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Shard allocation awareness

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Shard allocation awareness

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You can use custom node attributes as awareness attributes 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.

When shard allocation awareness is enabled with the 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, Elasticsearch considers each attribute separately when allocating shards.

The allocation awareness settings can be configured in elasticsearch.yml and updated dynamically with the cluster-update-settings API.

By default Elasticsearch uses adaptive replica selection to route search or GET requests. However, with the presence of allocation awareness attributes Elasticsearch 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.

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 shard allocation awareness

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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.
```
node.attr.rack_id: rack_one
```
You can also set custom attributes when you start a node:
```
`./bin/elasticsearch -Enode.attr.rack_id=rack_one`
```
Tell Elasticsearch 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.
```
cluster.routing.allocation.awareness.attributes: rack_id 
```
Specify multiple attributes as a comma-separated list.

You can also use the 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, Elasticsearch 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 Elasticsearch 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

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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:

cluster.routing.allocation.awareness.attributes: zone
cluster.routing.allocation.awareness.force.zone.values: zone1,zone2

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.

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