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Cluster Reroute

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Cluster Reroute

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The reroute command allows to explicitly execute a cluster reroute allocation command including specific commands. For example, a shard can be moved from one node to another explicitly, an allocation can be canceled, or an unassigned shard can be explicitly allocated on a specific node.

Here is a short example of how a simple reroute API call:

POST /_cluster/reroute
{
    "commands" : [
        {
            "move" : {
                "index" : "test", "shard" : 0,
                "from_node" : "node1", "to_node" : "node2"
            }
        },
        {
          "allocate_replica" : {
                "index" : "test", "shard" : 1,
                "node" : "node3"
          }
        }
    ]
}

An important aspect to remember is the fact that once when an allocation occurs, the cluster will aim at re-balancing its state back to an even state. For example, if the allocation includes moving a shard from node1 to node2, in an even state, then another shard will be moved from node2 to node1 to even things out.

The cluster can be set to disable allocations, which means that only the explicitly allocations will be performed. Obviously, only once all commands has been applied, the cluster will aim to be re-balance its state.

Another option is to run the commands in dry_run (as a URI flag, or in the request body). This will cause the commands to apply to the current cluster state, and return the resulting cluster after the commands (and re-balancing) has been applied.

If the explain parameter is specified, a detailed explanation of why the commands could or could not be executed is returned.

The commands supported are:

move: Move a started shard from one node to another node. Accepts index and shard for index name and shard number, from_node for the node to move the shard from, and to_node for the node to move the shard to.
cancel: Cancel allocation of a shard (or recovery). Accepts index and shard for index name and shard number, and node for the node to cancel the shard allocation on. It also accepts allow_primary flag to explicitly specify that it is allowed to cancel allocation for a primary shard. This can be used to force resynchronization of existing replicas from the primary shard by cancelling them and allowing them to be reinitialized through the standard reallocation process.
allocate_replica: Allocate an unassigned replica shard to a node. Accepts the index and shard for index name and shard number, and node to allocate the shard to. Takes allocation deciders into account.

Two more commands are available that allow the allocation of a primary shard to a node. These commands should however be used with extreme care, as primary shard allocation is usually fully automatically handled by Elasticsearch. Reasons why a primary shard cannot be automatically allocated include the following:

A new index was created but there is no node which satisfies the allocation deciders.
An up-to-date shard copy of the data cannot be found on the current data nodes in the cluster. To prevent data loss, the system does not automatically promote a stale shard copy to primary.

As a manual override, two commands to forcefully allocate primary shards are available:

allocate_stale_primary: Allocate a primary shard to a node that holds a stale copy. Accepts the index and shard for index name and shard number, and node to allocate the shard to. Using this command may lead to data loss for the provided shard id. If a node which has the good copy of the data rejoins the cluster later on, that data will be overwritten with the data of the stale copy that was forcefully allocated with this command. To ensure that these implications are well-understood, this command requires the special field accept_data_loss to be explicitly set to true for it to work.
allocate_empty_primary: Allocate an empty primary shard to a node. Accepts the index and shard for index name and shard number, and node to allocate the shard to. Using this command leads to a complete loss of all data that was indexed into this shard, if it was previously started. If a node which has a copy of the data rejoins the cluster later on, that data will be deleted! To ensure that these implications are well-understood, this command requires the special field accept_data_loss to be explicitly set to true for it to work.

Retry failed shards

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The cluster will attempt to allocate a shard a maximum of index.allocation.max_retries times in a row (defaults to 5), before giving up and leaving the shard unallocated. This scenario can be caused by structural problems such as having an analyzer which refers to a stopwords file which doesn’t exist on all nodes.

Once the problem has been corrected, allocation can be manually retried by calling the reroute API with ?retry_failed, which will attempt a single retry round for these shards.

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