Restart pods when configmap updates in Kubernetes?

How do I automatically restart Kubernetes pods and pods associated with deployments when their configmap is changed/updated?

---

I know there's been talk about the ability to automatically restart pods when a config maps changes but to my knowledge this is not yet available in Kubernetes 1.2.

So what (I think) I'd like to do is a "rolling restart" of the deployment resource associated with the pods consuming the config map. Is it possible, and if so how, to force a rolling restart of a deployment in Kubernetes without changing anything in the actual template? Is this currently the best way to do it or is there a better option?

The current best solution to this problem (referenced deep in https://github.com/kubernetes/kubernetes/issues/22368 linked in the sibling answer) is to use Deployments, and consider your ConfigMaps to be immutable.

When you want to change your config, create a new ConfigMap with the changes you want to make, and point your deployment at the new ConfigMap. If the new config is broken, the Deployment will refuse to scale down your working ReplicaSet. If the new config works, then your old ReplicaSet will be scaled to 0 replicas and deleted, and new pods will be started with the new config.

Not quite as quick as just editing the ConfigMap in place, but much safer.

Signalling a pod on config map update is a feature in the works (https://github.com/kubernetes/kubernetes/issues/22368).

You can always write a custom pid1 that notices the confimap has changed and restarts your app.

You can also eg: mount the same config map in 2 containers, expose a http health check in the second container that fails if the hash of config map contents changes, and shove that as the liveness probe of the first container (because containers in a pod share the same network namespace). The kubelet will restart your first container for you when the probe fails.

Of course if you don't care about which nodes the pods are on, you can simply delete them and the replication controller will "restart" them for you.

The best way I've found to do it is run Reloader

It allows you to define configmaps or secrets to watch, when they get updated, a rolling update of your deployment is performed. Here's an example:

You have a deployment foo and a ConfigMap called foo-configmap. You want to roll the pods of the deployment every time the configmap is changed. You need to run Reloader with:

kubectl apply -f https://raw.githubusercontent.com/stakater/Reloader/master/deployments/kubernetes/reloader.yaml

Then specify this annotation in your deployment:

kind: Deployment
metadata:
  annotations:
    configmap.reloader.stakater.com/reload: "foo-configmap"
  name: foo
...

Helm 3 doc page

Often times configmaps or secrets are injected as configuration files in containers. Depending on the application a restart may be required should those be updated with a subsequent helm upgrade, but if the deployment spec itself didn't change the application keeps running with the old configuration resulting in an inconsistent deployment.

The sha256sum function can be used together with the include function to ensure a deployments template section is updated if another spec changes:

kind: Deployment
spec:
  template:
    metadata:
      annotations:
        checksum/config: {{ include (print $.Template.BasePath "/secret.yaml") . | sha256sum }}
[...]

In my case, for some reasons, $.Template.BasePath didn't work but $.Chart.Name does:

spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: admin-app
      annotations:
        checksum/config: {{ include (print $.Chart.Name "/templates/" $.Chart.Name "-configmap.yaml") . | sha256sum }}

You can update a metadata annotation that is not relevant for your deployment. it will trigger a rolling-update

for example:

    spec:
      template:
        metadata:
          annotations:
            configmap-version: 1

If k8>1.15; then doing a rollout restart worked best for me as part of CI/CD with App configuration path hooked up with a volume-mount. A reloader plugin or setting restartPolicy: Always in deployment manifest YML did not work for me. No application code changes needed, worked for both static assets as well as Microservice.

kubectl rollout restart deployment/<deploymentName> -n <namespace> 

Had this problem where the Deployment was in a sub-chart and the values controlling it were in the parent chart's values file. This is what we used to trigger restart:

spec:
  template:
    metadata:
      annotations:
        checksum/config: {{ tpl (toYaml .Values) . | sha256sum }}

Obviously this will trigger restart on any value change but it works for our situation. What was originally in the child chart would only work if the config.yaml in the child chart itself changed:

    checksum/config: {{ include (print $.Template.BasePath "/config.yaml") . | sha256sum }}

I also banged my head around this problem for some time and wished to solve this in an elegant but quick way.

Here are my 20 cents:

• The answer using labels as mentioned here won't work if you are updating labels. But would work if you always add labels. More details here.

• The answer mentioned here is the most elegant way to do this quickly according to me but had the problem of handling deletes. I am adding on to this answer:

Solution

I am doing this in one of the Kubernetes Operator where only a single task is performed in one reconcilation loop.

• Compute the hash of the config map data. Say it comes as v2.
• Create ConfigMap cm-v2 having labels: version: v2 and product: prime if it does not exist and RETURN. If it exists GO BELOW.
• Find all the Deployments which have the label product: prime but do not have version: v2, If such deployments are found, DELETE them and RETURN. ELSE GO BELOW.
• Delete all ConfigMap which has the label product: prime but does not have version: v2 ELSE GO BELOW.
• Create Deployment deployment-v2 with labels product: prime and version: v2 and having config map attached as cm-v2 and RETURN, ELSE Do nothing.

