Note_Tech

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Kubernetes: Container - Sidecar

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• Kubernetes: Container - Sidecar
  • Sidecar container
  • Init Container vs Sidecar Container
    • Lab: Sidecar Container

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Sidecar container

• sidecar containers

  • a special case of init containers.(From offical doc)
  • the secondary containers that run along with the main application container within the same Pod.
  • used to enhance or to extend the functionality of the primary app container by providing additional services, or functionality such as logging, monitoring, security, or data synchronization, without directly altering the primary application code.

• Common use case:

  • ElasticSearch DB starts and continuously running
  • Kibana, the main application to display a dashboard, starts after

• specify a restartPolicy for containers listed in a Pod’s initContainers field.

  • make container independent from other init containers and from the main application container(s) within the same pod.
  • can be started, stopped, or restarted without affecting the main application container and other init containers.
  • start and remain running during the entire life of the Pod.

• Changing the image of a sidecar container will not cause the Pod to restart, but will trigger a container restart.

• sidecar containers support probes to control their lifecycle.

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• Manifest File

spec:
  initContainers:
    restartPolicy: Always # ensures containers are continuously running

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Init Container vs Sidecar Container

Dimension	Init (regular)	Sidecar (native)
Where declared	spec.initContainers	spec.initContainers
Key field	(no special restart policy)	restartPolicy: Always
Start order	Before all app containers	Starts before app containers (ordered), then app containers start
Duration	Runs to completion; then exits	Long-running; remains for Pod lifetime
On failure	Blocks Pod readiness; retried until success	Restarts automatically; Pod may be NotReady if probes fail
Probes	Regular inits don’t support liveness/readiness/startup probes	Sidecars can use readiness (affects Pod Ready) and other probes
Data/IPC	Can prep volumes/env only during init	Shares volumes/localhost with app during runtime
Shutdown	N/A (already exited)	Shuts down when Pod ends; for Jobs, native sidecars exit once primary containers finish so Jobs can complete

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Lab: Sidecar Container

# demo-pod-sidecar.yaml
apiVersion: v1
kind: Pod
metadata:
  name: demo-pod-sidecar
spec:
  containers:
    - name: main
      image: nginx
  initContainers:
    - name: sidecar # sidecar
      image: busybox
      command:
        - "sh"
        - "-c"
      args:
        - |
          echo "Sidecar starts.";
          while true; do
           echo "Sidecar running";
           sleep 1;
          done
      restartPolicy: Always # side car
    - name: init # init container
      image: busybox
      command:
        - "sh"
        - "-c"
      args:
        - |
          echo "init container starts.";
          sleep 10;
          echo "init container completed";

kubectl create -f demo-pod-sidecar.yaml
# pod/demo-pod-sidecar created

# confirm:
#  init: 2
#  ready: 2
kubectl get pod -w
# NAME               READY   STATUS               RESTARTS   AGE
# demo-pod-sidecar   0/2     Pending              0          0s
# demo-pod-sidecar   0/2     Pending              0          0s
# demo-pod-sidecar   0/2     Init:0/2             0          0s
# demo-pod-sidecar   1/2     Init:1/2             0          2s
# demo-pod-sidecar   1/2     Init:1/2             0          4s
# demo-pod-sidecar   1/2     PodInitializing      0          15s
# demo-pod-sidecar   2/2     Running              0          17s

# confirm sidecar is running
kubectl describe pod demo-pod-sidecar
# Init Containers:
#   sidecar:
#     State:          Running
#     Ready:          True
#   init:
#     State:          Terminated
#       Reason:       Completed
#       Exit Code:    0
#       Started:      Sat, 03 Jan 2026 20:43:57 -0500
#       Finished:     Sat, 03 Jan 2026 20:44:07 -0500
#     Ready:          True
# Containers:
#   main:
#     State:          Running
#       Started:      Sat, 03 Jan 2026 20:44:10 -0500
#     Ready:          True
# Events:
#   Type    Reason     Age   From               Message
#   ----    ------     ----  ----               -------
#   Normal  Scheduled  94s   default-scheduler  Successfully assigned default/demo-pod-sidecar to docker-desktop
#   Normal  Pulling    94s   kubelet            Pulling image "busybox"
#   Normal  Pulled     93s   kubelet            Successfully pulled image "busybox" in 1.155s (1.155s including waiting). Image size: 2224358 bytes.
#   Normal  Created    93s   kubelet            Created container: sidecar
#   Normal  Started    93s   kubelet            Started container sidecar
#   Normal  Pulling    93s   kubelet            Pulling image "busybox"
#   Normal  Pulled     92s   kubelet            Successfully pulled image "busybox" in 734ms (734ms including waiting). Image size: 2224358 bytes.
#   Normal  Created    92s   kubelet            Created container: init
#   Normal  Started    92s   kubelet            Started container init
#   Normal  Pulling    80s   kubelet            Pulling image "nginx"
#   Normal  Pulled     80s   kubelet            Successfully pulled image "nginx" in 863ms (863ms including waiting). Image size: 59797235 bytes.
#   Normal  Created    79s   kubelet            Created container: main
#   Normal  Started    79s   kubelet            Started container main

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