valkey:8.0.2-alpine
      command:
        - /bin/sh
        - /opt/entrypoint.sh
      volumeMounts:
        - mountPath: /opt
          name: client-entrypoint-volume
  volumes:
    - configMap:
        name: client-entrypoint
        items:
          - key: "entrypoint.sh"
            path: "entrypoint.sh"
      name: client-entrypoint-volume

running

podman kube play play.yml

creates a pod named valkey0 with a container named server running the Valkey server and another container named client running a small Bash script that increases a counter every 5 seconds.

Containers on different pods

        flowchart
    subgraph pod1[Pod 1]
    flask
    end

    subgraph pod2[Pod 2]
    postgres
    end

    subgraph network[Network]
    pod1
    pod2
    end

    subgraph host[Host]
    browser
    network
    pod1
    pod2
    end

    browser[Web browser] --> flask[Flask];
    flask .-> postgres[PostgreSQL];

    classDef classPod fill:lightblue,stroke:blue
    class pod1,pod2 classPod;

    classDef classNetwork stroke:black,stroke-dasharray: 5 5
    class network classNetwork;
    

Diagram of containers running on different pods.

The container can be reached by another container from a different pod if both pods are part of the same network. This allows the container to be reached by another container using the name of the pod.

Example

Important

When no network option is specified and host network mode is not configured in the YAML file, a new network stack is created and pods are attached to it making possible pod to pod communication.

First, create a new network by running

podman network valkey1

Given the following Kubernetes manifest file

play.yml

apiVersion: v1
kind: ConfigMap
metadata:
  name: client-entrypoint
data:
  entrypoint.sh: |
    valkey-cli -h valkey1-server set counter 0
    while [ -z "" ]
    do
      valkey-cli -h valkey1-server incr counter
      sleep 5s
    done
---
apiVersion: v1
kind: Pod
metadata:
  labels:
    app: valkey
  name: valkey1-server
spec:
  containers:
    - name: server
      image: docker.io/valkey/valkey:8.0.2-alpine
---
apiVersion: v1
kind: Pod
metadata:
  labels:
    app: valkey
  name: valkey1-client
spec:
  containers:
    - name: client
      image: docker.io/valkey/valkey:8.0.2-alpine
      command:
        - /bin/sh
        - /opt/entrypoint.sh
      volumeMounts:
        - mountPath: /opt
          name: client-entrypoint-volume
  volumes:
    - configMap:
        name: client-entrypoint
        items:
          - key: "entrypoint.sh"
            path: "entrypoint.sh"
      name: client-entrypoint-volume

running

podman kube play --net valkey1 play.yml

creates a pod named valkey1-server connected to the network valkey1 with a container named server running the Valkey server and another pod named valkey1-client connected to the network valkey1 with a container named client running a small Bash script that increases a counter every 5 seconds.