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This guide is for anyone wishing to specify safety constraints on pods or anyone wishing to write software (typically automation software) that respects those constraints.

Rationale

Various cluster management operations may voluntarily evict pods. “Voluntary” means an eviction can be safely delayed for a reasonable period of time. The principal examples today are draining a node for maintenance or upgrade (kubectl drain), and cluster autoscaling down. In the future the rescheduler may also perform voluntary evictions. By contrast, something like evicting pods because a node has become unreachable or reports NotReady, is not “voluntary.”

For voluntary evictions, it can be useful for applications to be able to limit the number of pods that are down simultaneously. 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, even temporarily. Or a web front end might want to ensure that the number of replicas serving load never falls below a certain percentage of the total, even briefly. PodDisruptionBudget is an API object that specifies the minimum number or percentage of replicas of a collection that must be up at a time. Components that wish to evict a pod subject to disruption budget use the /eviction subresource; unlike a regular pod deletion, this operation may be rejected by the API server if the eviction would cause a disruption budget to be violated.

Specifying a PodDisruptionBudget

A PodDisruptionBudget has two components: a label selector selector to specify the set of pods to which it applies, and minAvailable which is a description of the number of pods from that set that must still be available after the eviction, i.e. even in the absence of the evicted pod. minAvailable can be either an absolute number or a percentage. So for example, 100% means no voluntary evictions from the set are permitted. 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.

Note that 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.

Requesting an eviction

If you are writing infrastructure software that wants to produce these voluntary evictions, you will need to use the eviction API. The eviction subresource of a pod can be thought of as a kind of policy-controlled DELETE operation on the pod itself. To attempt an eviction (perhaps more REST-precisely, to attempt to create an eviction), you POST an attempted operation. Here’s an example:

{
  "apiVersion": "policy/v1alpha1",
  "kind": "Eviction",
  "metadata": {
    "name": "quux",
    "namespace": "default"
  }
}

You can attempt an eviction using curl:

$ curl -v -H 'Content-type: application/json' http://127.0.0.1:8080/api/v1/namespaces/default/pods/quux/eviction -d @eviction.json

The API can respond in one of three ways.

  1. If the eviction is granted, then the pod is deleted just as if you had sent a DELETE request to the pod’s URL and you get back 200 OK.
  2. If the current state of affairs wouldn’t allow an eviction by the rules set forth in the budget, you get back 429 Too Many Requests. This is typically used for generic rate limiting of any requests, but here we mean that this request isn’t allowed right now but it may be allowed later. Currently, callers do not get any Retry-After advice, but they may in future versions.
  3. If there is some kind of misconfiguration, like multiple budgets pointing at the same pod, you will get 500 Internal Server Error.

For a given eviction request, there are two cases.

  1. There is no budget that matches this pod. In this case, the server always returns 200 OK.
  2. There is at least one budget. In this case, any of the three above responses may apply.

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