132 lines
7.0 KiB
Plaintext
132 lines
7.0 KiB
Plaintext
[[modules-discovery-adding-removing-nodes]]
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=== Adding and removing nodes
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As nodes are added or removed Elasticsearch maintains an optimal level of fault
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tolerance by automatically updating the cluster's _voting configuration_, which
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is the set of <<master-node,master-eligible nodes>> whose responses are counted
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when making decisions such as electing a new master or committing a new cluster
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state.
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It is recommended to have a small and fixed number of master-eligible nodes in a
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cluster, and to scale the cluster up and down by adding and removing
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master-ineligible nodes only. However there are situations in which it may be
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desirable to add or remove some master-eligible nodes to or from a cluster.
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[[modules-discovery-adding-nodes]]
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==== Adding master-eligible nodes
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If you wish to add some nodes to your cluster, simply configure the new nodes
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to find the existing cluster and start them up. Elasticsearch adds the new nodes
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to the voting configuration if it is appropriate to do so.
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During master election or when joining an existing formed cluster, a node
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sends a join request to the master in order to be officially added to the
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cluster. You can use the `cluster.join.timeout` setting to configure how long a
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node waits after sending a request to join a cluster. Its default value is `30s`.
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See <<modules-discovery-settings>>.
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[[modules-discovery-removing-nodes]]
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==== Removing master-eligible nodes
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When removing master-eligible nodes, it is important not to remove too many all
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at the same time. For instance, if there are currently seven master-eligible
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nodes and you wish to reduce this to three, it is not possible simply to stop
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four of the nodes at once: to do so would leave only three nodes remaining,
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which is less than half of the voting configuration, which means the cluster
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cannot take any further actions.
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As long as there are at least three master-eligible nodes in the cluster, as a
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general rule it is best to remove nodes one-at-a-time, allowing enough time for
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the cluster to <<modules-discovery-quorums,automatically adjust>> the voting
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configuration and adapt the fault tolerance level to the new set of nodes.
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If there are only two master-eligible nodes remaining then neither node can be
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safely removed since both are required to reliably make progress. You must first
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inform Elasticsearch that one of the nodes should not be part of the voting
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configuration, and that the voting power should instead be given to other nodes.
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You can then take the excluded node offline without preventing the other node
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from making progress. A node which is added to a voting configuration exclusion
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list still works normally, but Elasticsearch tries to remove it from the voting
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configuration so its vote is no longer required. Importantly, Elasticsearch
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will never automatically move a node on the voting exclusions list back into the
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voting configuration. Once an excluded node has been successfully
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auto-reconfigured out of the voting configuration, it is safe to shut it down
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without affecting the cluster's master-level availability. A node can be added
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to the voting configuration exclusion list using the <<voting-config-exclusions>> API. For example:
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[source,js]
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--------------------------------------------------
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# Add node to voting configuration exclusions list and wait for the system to
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# auto-reconfigure the node out of the voting configuration up to the default
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# timeout of 30 seconds
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POST /_cluster/voting_config_exclusions/node_name
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# Add node to voting configuration exclusions list and wait for
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# auto-reconfiguration up to one minute
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POST /_cluster/voting_config_exclusions/node_name?timeout=1m
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--------------------------------------------------
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// CONSOLE
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// TEST[skip:this would break the test cluster if executed]
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The node that should be added to the exclusions list is specified using
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<<cluster-nodes,node filters>> in place of `node_name` here. If a call to the
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voting configuration exclusions API fails, you can safely retry it. Only a
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successful response guarantees that the node has actually been removed from the
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voting configuration and will not be reinstated. If it's the active master that
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was removed from the voting configuration, then it will abdicate to another
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master-eligible node that's still in the voting configuration, if such a node
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is available.
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Although the voting configuration exclusions API is most useful for down-scaling
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a two-node to a one-node cluster, it is also possible to use it to remove
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multiple master-eligible nodes all at the same time. Adding multiple nodes to
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the exclusions list has the system try to auto-reconfigure all of these nodes
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out of the voting configuration, allowing them to be safely shut down while
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keeping the cluster available. In the example described above, shrinking a
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seven-master-node cluster down to only have three master nodes, you could add
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four nodes to the exclusions list, wait for confirmation, and then shut them
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down simultaneously.
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NOTE: Voting exclusions are only required when removing at least half of the
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master-eligible nodes from a cluster in a short time period. They are not
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required when removing master-ineligible nodes, nor are they required when
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removing fewer than half of the master-eligible nodes.
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Adding an exclusion for a node creates an entry for that node in the voting
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configuration exclusions list, which has the system automatically try to
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reconfigure the voting configuration to remove that node and prevents it from
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returning to the voting configuration once it has removed. The current list of
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exclusions is stored in the cluster state and can be inspected as follows:
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[source,js]
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--------------------------------------------------
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GET /_cluster/state?filter_path=metadata.cluster_coordination.voting_config_exclusions
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--------------------------------------------------
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// CONSOLE
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This list is limited in size by the `cluster.max_voting_config_exclusions`
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setting, which defaults to `10`. See <<modules-discovery-settings>>. Since
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voting configuration exclusions are persistent and limited in number, they must
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be cleaned up. Normally an exclusion is added when performing some maintenance
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on the cluster, and the exclusions should be cleaned up when the maintenance is
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complete. Clusters should have no voting configuration exclusions in normal
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operation.
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If a node is excluded from the voting configuration because it is to be shut
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down permanently, its exclusion can be removed after it is shut down and removed
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from the cluster. Exclusions can also be cleared if they were created in error
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or were only required temporarily:
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[source,js]
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--------------------------------------------------
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# Wait for all the nodes with voting configuration exclusions to be removed from
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# the cluster and then remove all the exclusions, allowing any node to return to
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# the voting configuration in the future.
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DELETE /_cluster/voting_config_exclusions
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# Immediately remove all the voting configuration exclusions, allowing any node
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# to return to the voting configuration in the future.
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DELETE /_cluster/voting_config_exclusions?wait_for_removal=false
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--------------------------------------------------
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// CONSOLE
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