Update strategy

You can use the updateStrategy specification to limit the number of simultaneous changes, like for example in the following cases:

The operator takes a Pod down to restart a node and applies a new configuration value.
The operator must spin up Pods above what is currently in the specification to migrate to a new node set.

spec:
  updateStrategy:
    changeBudget:
      maxSurge: 3
      maxUnavailable: 1

maxSurge: Refers to the number of extra Pods that can be temporarily scheduled exceeding the number of Pods defined in the specification. This setting is useful for controlling the resource usage of the Kubernetes cluster when nodeSet configuration changes and new Pods need to be spun up to replace existing Pods. MaxSurge restricts the number of extra pods that can be running at any given point in time. If you have a large Elasticsearch cluster or a Kubernetes cluster running near capacity, not setting maxSurge could cause the newly created pods to temporarily use up all available spare resource capacity in the Kubernetes cluster and starve other workloads running there.

maxUnavailable: Refers to the number of Pods that can be unavailable out of the total number of Pods in the currently applied specification. A Pod is defined unavailable when it is not ready from a Kubernetes perspective.

The operator only tries to apply these constraints when a new specification is being applied. It is possible that the cluster state does not conform to the constraints at the beginning of the operation due to external factors. The operator will attempt to get to the desired state by adding or removing Pods as necessary while ensuring that the constraints are still satisfied.

For example, if a new specification defines a larger cluster with maxUnavailable: 0, the operator creates the missing Pods according to the best practices. Similarly, if a new specification defines a smaller cluster with maxSurge: 0, the operator safely removes the unnecessary Pods.

Specify changeBudget

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For both maxSurge and maxUnavailable you can specify the following values:

null - The default value is used.
non-negative - The value is used as is.
negative - The value is unbounded.

Default behavior

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When updateStrategy is not present in the specification, it defaults to the following:

spec:
  updateStrategy:
    changeBudget:
      maxSurge: -1
      maxUnavailable: 1

maxSurge is unbounded: This means that all the required Pods are created immediately. maxUnavailable defaults to 1: This ensures that the cluster has no more than one unavailable Pod at any given point in time.

Caveats

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With both maxSurge and maxUnavailable set to 0, the operator cannot bring down an existing Pod nor create a new Pod.
Due to the safety measures employed by the operator, certain changeBudget might prevent the operator from making any progress . For example, with maxSurge set to 0, you cannot remove the last data node from one nodeSet and add a data node to a different nodeSet. In this case, the operator cannot create the new node because maxSurge is 0, and it cannot remove the old node because there are no other data nodes to migrate the data to.
For certain complex configurations, the operator might not be able to deduce the optimal order of operations necessary to achieve the desired outcome. If progress is blocked, you may need to update the maxSurge setting to a higher value than the theoretical best to help the operator make progress in that case.

If any of the above occurs, the operator generates logs to indicate that upscaling or downscaling are limited by maxSurge or maxUnavailable settings.

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