[[shard-allocation-awareness]]
==== Shard allocation awareness

You can use custom node attributes as _awareness attributes_ to enable {es}
to take your physical hardware configuration into account when allocating shards.
If {es} knows which nodes are on the same physical server, in the same rack, or
in the same zone, it can distribute the primary shard and its replica shards to
minimise the risk of losing all shard copies in the event of a failure.

When shard allocation awareness is enabled with the
<<dynamic-cluster-setting,dynamic>>
`cluster.routing.allocation.awareness.attributes` setting, shards are only
allocated to nodes that have values set for the specified awareness attributes.
If you use multiple awareness attributes, {es} considers each attribute
separately when allocating shards.

By default {es} uses <<search-adaptive-replica,adaptive replica selection>>
to route search or GET requests. However, with the presence of allocation awareness
attributes {es} will prefer using shards in the same location (with the same
awareness attribute values) to process these requests. This behavior can be
disabled by specifying `export ES_JAVA_OPTS="$ES_JAVA_OPTS -Des.search.ignore_awareness_attributes=true"`
system property on every node that is part of the cluster.

NOTE: The number of attribute values determines how many shard copies are
allocated in each location. If the number of nodes in each location is
unbalanced and there are a lot of replicas, replica shards might be left
unassigned.

[[enabling-awareness]]
===== Enabling shard allocation awareness

To enable shard allocation awareness:

. Specify the location of each node with a custom node attribute. For example,
if you want Elasticsearch to distribute shards across different racks, you might
set an awareness attribute called `rack_id` in each node's `elasticsearch.yml`
config file.
+
[source,yaml]
--------------------------------------------------------
node.attr.rack_id: rack_one
--------------------------------------------------------
+
You can also set custom attributes when you start a node:
+
[source,sh]
--------------------------------------------------------
`./bin/elasticsearch -Enode.attr.rack_id=rack_one`
--------------------------------------------------------

. Tell {es} to take one or more awareness attributes into account when
allocating shards by setting
`cluster.routing.allocation.awareness.attributes` in *every* master-eligible
node's `elasticsearch.yml` config file.
+
--
[source,yaml]
--------------------------------------------------------
cluster.routing.allocation.awareness.attributes: rack_id <1>
--------------------------------------------------------
<1> Specify multiple attributes as a comma-separated list.
--
+
You can also use the
<<cluster-update-settings,cluster-update-settings>> API to set or update
a cluster's awareness attributes.

With this example configuration, if you start two nodes with
`node.attr.rack_id` set to `rack_one` and create an index with 5 primary
shards and 1 replica of each primary, all primaries and replicas are
allocated across the two nodes.

If you add two nodes with `node.attr.rack_id` set to `rack_two`,
{es} moves shards to the new nodes, ensuring (if possible)
that no two copies of the same shard are in the same rack.

If `rack_two` fails and takes down both its nodes, by default {es}
allocates the lost shard copies to nodes in `rack_one`. To prevent multiple
copies of a particular shard from being allocated in the same location, you can
enable forced awareness.

[[forced-awareness]]
===== Forced awareness

By default, if one location fails, Elasticsearch assigns all of the missing
replica shards to the remaining locations. While you might have sufficient
resources across all locations to host your primary and replica shards, a single
location might be unable to host *ALL* of the shards.

To prevent a single location from being overloaded in the event of a failure,
you can set `cluster.routing.allocation.awareness.force` so no replicas are
allocated until nodes are available in another location.

For example, if you have an awareness attribute called `zone` and configure nodes
in `zone1` and `zone2`, you can use forced awareness to prevent Elasticsearch
from allocating replicas if only one zone is available:

[source,yaml]
-------------------------------------------------------------------
cluster.routing.allocation.awareness.attributes: zone
cluster.routing.allocation.awareness.force.zone.values: zone1,zone2 <1>
-------------------------------------------------------------------
<1> Specify all possible values for the awareness attribute.

With this example configuration, if you start two nodes with `node.attr.zone` set
to `zone1` and create an index with 5 shards and 1 replica, Elasticsearch creates
the index and allocates the 5 primary shards but no replicas. Replicas are
only allocated once nodes with `node.attr.zone` set to `zone2` are available.
