[[cluster-reroute]]
== Cluster Reroute

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:

[source,js]
--------------------------------------------------
curl -XPOST 'localhost:9200/_cluster/reroute' -d '{
    "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 <<modules-cluster,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.

[float]
=== Retry failed shards

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 <<cluster-reroute,`_reroute`>> API with `?retry_failed`, which
will attempt a single retry round for these shards.