50 lines
1.8 KiB
Markdown
50 lines
1.8 KiB
Markdown
# Sidecar containers: what, why, and how
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## Introduction
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In kubernetes a pod can contain multiple containers:
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```yml
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apiVersion: v1
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kind: Pod
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metadata:
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name: simple-pod
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spec:
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containers:
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- name: container1
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image: nginx
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- name: container2
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image: busybox
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```
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Here we have 2 containers: `container1` and `container2`.
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When you have multiple containers in a pod we call them sidecar containers. Most of the time you have a "primary" container, the one with containing your application, and "secondary", hence the sidecar terminology.
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All the containers of a given pod share the same network, and can share the same volumes.
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## Use cases
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Sidecars are useful for containers that are tightly coupled. A good use case is when you migrate an app from one machine to containers. The application will have a lot of `localhost` or `127.0.0.1` hardcoded. So with sidecars you can work around this.
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Another use case is to have sidecars helping the main container, like sending logs to a centralized system, sending the metrics to a specific system, doing SSL termination, etc.
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Istio, the service mesh tool, installs a sidecar container to do its job: https://istio.io/docs/setup/kubernetes/additional-setup/sidecar-injection/
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## Exercices
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Review and apply the file [01-sidecar.yml](01-sidecar.yml). Connect to the `nginx` container and look at the file system in `/usr/share/nginx/html`.
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This exercice is taken from the [official Kubernetes documentation](https://kubernetes.io/docs/tasks/access-application-cluster/communicate-containers-same-pod-shared-volume/#creating-a-pod-that-runs-two-containers).
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## Clean up
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```sh
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kubectl delete service,deployment,pod --all
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```
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## Links
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* https://docs.microsoft.com/en-us/azure/architecture/patterns/sidecar
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* https://medium.com/@dwdraju/sidecar-pattern-with-use-case-examples-ed6642e5eaf7
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