[[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" : {
              "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`::
    Allocate an unassigned shard to a node. Accepts the
    `index` and `shard` for index name and shard number, and `node` to
    allocate the shard to. It also accepts `allow_primary` flag to
    explicitly specify that it is allowed to explicitly allocate a primary
    shard (might result in data loss).

WARNING: The `allow_primary` parameter will force a new _empty_ primary shard
to be allocated *without any data*. If a node which has a copy of the original
primary shard (including data) rejoins the cluster later on, that data will be
deleted: the old shard copy will be replaced by the new live shard copy.