When running nodes on multiple VMs on the same physical server, on multiple racks, or across multiple zones or domains, it is more likely that two nodes on the same physical server, in the same rack, or in the same zone or domain will crash at the same time, rather than two unrelated nodes crashing simultaneously.

If Elasticsearch is aware of the physical configuration of your hardware, it can ensure that the primary shard and its replica shards are spread across different physical servers, racks, or zones, to minimise the risk of losing all shard copies at the same time.

The shard allocation awareness settings allow you to tell Elasticsearch about your hardware configuration.

As an example, let’s assume we have several racks. When we start a node, we can tell it which rack it is in by assigning it an arbitrary metadata attribute called rack_id — we could use any attribute name. For example:

./bin/elasticsearch -Enode.attr.rack_id=rack_one 

Now, we need to set up shard allocation awareness by telling Elasticsearch which attributes to use. This can be configured in the elasticsearch.yml file on all master-eligible nodes, or it can be set (and changed) with the cluster-update-settings API.

For our example, we’ll set the value in the config file:

cluster.routing.allocation.awareness.attributes: rack_id

With this config in place, let’s say we start two nodes with node.attr.rack_id set to rack_one, and we create an index with 5 primary shards and 1 replica of each primary. All primaries and replicas are allocated across the two nodes.

Now, if we start two more nodes with node.attr.rack_id set to rack_two, Elasticsearch will move shards across to the new nodes, ensuring (if possible) that no two copies of the same shard will be in the same rack. However if rack_two were to fail, taking down both of its nodes, Elasticsearch will still allocate the lost shard copies to nodes in rack_one.

Multiple awareness attributes can be specified, in which case each attribute is considered separately when deciding where to allocate the shards.

cluster.routing.allocation.awareness.attributes: rack_id,zone

When using awareness attributes, shards will not be allocated to nodes that don’t have values set for those attributes.

Number of primary/replica of a shard allocated on a specific group of nodes with the same awareness attribute value is determined by the number of attribute values. When the number of nodes in groups is unbalanced and there are many replicas, replica shards may be left unassigned.

Forced Awarenessedit

Imagine that you have two zones and enough hardware across the two zones to host all of your primary and replica shards. But perhaps the hardware in a single zone, while sufficient to host half the shards, would be unable to host ALL the shards.

With ordinary awareness, if one zone lost contact with the other zone, Elasticsearch would assign all of the missing replica shards to a single zone. But in this example, this sudden extra load would cause the hardware in the remaining zone to be overloaded.

Forced awareness solves this problem by NEVER allowing copies of the same shard to be allocated to the same zone.

For example, lets say we have an awareness attribute called zone, and we know we are going to have two zones, zone1 and zone2. Here is how we can force awareness on a node:

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

Now, if we start 2 nodes with node.attr.zone set to zone1 and create an index with 5 shards and 1 replica. The index will be created, but only the 5 primary shards will be allocated (with no replicas). Only when we start more nodes with node.attr.zone set to zone2 will the replicas be allocated.

The cluster.routing.allocation.awareness.* settings can all be updated dynamically on a live cluster with the cluster-update-settings API.