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Specifying a Disruption Budget for your Application

This page shows how to limit the number of concurrent disruptions that your application experiences, allowing for higher availability while permitting the cluster administrator to manage the clusters nodes.

Before you begin

Protecting an Application with a PodDisruptionBudget

  1. Identify what application you want to protect with a PodDisruptionBudget (PDB).
  2. Think about how your application reacts to disruptions.
  3. Create a PDB definition as a YAML file.
  4. Create the PDB object from the YAML file.

Identify an Application to Protect

The most common use case when you want to protect an application specified by one of the built-in Kubernetes controllers:

In this case, make a note of the controller’s .spec.selector; the same selector goes into the PDBs .spec.selector.

You can also use PDBs with pods which are not controlled by one of the above controllers, or arbitrary groups of pods, but there are some restrictions, described in Arbitrary Controllers and Selectors.

Think about how your application reacts to disruptions

Decide how many instances can be down at the same time for a short period due to a voluntary disruption.

Rounding logic when specifying percentages

Values for minAvailable or maxUnavailable can be expressed as integers or as a percentage.

When you specify the value as a percentage, it may not map to an exact number of Pods. For example, if you have 7 Pods and you set minAvailable to "50%", it’s not immediately obvious whether that means 3 Pods or 4 Pods must be available. Kubernetes rounds up to the nearest integer, so in this case, 4 Pods must be available. You can examine the code that controls this behavior.

Specifying a PodDisruptionBudget

A PodDisruptionBudget has three fields:

Note: For versions 1.8 and earlier: When creating a PodDisruptionBudget object using the kubectl command line tool, the minAvailable field has a default value of 1 if neither minAvailable nor maxUnavailable is specified.

You can specify only one of maxUnavailable and minAvailable in a single PodDisruptionBudget. maxUnavailable can only be used to control the eviction of pods that have an associated controller managing them. In the examples below, “desired replicas” is the scale of the controller managing the pods being selected by the PodDisruptionBudget.

Example 1: With a minAvailable of 5, evictions are allowed as long as they leave behind 5 or more healthy pods among those selected by the PodDisruptionBudget’s selector.

Example 2: With a minAvailable of 30%, evictions are allowed as long as at least 30% of the number of desired replicas are healthy.

Example 3: With a maxUnavailable of 5, evictions are allowed as long as there are at most 5 unhealthy replicas among the total number of desired replicas.

Example 4: With a maxUnavailable of 30%, evictions are allowed as long as no more than 30% of the desired replicas are unhealthy.

In typical usage, a single budget would be used for a collection of pods managed by a controller—for example, the pods in a single ReplicaSet or StatefulSet.

Note: A disruption budget does not truly guarantee that the specified number/percentage of pods will always be up. For example, a node that hosts a pod from the collection may fail when the collection is at the minimum size specified in the budget, thus bringing the number of available pods from the collection below the specified size. The budget can only protect against voluntary evictions, not all causes of unavailability.

A maxUnavailable of 0% (or 0) or a minAvailable of 100% (or equal to the number of replicas) may block node drains entirely. This is permitted as per the semantics of PodDisruptionBudget.

You can find examples of pod disruption budgets defined below. They match pods with the label app: zookeeper.

Example PDB Using minAvailable:

policy/zookeeper-pod-disruption-budget-minavailable.yaml 
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):

policy/zookeeper-pod-disruption-budget-maxunavailable.yaml 
apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
  name: zk-pdb
spec:
  maxUnavailable: 1
  selector:
    matchLabels:
      app: zookeeper

For example, if the above zk-pdb object selects the pods of a StatefulSet of size 3, both specifications have the exact same meaning. The use of maxUnavailable is recommended as it automatically responds to changes in the number of replicas of the corresponding controller.

Create the PDB object

You can create the PDB object with a command like kubectl apply -f mypdb.yaml.

You cannot update PDB objects. They must be deleted and re-created.

Check the status of the PDB

Use kubectl to check that your PDB is created.

Assuming you don’t actually have pods matching app: zookeeper in your namespace, then you’ll see something like this:

kubectl get poddisruptionbudgets
NAME      MIN-AVAILABLE   ALLOWED-DISRUPTIONS   AGE
zk-pdb    2               0                     7s

If there are matching pods (say, 3), then you would see something like this:

kubectl get poddisruptionbudgets
NAME      MIN-AVAILABLE   ALLOWED-DISRUPTIONS   AGE
zk-pdb    2               1                     7s

The non-zero value for ALLOWED-DISRUPTIONS means that the disruption controller has seen the pods, counted the matching pods, and updated the status of the PDB.

You can get more information about the status of a PDB with this command:

kubectl get poddisruptionbudgets zk-pdb -o yaml
apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
  creationTimestamp: 2017-08-28T02:38:26Z
  generation: 1
  name: zk-pdb
…
status:
  currentHealthy: 3
  desiredHealthy: 3
  disruptedPods: null
  disruptionsAllowed: 1
  expectedPods: 3
  observedGeneration: 1

Arbitrary Controllers and Selectors

You can skip this section if you only use PDBs with the built-in application controllers (Deployment, ReplicationController, ReplicaSet, and StatefulSet), with the PDB selector matching the controller’s selector.

You can use a PDB with pods controlled by another type of controller, by an “operator”, or bare pods, but with these restrictions:

You can use a selector which selects a subset or superset of the pods belonging to a built-in controller. However, when there are multiple PDBs in a namespace, you must be careful not to create PDBs whose selectors overlap.

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