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
- Identify an Application to Protect
- Think about how your application reacts to disruptions
- Specifying a PodDisruptionBudget
- Create the PDB object
- Check the status of the PDB
- Arbitrary Controllers and Selectors
Before you begin
- You are the owner of an application running on a Kubernetes cluster that requires high availability.
- You should know how to deploy Replicated Stateless Applications and/or Replicated Stateful Applications.
- You should have read about Pod Disruptions.
- You should confirm with your cluster owner or service provider that they respect Pod Disruption Budgets.
Protecting an Application with a PodDisruptionBudget
- Identify what application you want to protect with a PodDisruptionBudget (PDB).
- Think about how your application reacts to disruptions.
- Create a PDB definition as a YAML file.
- 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
From version 1.15 PDBs support custom controllers where the scale subresource is enabled.
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.
- Stateless frontends:
- Concern: don’t reduce serving capacity by more than 10%.
- Solution: use PDB with minAvailable 90% for example.
- Single-instance Stateful Application:
- Concern: do not terminate this application without talking to me.
- Possible Solution 1: Do not use a PDB and tolerate occasional downtime.
- Possible Solution 2: Set PDB with maxUnavailable=0. Have an understanding (outside of Kubernetes) that the cluster operator needs to consult you before termination. When the cluster operator contacts you, prepare for downtime, and then delete the PDB to indicate readiness for disruption. Recreate afterwards.
- Multiple-instance Stateful application such as Consul, ZooKeeper, or etcd:
- Concern: Do not reduce number of instances below quorum, otherwise writes fail.
- Possible Solution 1: set maxUnavailable to 1 (works with varying scale of application).
- Possible Solution 2: set minAvailable to quorum-size (e.g. 3 when scale is 5). (Allows more disruptions at once).
- Restartable Batch Job:
- Concern: Job needs to complete in case of voluntary disruption.
- Possible solution: Do not create a PDB. The Job controller will create a replacement pod.
Rounding logic when specifying percentages
maxUnavailable can be expressed as integers or as a percentage.
- When you specify an integer, it represents a number of Pods. For instance, if you set
minAvailableto 10, then 10 Pods must always be available, even during a disruption.
- When you specify a percentage by setting the value to a string representation of a percentage (eg.
"50%"), it represents a percentage of total Pods. For instance, if you set
"50%", then only 50% of the Pods can be unavailable during a disruption.
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
"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
that controls this behavior.
Specifying a PodDisruptionBudget
PodDisruptionBudget has three fields:
- A label selector
.spec.selectorto specify the set of pods to which it applies. This field is required.
.spec.minAvailablewhich is a description of the number of pods from that set that must still be available after the eviction, even in the absence of the evicted pod.
minAvailablecan be either an absolute number or a percentage.
.spec.maxUnavailable(available in Kubernetes 1.7 and higher) which is a description of the number of pods from that set that can be unavailable after the eviction. It can be either an absolute number or a percentage.
Note: For versions 1.8 and earlier: When creating a
PodDisruptionBudgetobject using the
kubectlcommand line tool, the
minAvailablefield has a default value of 1 if neither
You can specify only one of
minAvailable in a single
maxUnavailable can only be used to control the eviction of pods
that have an associated controller managing them. In the examples below, “desired replicas”
scale of the controller managing the pods being selected by the
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
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.
If you set
maxUnavailable to 0% or 0, or you set
minAvailable to 100% or the number of replicas,
you are requiring zero voluntary evictions. When you set zero voluntary evictions for a workload
object such as ReplicaSet, then you cannot successfully drain a Node running one of those Pods.
If you try to drain a Node where an unevictable Pod is running, the drain never completes. This is permitted as per the
You can find examples of pod disruption budgets defined below. They match pods with the label
Example PDB Using minAvailable:
Example PDB Using maxUnavailable (Kubernetes 1.7 or higher):
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 or update the PDB object with a command like
kubectl apply -f mypdb.yaml.
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:
.spec.minAvailablecan be used, not
- only an integer value can be used with
.spec.minAvailable, not a percentage.
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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