Using sidecar containers

The Operator allows you to deploy additional (so-called sidecar) containers to the Pod. You can use this feature to run debugging tools, some specific monitoring solutions, etc.

Note

Custom sidecar containers can easily access other components of your cluster. Therefore they should be used carefully and by experienced users only.

Adding a sidecar container

You can add sidecar containers to Percona Distribution for MongoDB Replica Set, Config Servers, and mongos Pods. Just use sidecars subsection in the replsets, sharding.configsvrReplSet, and sharding.mongos of the deploy/cr.yaml configuration file. In this subsection, you should specify the name and image of your container and possibly a command to run:

spec:
  replsets:
    ....
    sidecars:
    - image: busybox
      command: ["/bin/sh"]
      args: ["-c", "while true; do echo echo $(date -u) 'test' >> /dev/null; sleep 5; done"]
      name: rs-sidecar-0
    ....

Apply your modifications as usual:

$ kubectl apply -f deploy/cr.yaml

Running kubectl describe command for the appropriate Pod can bring you the information about the newly created container:

$ kubectl describe pod my-cluster-name-rs0-0

Expected output

....
Containers:
....
rs-sidecar-0:
  Container ID:  docker://f0c3437295d0ec819753c581aae174a0b8d062337f80897144eb8148249ba742
  Image:         busybox
  Image ID:      docker-pullable://busybox@sha256:139abcf41943b8bcd4bc5c42ee71ddc9402c7ad69ad9e177b0a9bc4541f14924
  Port:          <none>
  Host Port:     <none>
  Command:
    /bin/sh
  Args:
    -c
    while true; do echo echo $(date -u) 'test' >> /dev/null; sleep 5; done
  State:          Running
    Started:      Thu, 11 Nov 2021 10:38:15 +0300
  Ready:          True
  Restart Count:  0
  Environment:    <none>
  Mounts:
    /var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-fbrbn (ro)
....

Getting shell access to a sidecar container

You can login to your sidecar container as follows:

$ kubectl exec -it my-cluster-name-rs0-0 -c rs-sidecar-0 -- sh
/ #

Mount volumes into sidecar containers

It is possible to mount volumes into sidecar containers.

Following subsections describe different volume types, which were tested with sidecar containers and are known to work.

Persistent Volume

You can use Persistent volumes when you need dynamically provisioned storage which doesn’t depend on the Pod lifecycle. To use such volume, you should claim durable storage with persistentVolumeClaim without specifying any non-important details.

The following example requests 1G storage with sidecar-volume-claim PersistentVolumeClaim, and mounts the correspondent Persistent Volume to the rs-sidecar-0 container’s filesystem under the /volume0 directory:

...
sidecars:
- image: busybox
  command: ["/bin/sh"]
  args: ["-c", "while true; do echo echo $(date -u) 'test' >> /dev/null; sleep 5; done"]
  name: rs-sidecar-0
  volumeMounts:
  - mountPath: /volume0
    name: sidecar-volume-claim
sidecarPVCs:
- apiVersion: v1
  kind: PersistentVolumeClaim
  metadata:
    name: sidecar-volume-claim
  spec:
    resources:
      requests:
        storage: 1Gi
    volumeMode: Filesystem
    accessModes:
      - ReadWriteOnce

Note

Sidecar containers for mongos Pods have limited Persistent volumes support: sharding.mongos.sidecarPVCs option can be used if there is a single mongos in deployment or when ReadWriteMany/ReadOnlyMany access modes are used (but these modes are available not in every storage).

Secret

You can use a secret volume to pass the information which needs additional protection (e.g. passwords), to the container. Secrets are stored with the Kubernetes API and mounted to the container as RAM-stored files.

You can mount a secret volume as follows:

...
sidecars:
- image: busybox
  command: ["/bin/sh"]
  args: ["-c", "while true; do echo echo $(date -u) 'test' >> /dev/null; sleep 5; done"]
  name: rs-sidecar-0
  volumeMounts:
  - mountPath: /secret
    name: sidecar-secret
sidecarVolumes:
- name: sidecar-secret
  secret:
    secretName: mysecret

The above example creates a sidecar-secret volume (based on already existing mysecret Secret object) and mounts it to the rs-sidecar-0 container’s filesystem under the /secret directory.

Note

Don’t forget you need to create a Secret Object before you can use it.

configMap

You can use a configMap volume to pass some configuration data to the container. Secrets are stored with the Kubernetes API and mounted to the container as RAM-stored files.

You can mount a configMap volume as follows:

...
sidecars:
- image: busybox
  command: ["/bin/sh"]
  args: ["-c", "while true; do echo echo $(date -u) 'test' >> /dev/null; sleep 5; done"]
  name: rs-sidecar-0
  volumeMounts:
  - mountPath: /config
    name: sidecar-config
sidecarVolumes:
- name: sidecar-config
  configMap:
    name: myconfigmap

The above example creates a sidecar-config volume (based on already existing myconfigmap configMap object) and mounts it to the rs-sidecar-0 container’s filesystem under the /config directory.

Note

Don’t forget you need to create a configMap Object before you can use it.

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Last update: 2023-09-29