[[index-modules-allocation]]
== Index Shard Allocation

[float]
=== Shard Allocation Filtering

Allow to control allocation if indices on nodes based on include/exclude
filters. The filters can be set both on the index level and on the
cluster level. Lets start with an example of setting it on the cluster
level:

Lets say we have 4 nodes, each has specific attribute called `tag`
associated with it (the name of the attribute can be any name). Each
node has a specific value associated with `tag`. Node 1 has a setting
`node.tag: value1`, Node 2 a setting of `node.tag: value2`, and so on.

We can create an index that will only deploy on nodes that have `tag`
set to `value1` and `value2` by setting
`index.routing.allocation.include.tag` to `value1,value2`. For example:

[source,js]
--------------------------------------------------
curl -XPUT localhost:9200/test/_settings -d '{
    "index.routing.allocation.include.tag" : "value1,value2"
}'
--------------------------------------------------

On the other hand, we can create an index that will be deployed on all
nodes except for nodes with a `tag` of value `value3` by setting
`index.routing.allocation.exclude.tag` to `value3`. For example:

[source,js]
--------------------------------------------------
curl -XPUT localhost:9200/test/_settings -d '{
    "index.routing.allocation.exclude.tag" : "value3"
}'
--------------------------------------------------

From version 0.90, `index.routing.allocation.require.*` can be used to 
specify a number of rules, all of which MUST match in order for a shard
to be allocated to a node. This is in contrast to `include` which will
include a node if ANY rule matches.

The `include`, `exclude` and `require` values can have generic simple
matching wildcards, for example, `value1*`. A special attribute name
called `_ip` can be used to match on node ip values.

Obviously a node can have several attributes associated with it, and
both the attribute name and value are controlled in the setting. For
example, here is a sample of several node configurations:

[source,js]
--------------------------------------------------
node.group1: group1_value1
node.group2: group2_value4
--------------------------------------------------

In the same manner, `include`, `exclude` and `require` can work against
several attributes, for example:

[source,js]
--------------------------------------------------
curl -XPUT localhost:9200/test/_settings -d '{
    "index.routing.allocation.include.group1" : "xxx"
    "index.routing.allocation.include.group2" : "yyy",
    "index.routing.allocation.exclude.group3" : "zzz",
    "index.routing.allocation.require.group4" : "aaa",
}'
--------------------------------------------------

The provided settings can also be updated in real time using the update
settings API, allowing to "move" indices (shards) around in realtime.

Cluster wide filtering can also be defined, and be updated in real time
using the cluster update settings API. This setting can come in handy
for things like decommissioning nodes (even if the replica count is set
to 0). Here is a sample of how to decommission a node based on `_ip`
address:

[source,js]
--------------------------------------------------
curl -XPUT localhost:9200/_cluster/settings -d '{
    "transient" : {
        "cluster.routing.allocation.exclude._ip" : "10.0.0.1"
    }
}'
--------------------------------------------------

[float]
=== Total Shards Per Node

The `index.routing.allocation.total_shards_per_node` setting allows to
control how many total shards for an index will be allocated per node.
It can be dynamically set on a live index using the update index
settings API.