That's it! It looks long, but this could be the fastest implementation and is in principle with treating infrastructure as Cattle (immutability).

Also, the above solution works when your Kubernetes Deployment has Recreate update strategy. Logic may require little tweaks for other scenarios.

Consider using kustomize (or kubectl apply -k) and then leveraging it's powerful configMapGenerator feature. For example, from: https://kubectl.docs.kubernetes.io/references/kustomize/kustomization/configmapgenerator/

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

# Just one example of many...
- name: my-app-config
  literals:
  - JAVA_HOME=/opt/java/jdk
  - JAVA_TOOL_OPTIONS=-agentlib:hprof

  # Explanation below...
  - SECRETS_VERSION=1

Then simply reference my-app-config in your deployments. When building with kustomize, it'll automatically find and update references to my-app-config with an updated suffix, e.g. my-app-config-f7mm6mhf59.

Bonus, updating secrets: I also use this technique for forcing a reload of secrets (since they're affected in the same way). While I personally manage my secrets completely separately (using Mozilla sops), you can bundle a config map alongside your secrets, so for example in your deployment:

# ...
spec:
  template:
    spec:
      containers:
        - name: my-app
          image: my-app:tag
          envFrom:
            # For any NON-secret environment variables. Name is automatically updated by Kustomize
            - configMapRef:
                name: my-app-config

            # Defined separately OUTSIDE of Kustomize. Just modify SECRETS_VERSION=[number] in the my-app-config ConfigMap
            # to trigger an update in both the config as well as the secrets (since the pod will get restarted).
            - secretRef:
                name: my-app-secrets

Then, just add a variable like SECRETS_VERSION into your ConfigMap like I did above. Then, each time you change my-app-secrets, just increment the value of SECRETS_VERSION, which serves no other purpose except to trigger a change in the kustomize'd ConfigMap name, which should also result in a restart of your pod. So then it becomes:

Adding the immutable property to the config map totally avoids the problem. Using config hashing helps in a seamless rolling update but it does not help in a rollback. You can take a look at this open-source project - 'Configurator' - https://github.com/gopaddle-io/configurator.git .'Configurator' works by the following using the custom resources :

1. Configurator ties the deployment lifecycle with the configMap. When the config map is updated, a new version is created for that configMap. All the deployments that were attached to the configMap get a rolling update with the latest configMap version tied to it.

2. When you roll back the deployment to an older version, it bounces to configMap version it had before doing the rolling update.

This way you can maintain versions to the config map and facilitate rolling and rollback to your deployment along with the config map.

> How do I automatically restart Kubernetes pods and pods associated > with deployments when their configmap is changed/updated?

If you are using configmap as Environment you have to use the external option.

• Reloader
• Kube watcher
• Configurator

Kubernetes auto-reload the config map if it's mounted as volume (If subpath there it won't work with that).

> When a ConfigMap currently consumed in a volume is updated, projected > keys are eventually updated as well. The kubelet checks whether the > mounted ConfigMap is fresh on every periodic sync. However, the > kubelet uses its local cache for getting the current value of the > ConfigMap. The type of the cache is configurable using the > ConfigMapAndSecretChangeDetectionStrategy field in the > KubeletConfiguration struct. A ConfigMap can be either propagated by > watch (default), ttl-based, or by redirecting all requests directly to > the API server. As a result, the total delay from the moment when the > ConfigMap is updated to the moment when new keys are projected to the > Pod can be as long as the kubelet sync period + cache propagation > delay, where the cache propagation delay depends on the chosen cache > type (it equals to watch propagation delay, ttl of cache, or zero > correspondingly).

Official document : https://kubernetes.io/docs/concepts/configuration/configmap/#mounted-configmaps-are-updated-automatically

ConfigMaps consumed as environment variables are not updated automatically and require a pod restart.

Simple example Configmap

apiVersion: v1
kind: ConfigMap
metadata:
  name: config
  namespace: default
data:
  foo: bar

POD config

spec:
  containers:
  - name: configmaptestapp
    image: <Image>
    volumeMounts:
    - mountPath: /config
      name: configmap-data-volume
    ports:
    - containerPort: 8080
  volumes:
    - name: configmap-data-volume
      configMap:
        name: config

Example : https://medium.com/@harsh.manvar111/update-configmap-without-restarting-pod-56801dce3388

Another way is to stick it into the command section of the Deployment:

...
command: [ "echo", "
  option = value\n
  other_option = value\n
" ]
...

Alternatively, to make it more ConfigMap-like, use an additional Deployment that will just host that config in the command section and execute kubectl create on it while adding an unique 'version' to its name (like calculating a hash of the content) and modifying all the deployments that use that config:

...
command: [ "/usr/sbin/kubectl-apply-config.sh", "
  option = value\n
  other_option = value\n
" ]
...

I'll probably post kubectl-apply-config.sh if it ends up working.

(don't do that; it looks too bad)