feat: add sidecars (#20)

16-sidecar-containers/01-sidecar.yml

@@ -0,0 +1,21 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: two-containers
+spec:
+  volumes:
+  - name: shared-data
+    emptyDir: {}
+  containers:
+  - name: nginx
+    image: nginx
+    volumeMounts:
+    - name: data
+      mountPath: /usr/share/nginx/html
+  - name: debian
+    image: debian
+    volumeMounts:
+    - name: data
+      mountPath: /pod-data
+    command: ["/bin/sh"]
+    args: ["-c", "echo Hello from the debian container > /pod-data/index.html"]

16-sidecar-containers/README.md

@@ -2,9 +2,40 @@
 
 ## Introduction
 
+In kubernetes a pod can contain multiple containers:
+
+```yml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: simple-pod
+spec:
+  containers:
+  - name: container1
+    image: nginx
+  - name: container2
+    image: busybox
+```
+
+Here we have 2 containers: `container1` and `container2`.
+
+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.
+
+All the containers of a given pod share the same network, and can share the same volumes.
+
+## Use cases
+
+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` harcoded. So with sidecars you can work around this.
+
+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.
+
+Istio, the service mesh tool, installs a sidecar container to do its job: https://istio.io/docs/setup/kubernetes/additional-setup/sidecar-injection/
+
 ## Exercices
 
-Nothing to see here.
+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`.
+
+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).
 
 ## Clean up
 
@@ -13,3 +44,6 @@ kubectl delete service,deployment,pod --all
 ```
 
 ## Links
+
+* https://docs.microsoft.com/en-us/azure/architecture/patterns/sidecar
+* https://medium.com/@dwdraju/sidecar-pattern-with-use-case-examples-ed6642e5eaf7