Kubernetes in production — Pod Disruption Budget

Dealing with Disruptions

Here are some ways to mitigate involuntary disruptions:

• Ensure your pod requests the resources it needs.
• Replicate your application if you need higher availability. (Learn about running replicated stateless and stateful applications.)
• For even higher availability when running replicated applications, spread applications across racks (using anti-affinity) or across zones (if using a multi-zone cluster.)

PodDisruptionBudget

An Application Owner can create a PodDisruptionBudget object (PDB) for each application. A PDB limits the number pods of a replicated application that are down simultaneously from voluntary disruptions. For example, a quorum-based application would like to ensure that the number of replicas running is never brought below the number needed for a quorum. A web front end might want to ensure that the number of replicas serving load never falls below a certain percentage of the total.

Cluster managers and hosting providers should use tools which respect Pod Disruption Budgets by calling the Eviction API instead of directly deleting pods. Examples are the kubectl drain command and the Kubernetes-on-GCE cluster upgrade script (cluster/gce/upgrade.sh).

When a cluster administrator wants to drain a node they use the kubectl drain command. That tool tries to evict all the pods on the machine. The eviction request may be temporarily rejected, and the tool periodically retries all failed requests until all pods are terminated, or until a configurable timeout is reached.

Example PDB Using minAvailable:

apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
  name: zk-pdb
spec:
  minAvailable: 2
  selector:
    matchLabels:
      app: zookeeper

Example PDB Using maxUnavailable (Kubernetes 1.7 or higher):

apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
  name: zk-pdb
spec:
  maxUnavailable: 1
  selector:
    matchLabels:
      app: zookeeper

Helm

apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
 name: 
 namespace: 
 labels:
 app: 
 chart: -
 release: 
 heritage: 
spec:
 selector:
 matchLabels:
 app: 
 env: 
 minAvailable: 

Imagine you have a service with 2 replicas and you need at least 1 to be available even during node upgrades and other ops tasks.

install / upgrade your release:

helm upgrade --install --debug "$RELEASE_NAME" -f helm/values.yaml \ 
--set replicas=2,budget.minAvailable=1 myrepo/mychart

kubectl describe pdb “$RELEASE_NAME”

Name: mysvc-prod
Namespace: prod
Min available: 1
Selector: app=myservice,env=prod
Status:
 Allowed disruptions: 1
 Current: 2
 Desired: 1
 Total: 2
Events: <none>

drain a node with one of your pods running:

kubectl drain --delete-local-data --force --ignore-daemonsets gke-mycluster-prod-pool-2fca4c85-k6g5node
 "gke-mycluster-prod-pool-2fca4c85-k6g5" already cordoned
WARNING: Deleting pods with local storage: sqlproxy-67f695889d-t778w; 
Ignoring DaemonSet-managed pods: fluentd-gcp-v3.0.0-llp5s; 
Deleting pods not managed by ReplicationController, ReplicaSet, Job, DaemonSet or StatefulSet:
 kube-proxy-gke-testing-dev-pool-2fca4c85-k6g5
pod "tiller-deploy-7b7b795779-rcvkd" evicted
pod "mysvc-prod-6856d59f9b-lzrtf" evicted
node "gke-mycluster-prod-pool-2fca4c85-k6g5" drained

again: kubectl describe pdb “$RELEASE_NAME”

Name: mysvc-prod
Namespace: prod
Min available: 1
Selector: app=myservice,env=prod
Status:
 Allowed disruptions: 0
 Current: 1
 Desired: 1
 Total: 2
Events: <none>

Tadaaa! We drained a node without any disruptions of our service.

PDB with 1 replica only?

If we had 1 replica only, the kubectl drain would get stuck always. Node drains / upgrades would need to be solved manually.

You might expect the eviction API would try to surge a replica to comply with the minAvailable condition, instead the drain gets stuck and it is your responsibility to solve this situation by yourself. Is it a bug or a feature? The Kubernetes community says you shouldn’t use 1 replica in production at all if you want HA, which is fair :)

It does what is expected of it, though.

If you don’t want your kubectl drains to get stuck, you might want to use PDB for deployments with more than 1 replica.

Edit your templates/pdb.yaml:

apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
...