Creating a Pod that has multiple containers
In this article I will create a Kubernetes pod that contains two containers. First writing a manifest file like this1:
kind: Pod
apiVersion: v1
metadata:
name: mc
spec:
containers:
- name: c0
image: alpine/socat
command: [ "/bin/sh", "-c", "/usr/bin/socat tcp-l:1234,fork exec:'echo c0'" ]
- name: c1
image: alpine/socat
command: [ "/bin/sh", "-c", "/usr/bin/socat tcp-l:1235,fork exec:'echo c1'" ]
The above manifest file describes a pod named mc, and it contains two containers called c0 and c1. The container uses the image alpine/socat, which is convenient to start a TCP server. In the above pod, there are two containers, and each container will run a tcp server that output c0 and c1 specifically, and one service listens to the port 1234 and the other listens to 1235. To create the above pod, run the following command:
➤ kubectl apply -f mc.yml
pod/mc created
If everything goes fine, after a while the pod should be created and run. Using the following command to check the status of the pod:
➤ kubectl get po | grep mc
mc 2/2 Running 0 4m9s
From the above output, we can see the mc pod is running and there are 2/2 containers running. And we can see the details of the containers with the following command:
➤ kubectl describe po mc | grep -A34 'Containers:' 23:23:42
Containers:
c0:
Container ID: docker://8e754fd7ab1807881d314af6cb186d16c210bb7900e99405ecc7a1bc6f322be1
Image: alpine/socat
Image ID: docker-pullable://alpine/socat@sha256:0f359f1a2fabf445765eb5762ff565d1c1f7c360a5fa5e788848e68eb193c349
Port: <none>
Host Port: <none>
Command:
/bin/sh
-c
/usr/bin/socat tcp-l:1234,fork exec:'echo c0'
State: Running
Started: Wed, 05 Jul 2023 23:08:03 +0800
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-kthb6 (ro)
c1:
Container ID: docker://bf65ca4ef733b07781114e1556f942969308a946bc73ab5bce8e9fa55e2958bd
Image: alpine/socat
Image ID: docker-pullable://alpine/socat@sha256:0f359f1a2fabf445765eb5762ff565d1c1f7c360a5fa5e788848e68eb193c349
Port: <none>
Host Port: <none>
Command:
/bin/sh
-c
/usr/bin/socat tcp-l:1235,fork exec:'echo c1'
State: Running
Started: Wed, 05 Jul 2023 23:08:09 +0800
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-kthb6 (ro)
From the above output, we can see there are two containers c0 and c1. We can login to c0 with the following command2:
➤ kubectl exec -i -t mc --container c0 -- /bin/sh
/ #
Now we are entering the terminal of c0, then we need to add the telnet command to access the services provide by the containers. We can use the apk tool provided by the container(which is Alpine Linux based) to install the telnet command. First update the Alpine Linux repo with the following command:
/ # apk update
fetch https://dl-cdn.alpinelinux.org/alpine/v3.18/main/x86_64/APKINDEX.tar.gz
481BDE4E0F7F0000:error:0A000126:SSL routines:ssl3_read_n:unexpected eof while reading:ssl/record/rec_layer_s3.c:303:
WARNING: updating https://dl-cdn.alpinelinux.org/alpine/v3.18/main: Permission denied
fetch https://dl-cdn.alpinelinux.org/alpine/v3.18/community/x86_64/APKINDEX.tar.gz
v3.18.2-273-g8291981acc0 [https://dl-cdn.alpinelinux.org/alpine/v3.18/main]
v3.18.2-274-ga199b869491 [https://dl-cdn.alpinelinux.org/alpine/v3.18/community]
0 unavailable, 1 stale; 20062 distinct packages available
/ #
And then install the busybox-extras which contains the telnet command:
/ # apk add busybox-extras
(1/1) Installing busybox-extras (1.36.1-r0)
Executing busybox-extras-1.36.1-r0.post-install
Executing busybox-1.36.1-r0.trigger
OK: 9 MiB in 20 packages
/ #
Until now we have installed the telnet command in container c0, and we can access the service provided by the c0 container like this:
/ # telnet localhost 1234
Connected to localhost
c0
Connection closed by foreign host
From the above output we can see the c0 string is output by the service created by the socat command defined in the pod manifest. In addition, because the two containers share the same network3, so we can directly access the service provided by c1 in c0 like this:
/ # telnet localhost 1235
Connected to localhost
c1
Connection closed by foreign host
/ #
Until now, we have seen how to create and access the containers inside a pod.