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.
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.
Decide how many instances can be down at the same time for a short period due to a voluntary disruption.
A PodDisruptionBudget
has three fields:
.spec.selector
to specify the set of
pods to which it applies. This field is required..spec.minAvailable
which 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. minAvailable
can 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 aPodDisruptionBudget
object using thekubectl
command line tool, theminAvailable
field has a default value of 1 if neitherminAvailable
normaxUnavailable
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:
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
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.
You can create the PDB object with a command like kubectl create -f mypdb.yaml
.
You cannot update PDB objects. They must be deleted and re-created.
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
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.minAvailable
can be used, not .spec.maxUnavailable
..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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