[[node-tool]] == elasticsearch-node The `elasticsearch-node` command enables you to perform unsafe operations that risk data loss but which may help to recover some data in a disaster. [float] === Synopsis [source,shell] -------------------------------------------------- bin/elasticsearch-node unsafe-bootstrap|detach-cluster [--ordinal ] [-E ] [-h, --help] ([-s, --silent] | [-v, --verbose]) -------------------------------------------------- [float] === Description Sometimes {es} nodes are temporarily stopped, perhaps because of the need to perform some maintenance activity or perhaps because of a hardware failure. After you resolve the temporary condition and restart the node, it will rejoin the cluster and continue normally. Depending on your configuration, your cluster may be able to remain completely available even while one or more of its nodes are stopped. Sometimes it might not be possible to restart a node after it has stopped. For example, the node's host may suffer from a hardware problem that cannot be repaired. If the cluster is still available then you can start up a fresh node on another host and {es} will bring this node into the cluster in place of the failed node. Each node stores its data in the data directories defined by the <>. This means that in a disaster you can also restart a node by moving its data directories to another host, presuming that those data directories can be recovered from the faulty host. {es} <> in order to elect a master and to update the cluster state. This means that if you have three master-eligible nodes then the cluster will remain available even if one of them has failed. However if two of the three master-eligible nodes fail then the cluster will be unavailable until at least one of them is restarted. In very rare circumstances it may not be possible to restart enough nodes to restore the cluster's availability. If such a disaster occurs, you should build a new cluster from a recent snapshot and re-import any data that was ingested since that snapshot was taken. However, if the disaster is serious enough then it may not be possible to recover from a recent snapshot either. Unfortunately in this case there is no way forward that does not risk data loss, but it may be possible to use the `elasticsearch-node` tool to construct a new cluster that contains some of the data from the failed cluster. This tool has two modes: * `elastisearch-node unsafe-bootstap` can be used if there is at least one remaining master-eligible node. It forces one of the remaining nodes to form a brand-new cluster on its own, using its local copy of the cluster metadata. This is known as _unsafe cluster bootstrapping_. * `elastisearch-node detach-cluster` enables you to move nodes from one cluster to another. This can be used to move nodes into the new cluster created with the `elastisearch-node unsafe-bootstap` command. If unsafe cluster bootstrapping was not possible, it also enables you to move nodes into a brand-new cluster. [[node-tool-unsafe-bootstrap]] [float] ==== Unsafe cluster bootstrapping If there is at least one remaining master-eligible node, but it is not possible to restart a majority of them, then the `elasticsearch-node unsafe-bootstrap` command will unsafely override the cluster's <> as if performing another <>. The target node can then form a new cluster on its own by using the cluster metadata held locally on the target node. [WARNING] These steps can lead to arbitrary data loss since the target node may not hold the latest cluster metadata, and this out-of-date metadata may make it impossible to use some or all of the indices in the cluster. Since unsafe bootstrapping forms a new cluster containing a single node, once you have run it you must use the <> to migrate any other surviving nodes from the failed cluster into this new cluster. When you run the `elasticsearch-node unsafe-bootstrap` tool it will analyse the state of the node and ask for confirmation before taking any action. Before asking for confirmation it reports the term and version of the cluster state on the node on which it runs as follows: [source,txt] ---- Current node cluster state (term, version) pair is (4, 12) ---- If you have a choice of nodes on which to run this tool then you should choose one with a term that is as large as possible. If there is more than one node with the same term, pick the one with the largest version. This information identifies the node with the freshest cluster state, which minimizes the quantity of data that might be lost. For example, if the first node reports `(4, 12)` and a second node reports `(5, 3)`, then the second node is preferred since its term is larger. However if the second node reports `(3, 17)` then the first node is preferred since its term is larger. If the second node reports `(4, 10)` then it has the same term as the first node, but has a smaller version, so the first node is preferred. [WARNING] Running this command can lead to arbitrary data loss. Only run this tool if you understand and accept the possible consequences and have exhausted all other possibilities for recovery of your cluster. The sequence of operations for using this tool are as follows: 1. Make sure you have really lost access to at least half of the master-eligible nodes in the cluster, and they cannot be repaired or recovered by moving their data paths to healthy hardware. 2. Stop **all** remaining nodes. 3. Choose one of the remaining master-eligible nodes to become the new elected master as described above. 4. On this node, run the `elasticsearch-node unsafe-bootstrap` command as shown below. Verify that the tool reported `Master node was successfully bootstrapped`. 5. Start this node and verify that it is elected as the master node. 6. Run the <>, described below, on every other node in the cluster. 7. Start all other nodes and verify that each one joins the cluster. 