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<h2 class="title"><a id="allocation-awareness"></a>Shard allocation awareness<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.2/docs/reference/modules/cluster/allocation_awareness.asciidoc">edit</a></h2>
</div></div></div>
<p>You can use custom node attributes as <em>awareness attributes</em> to enable Elasticsearch
to take your physical hardware configuration into account when allocating shards.
If Elasticsearch 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.</p>
<p>When shard allocation awareness is enabled with the
<code class="literal">cluster.routing.allocation.awareness.attributes</code> setting, shards are only
allocated to nodes that have values set for the specified awareness
attributes. If you use multiple awareness attributes, Elasticsearch considers
each attribute separately when allocating shards.</p>
<p>The allocation awareness settings can be configured in
<code class="literal">elasticsearch.yml</code> and updated dynamically with the
<a class="xref" href="cluster-update-settings.html" title="Cluster Update Settings">cluster-update-settings</a> API.</p>
<p>Elasticsearch prefers using shards in the same location (with the same
awareness attribute values) to process search or GET requests. Using local
shards is usually faster than crossing rack or zone boundaries.</p>
<div class="note admon">
<div class="icon"></div>
<div class="admon_content">
<p>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.</p>
</div>
</div>
<h4><a id="enabling-awareness"></a>Enabling shard allocation awareness<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.2/docs/reference/modules/cluster/allocation_awareness.asciidoc">edit</a></h4>
<p>To enable shard allocation awareness:</p>
<div class="olist orderedlist">
<ol class="orderedlist">
<li class="listitem">
<p>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 <code class="literal">rack_id</code> in each node’s <code class="literal">elasticsearch.yml</code>
config file.</p>
<div class="pre_wrapper lang-yaml">
<pre class="programlisting prettyprint lang-yaml">node.attr.rack_id: rack_one</pre>
</div>
<p>You can also set custom attributes when you start a node:</p>
<div class="pre_wrapper lang-sh">
<pre class="programlisting prettyprint lang-sh">`./bin/elasticsearch -Enode.attr.rack_id=rack_one`</pre>
</div>
</li>
<li class="listitem">
<p>Tell Elasticsearch to take one or more awareness attributes into account when
allocating shards by setting
<code class="literal">cluster.routing.allocation.awareness.attributes</code> in <span class="strong strong"><strong>every</strong></span> master-eligible
node’s <code class="literal">elasticsearch.yml</code> config file.</p>
<div class="pre_wrapper lang-yaml">
<pre class="programlisting prettyprint lang-yaml">cluster.routing.allocation.awareness.attributes: rack_id <a id="CO355-1"></a><i class="conum" data-value="1"></i></pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO355-1"><i class="conum" data-value="1"></i></a></p>
</td>
<td align="left" valign="top">
<p>Specify multiple attributes as a comma-separated list.</p>
</td>
</tr>
</table>
</div>
<p>You can also use the
<a class="xref" href="cluster-update-settings.html" title="Cluster Update Settings">cluster-update-settings</a> API to set or update
a cluster’s awareness attributes.</p>
</li>
</ol>
</div>
<p>With this example configuration, if you start two nodes with
<code class="literal">node.attr.rack_id</code> set to <code class="literal">rack_one</code> and create an index with 5 primary
shards and 1 replica of each primary, all primaries and replicas are
allocated across the two nodes.</p>
<p>If you add two nodes with <code class="literal">node.attr.rack_id</code> set to <code class="literal">rack_two</code>,
Elasticsearch moves shards to the new nodes, ensuring (if possible)
that no two copies of the same shard are in the same rack.</p>
<p>If <code class="literal">rack_two</code> fails and takes down both its nodes, by default Elasticsearch
allocates the lost shard copies to nodes in <code class="literal">rack_one</code>. To prevent multiple
copies of a particular shard from being allocated in the same location, you can
enable forced awareness.</p>
<h4><a id="forced-awareness"></a>Forced awareness<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.2/docs/reference/modules/cluster/allocation_awareness.asciidoc">edit</a></h4>
<p>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 <span class="strong strong"><strong>ALL</strong></span> of the shards.</p>
<p>To prevent a single location from being overloaded in the event of a failure,
you can set <code class="literal">cluster.routing.allocation.awareness.force</code> so no replicas are
allocated until nodes are available in another location.</p>
<p>For example, if you have an awareness attribute called <code class="literal">zone</code> and configure nodes
in <code class="literal">zone1</code> and <code class="literal">zone2</code>, you can use forced awareness to prevent Elasticsearch
from allocating replicas if only one zone is available:</p>
<div class="pre_wrapper lang-yaml">
<pre class="programlisting prettyprint lang-yaml">cluster.routing.allocation.awareness.attributes: zone
cluster.routing.allocation.awareness.force.zone.values: zone1,zone2 <a id="CO356-1"></a><i class="conum" data-value="1"></i></pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO356-1"><i class="conum" data-value="1"></i></a></p>
</td>
<td align="left" valign="top">
<p>Specify all possible values for the awareness attribute.</p>
</td>
</tr>
</table>
</div>
<p>With this example configuration, if you start two nodes with <code class="literal">node.attr.zone</code> set
to <code class="literal">zone1</code> 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 <code class="literal">node.attr.zone</code> set to <code class="literal">zone2</code> are available.</p>
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