# `MachineDisruptionBudget` A `MachineDisruptionBudget` limits how many of the machines it selects can be out of service at the same time. The budget named `default` covers the whole fleet. Add a budget with a selector to give a group of machines a stricter limit. `kubectl get mdb` lists each budget and how many more machines it allows out now. [How `liken` works](/docs/concepts/how-liken-works/#the-reboot-turn) explains how the budgets decide each reboot turn, and [`Disruption`](/docs/reference/disruption/) describes the requests that wait for them. A MachineDisruptionBudget limits how many of the machines it selects may be out of service at the same time, planned and unplanned together. A machine takes a reboot turn only when every budget that selects it has room. The budget named default selects every machine, and cluster-operator creates it with a limit of one machine when it is missing. Whatever the budgets allow, only one leader is out of service at a time. ## spec The machines the budget covers, and the limit. | Field | Type | Required | Description | | --- | --- | --- | --- | | `selector` | [object](#specselector) | no | Selects machines by the labels on their Machine objects, with the grammar of a Kubernetes label selector. An empty selector selects every machine. | | `maxUnavailable` | integer | yes | How many of the selected machines can be out of service at the same time. A machine counts when it holds a reboot turn, or when it is Lost, Degraded, or otherwise unavailable without one. | ### spec.selector Selects machines by the labels on their Machine objects, with the grammar of a Kubernetes label selector. An empty selector selects every machine. | Field | Type | Required | Description | | --- | --- | --- | --- | | `matchLabels` | map[string]string | no | Each label a selected machine carries, with its value. | | `matchExpressions` | [\[\]object](#specselectormatchexpressions) | no | Each expression a selected machine's labels meet. | #### spec.selector.matchExpressions[] Each expression a selected machine's labels meet. | Field | Type | Required | Description | | --- | --- | --- | --- | | `key` | string | yes | | | `operator` | string | yes | One of: `In`, `NotIn`, `Exists`, `DoesNotExist`. | | `values` | []string | no | | ## status What the budget counted on cluster-operator's last sweep. | Field | Type | Required | Description | | --- | --- | --- | --- | | `observedGeneration` | integer | no | | | `selected` | integer | no | How many machines the selector selects. | | `unavailable` | integer | no | How many of the selected machines are out of service. | | `allowed` | integer | no | How many more of the selected machines may go out of service now. | | `conditions` | [\[\]object](#statusconditions) | no | DisruptionAllowed is True while the budget has room for one more machine, and its message names the machines that fill it. | ### status.conditions[] DisruptionAllowed is True while the budget has room for one more machine, and its message names the machines that fill it. | Field | Type | Required | Description | | --- | --- | --- | --- | | `type` | string | yes | | | `status` | string | yes | One of: `True`, `False`, `Unknown`. | | `observedGeneration` | integer | no | | | `lastTransitionTime` | string | yes | | | `reason` | string | yes | | | `message` | string | no | |