8. Investigate the data in the cluster to discover if any was lost during this process. When you run the tool it will make sure that the node that is being used to bootstrap the cluster is not running. It is important that all other master-eligible nodes are also stopped while this tool is running, but the tool does not check this. The message `Master node was successfully bootstrapped` does not mean that there has been no data loss, it just means that tool was able to complete its job. [[node-tool-detach-cluster]] [float] ==== Detaching nodes from their cluster It is unsafe for nodes to move between clusters, because different clusters have completely different cluster metadata. There is no way to safely merge the metadata from two clusters together. To protect against inadvertently joining the wrong cluster, each cluster creates a unique identifier, known as the _cluster UUID_, when it first starts up. Every node records the UUID of its cluster and refuses to join a cluster with a different UUID. However, if a node's cluster has permanently failed then it may be desirable to try and move it into a new cluster. The `elasticsearch-node detach-cluster` command lets you detach a node from its cluster by resetting its cluster UUID. It can then join another cluster with a different UUID. For example, after unsafe cluster bootstrapping you will need to detach all the other surviving nodes from their old cluster so they can join the new, unsafely-bootstrapped cluster. Unsafe cluster bootstrapping is only possible if there is at least one surviving master-eligible node. If there are no remaining master-eligible nodes then the cluster metadata is completely lost. However, the individual data nodes also contain a copy of the index metadata corresponding with their shards. This sometimes allows a new cluster to import these shards as <>. You can sometimes recover some indices after the loss of all master-eligible nodes in a cluster by creating a new cluster and then using the `elasticsearch-node detach-cluster` command to move any surviving nodes into this new cluster. There is a risk of data loss when importing a dangling index because data nodes may not have the most recent copy of the index metadata and do not have any information about <>. This means that a stale shard copy may be selected to be the primary, and some of the shards may be incompatible with the imported mapping. [WARNING] Execution of this command can lead to arbitrary data loss. Only run this tool if you understand and accept the possible consequences and have exhausted all other possibilities for recovery of your cluster. The sequence of operations for using this tool are as follows: 1. Make sure you have really lost access to every one of the master-eligible nodes in the cluster, and they cannot be repaired or recovered by moving their data paths to healthy hardware. 2. Start a new cluster and verify that it is healthy. This cluster may comprise one or more brand-new master-eligible nodes, or may be an unsafely-bootstrapped cluster formed as described above. 3. Stop **all** remaining data nodes. 4. On each data node, run the `elasticsearch-node detach-cluster` tool as shown below. Verify that the tool reported `Node was successfully detached from the cluster`. 5. If necessary, configure each data node to <>. 6. Start each data node and verify that it has joined the new cluster. 7. Wait for all recoveries to have completed, and investigate the data in the cluster to discover if any was lost during this process. The message `Node was successfully detached from the cluster` does not mean that there has been no data loss, it just means that tool was able to complete its job. [float] === Parameters `unsafe-bootstrap`:: Specifies to unsafely bootstrap this node as a new one-node cluster. `detach-cluster`:: Specifies to unsafely detach this node from its cluster so it can join a different cluster. `--ordinal `:: If there is <> then this specifies which node to target. Defaults to `0`, meaning to use the first node in the data path. `-E `:: Configures a setting. `-h, --help`:: Returns all of the command parameters. `-s, --silent`:: Shows minimal output. `-v, --verbose`:: Shows verbose output. [float] === Examples [float] ==== Unsafe cluster bootstrapping Suppose your cluster had five master-eligible nodes and you have permanently lost three of them, leaving two nodes remaining. * Run the tool on the first remaining node, but answer `n` at the confirmation step. [source,txt] ---- node_1$ ./bin/elasticsearch-node unsafe-bootstrap WARNING: Elasticsearch MUST be stopped before running this tool. Current node cluster state (term, version) pair is (4, 12) You should only run this tool if you have permanently lost half or more of the master-eligible nodes in this cluster, and you cannot restore the cluster from a snapshot. This tool can cause arbitrary data loss and its use should be your last resort. If you have multiple surviving master eligible nodes, you should run this tool on the node with the highest cluster state (term, version) pair. Do you want to proceed? Confirm [y/N] n ---- * Run the tool on the second remaining node, and again answer `n` at the confirmation step. [source,txt] ---- node_2$ ./bin/elasticsearch-node unsafe-bootstrap WARNING: Elasticsearch MUST be stopped before running this tool. Current node cluster state (term, version) pair is (5, 3) You should only run this tool if you have permanently lost half or more of the master-eligible nodes in this cluster, and you cannot restore the cluster from a snapshot. This tool can cause arbitrary data loss and its use should be your last resort. If you have multiple surviving master eligible nodes, you should run this tool on the node with the highest cluster state (term, version) pair. Do you want to proceed? Confirm [y/N] n ---- * Since the second node has a greater term it has a fresher cluster state, so it is better to unsafely bootstrap the cluster using this node: [source,txt] ---- node_2$ ./bin/elasticsearch-node unsafe-bootstrap WARNING: Elasticsearch MUST be stopped before running this tool. Current node cluster state (term, version) pair is (5, 3) You should only run this tool if you have permanently lost half or more of the master-eligible nodes in this cluster, and you cannot restore the cluster from a snapshot. This tool can cause arbitrary data loss and its use should be your last resort. If you have multiple surviving master eligible nodes, you should run this tool on the node with the highest cluster state (term, version) pair. Do you want to proceed? Confirm [y/N] y Master node was successfully bootstrapped ---- [float] ==== Detaching nodes from their cluster After unsafely bootstrapping a new cluster, run the `elasticsearch-node detach-cluster` command to detach all remaining nodes from the failed cluster so they can join the new cluster: [source, txt] ---- node_3$ ./bin/elasticsearch-node detach-cluster WARNING: Elasticsearch MUST be stopped before running this tool. You should only run this tool if you have permanently lost all of the master-eligible nodes in this cluster and you cannot restore the cluster from a snapshot, or you have already unsafely bootstrapped a new cluster by running `elasticsearch-node unsafe-bootstrap` on a master-eligible node that belonged to the same cluster as this node. This tool can cause arbitrary data loss and its use should be your last resort. Do you want to proceed? Confirm [y/N] y Node was successfully detached from the cluster ----