Nodes orchestration

NodeSets overview

The Elasticsearch cluster is specified using a list of NodeSets. Each NodeSet represents a group of Elasticsearch nodes sharing the same specification (both Elasticsearch configuration and Kubernetes Pod configuration).

apiVersion: elasticsearch.k8s.elastic.co/v1beta1
kind: Elasticsearch
metadata:
  name: quickstart
spec:
  version: 8.16.1
  nodeSets:
  - name: master-nodes
    count: 3
    config:
      master: true
      data: false
    volumeClaimTemplates:
    - metadata:
        name: elasticsearch-data
      spec:
        accessModes:
        - ReadWriteOnce
        resources:
          requests:
            storage: 10Gi
        storageClassName: standard
  - name: data-nodes
    count: 10
    config:
      master: false
      data: true
    volumeClaimTemplates:
    - metadata:
        name: elasticsearch-data
      spec:
        accessModes:
        - ReadWriteOnce
        resources:
          requests:
            storage: 1000Gi
        storageClassName: standard

The Elasticsearch resource above defines two NodeSets: one for master nodes, using 10Gi volumes, and one for data nodes, using 1000Gi volumes. The Elasticsearch cluster is composed of 13 nodes: 3 master nodes and 10 data nodes.

Upgrading the cluster

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ECK handles smooth upgrades from one cluster specification to another. You can apply a new Elasticsearch specification at any time.

Here are a few examples based on the Elasticsearch specification above:

To add five additional Elasticsearch data nodes: change count: 10 to count: 15 in the data-nodes NodeSet.
To increase the RAM memory limit of data nodes to 32Gi: set a different resources limits in the existing data-nodes NodeSet PodTemplate.
To replace dedicated master and dedicated data nodes by nodes having both master and data roles: replace the 2 existing NodeSets by a single one with a different name and the corresponding Elasticsearch configuration settings.
To upgrade Elasticsearch from version 7.2.0 to 7.3.0: change the value in the version field.

ECK orchestrates NodeSet changes with no downtime and makes sure that:

Before a node is removed, the relevant data is migrated to other nodes.
When a cluster topology changes, the Elasticsearch orchestration settings discovery.seed_hosts, cluster.initial_master_nodes, discovery.zen.minimum_master_nodes, _cluster/voting_config_exclusions are adjusted accordingly.
Rolling upgrades are performed safely, reusing the PersistentVolumes of the upgraded Elasticsearch nodes.

StatefulSets orchestration

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Behind the scenes, ECK translates each NodeSet specified in the Elasticsearch resource into a StatefulSet in Kubernetes. The StatefulSet specification is based on the NodeSet specification:

count corresponds to the number of replicas in the StatefulSet, each replica leading to the creation of a Pod, which corresponds to a single Elasticsearch node
podTemplate can be used to specify custom settings for the Elasticsearch Pod, overriding the default ones set by ECK on the generated StatefulSet specification
the StatefulSet name is built from the Elasticsearch resource name and the NodeSet name. Each Pod will be assigned the StatefulSet name suffixed by an ordinal. The corresponding Elasticsearch node has the same name as the Pod.

The actual Pod creation is handled by the StatefulSet controller in Kubernetes. ECK relies on the OnDelete StatefulSet update strategy since it needs full control over when and how Pods get upgraded to a new revision.

When a Pod is removed and recreated (maybe with a newer revision), the StatefulSet controller makes sure that the PersistentVolumes attached to the original Pod are then attached to the new Pod.

Cluster upgrade patterns

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Depending on how the NodeSets are updated, ECK handles the Kubernetes resources reconciliation in various ways.

When a new NodeSet is added to the Elasticsearch resource, ECK creates the corresponding StatefulSet. It also sets up Secrets and ConfigMaps to hold the TLS certificates and Elasticsearch configuration files.
When the node count of an existing NodeSet is increased, ECK increases the replicas of the corresponding StatefulSet.
When the node count of an existing NodeSet is decreased, ECK migrates data away from the corresponding Elasticsearch nodes to remove, then decreases the replicas of the corresponding StatefulSet, once data migration is over. Corresponding PersistentVolumeClaims are automatically removed.
When an existing NodeSet is removed, ECK migrates data away from the corresponding Elasticsearch nodes to remove, decreases the StatefulSet replicas accordingly, then finally removes the corresponding StatefulSet.
When the specification of an existing NodeSet is updated (for example the Elasticsearch configuration, or the PodTemplate resources requirements), ECK performs a rolling upgrade of the corresponding Elasticsearch nodes. In order to do so, it follows Elasticsearch rolling upgrade best practices, to slowly upgrade Pods to the newest revision while preventing unavailability of the Elasticsearch cluster. In most cases, it corresponds to restarting Elasticsearch nodes one by one and reusing the same PersistentVolume data. Note that some cluster topologies may cause the cluster to be unavailable during the upgrade.
When an existing NodeSet is renamed, ECK performs the creation of a new NodeSet with the new name, and the removal of the old NodeSet, according to the NodeSet creation and removal patterns described above. Elasticsearch data is migrated away from the deprecated NodeSet before removal. The Elasticsearch resource update strategy controls how many nodes can exist above or below the target node count during the upgrade.

In all these cases, ECK handles StatefulSet operations according to the Elasticsearch orchestration best practices, by adjusting the orchestration settings discovery.seed_hosts, cluster.initial_master_nodes, discovery.zen.minimum_master_nodes, and _cluster/voting_config_exclusions accordingly.

Limitations

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Based on how Kubernetes and StatefulSets operate, ECK orchestration has the following limitations:

Storage requirements (including volume size) of an existing NodeSet cannot be updated. StatefulSet volumes expansion is not available in Kubernetes yet. To upgrade the storage size, you can create a new NodeSet, or rename an existing one. Renaming a NodeSet automatically creates a new StatefulSet with the specified storage size. The original StatefulSet is removed once the Elasticsearch data is migrated to the nodes of the new StatefulSet.
Cluster availability is not be guaranteed in the following cases:
- During the rolling upgrade of single-node clusters
- For clusters that have indices with no replicas

If an Elasticsearch node holds the only copy of a shard, this shard becomes unavailable while the node is upgraded. Clusters with more than one node and at least one replica per index are considered best practice.

Elasticsearch Pods may stay Pending during a rolling upgrade if the Kubernetes scheduler cannot re-schedule them back. This is especially important when using local PersistentVolumes. If the Kubernetes node bound to a local PersistentVolume does not have enough capacity to host an upgraded Pod which was temporarily removed, that Pod will stay Pending.
Rolling upgrades can only make progress if the Elasticsearch cluster health is green. It is risky to attempt upgrading a cluster in the yellow state as some shards could become completely unavailable and degrade the cluster health to red. ECK takes the cautionary approach of waiting for green health before progressing but advanced users may force an upgrade by manually deleting Pods themselves. The deleted Pods will be automatically recreated at the latest revision. There are two exceptions to this rule:
- If all the Elasticsearch nodes of a NodeSet are unavailable, probably caused by a misconfiguration, the operator ignores the cluster health and upgrades nodes of the NodeSet.
- If an Elasticsearch node to upgrade is not healthy, and not part of the Elasticsearch cluster, the operator ignores the cluster health and upgrades the Elasticsearch node.
Elasticsearch versions cannot be downgraded. For example it is impossible to downgrade an existing cluster from version 7.3.0 to 7.2.0. This is not supported by Elasticsearch.

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