--- title: Specifying a Disruption Budget for your Application content_template: templates/task --- {{% capture overview %}} 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. {{% /capture %}} {{% capture prerequisites %}} * 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](/docs/tasks/run-application/run-stateless-application-deployment/) and/or [Replicated Stateful Applications](/docs/tasks/run-application/run-replicated-stateful-application/). * You should have read about [Pod Disruptions](/docs/concepts/workloads/pods/disruptions/). * You should confirm with your cluster owner or service provider that they respect Pod Disruption Budgets. {{% /capture %}} {{% capture steps %}} ## Protecting an Application with a PodDisruptionBudget 1. Identify what application you want to protect with a PodDisruptionBudget (PDB). 1. Think about how your application reacts to disruptions. 1. Create a PDB definition as a YAML file. 1. Create the PDB object from the YAML file. {{% /capture %}} {{% capture discussion %}} ## 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: - Deployment - ReplicationController - ReplicaSet - StatefulSet 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](#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. ## Specifying a PodDisruptionBudget A `PodDisruptionBudget` has three fields: * A label selector `.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 >}} **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. {{< /note >}} 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: ```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): ```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 create -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: ```shell $ 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: ```shell $ 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 update the status of the PDB. You can get more information about the status of a PDB with this command: ```shell $ 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: - only `.spec.minAvailable` can be used, not `.spec.maxUnavailable`. - 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. {{% /capture %}}