Shard allocation awareness in Elasticsearch - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the effect of this shard allocation awareness setting?

Given this Elasticsearch cluster setting, what will be the output of GET _cluster/settings?include_defaults=true regarding awareness attributes?

Elasticsearch

PUT _cluster/settings
{
  "persistent": {
    "cluster.routing.allocation.awareness.attributes": "rack_id"
  }
}

A The cluster will distribute shards evenly across nodes with different rack_id values.

B The cluster will allocate all shards only to nodes with the same rack_id.

C The cluster will ignore rack_id and allocate shards randomly.

D The cluster will disable shard allocation awareness.

💡 Hint

Think about what shard allocation awareness does with the specified attribute.

Setting cluster.routing.allocation.awareness.attributes to rack_id tells Elasticsearch to spread shards across nodes with different rack_id values to improve fault tolerance.

🧠 Conceptual

intermediate

What happens if a node lacks the awareness attribute?

In an Elasticsearch cluster with shard allocation awareness set to zone, what happens if a node does not have the zone attribute configured?

A The node will receive only replica shards.

B The node will receive shards normally, ignoring awareness settings.

C The node will not receive any shards because it lacks the awareness attribute.

D The cluster will fail to allocate shards and throw an error.

💡 Hint

Consider how awareness attributes control shard distribution.

Nodes without the configured awareness attribute are excluded from shard allocation to ensure shards are spread across distinct attribute values.

❓ Predict Output

advanced

What is the output of this shard allocation command?

What will be the output of this command in an Elasticsearch cluster with awareness attribute rack set, when trying to allocate a shard to a node without the rack attribute?

Elasticsearch

POST /_cluster/reroute
{
  "commands": [
    {
      "allocate": {
        "index": "my_index",
        "shard": 0,
        "node": "node_without_rack",
        "allow_primary": true
      }
    }
  ]
}

A {"acknowledged": false, "error": "allocation failed due to awareness attribute mismatch"}

B {"acknowledged": true, "explanation": "shard allocated successfully"}

C {"acknowledged": false, "error": "node not found"}

D {"acknowledged": true, "explanation": "shard allocated ignoring awareness"}

💡 Hint

Think about how awareness attributes restrict shard allocation.

Elasticsearch prevents allocating shards to nodes missing the required awareness attribute to maintain balanced distribution.

🧠 Conceptual

advanced

How does forced awareness affect shard allocation?

If cluster.routing.allocation.awareness.force.zone.values is set to ["zone1", "zone2"], what is the effect on shard allocation?

A Shards will be allocated randomly without considering zones.

B Shards will be allocated to any zone, but prefer zone1 and zone2.

C Shards will be allocated only if all zones have at least one node available.

D Shards will only be allocated to nodes in zone1 and zone2, ignoring other zones.

💡 Hint

Forced awareness restricts allocation to specified attribute values.

Forced awareness limits shard allocation strictly to the listed attribute values, ignoring nodes outside those values.

❓ Predict Output

expert

What is the number of shards allocated after these settings?

Given a cluster with 3 nodes having rack attributes: node1 (rack: a), node2 (rack: b), node3 (rack: a), and an index with 3 primary shards and 1 replica each, with awareness attribute set to rack, how many shards will be allocated on node3?

Elasticsearch

PUT _cluster/settings
{
  "persistent": {
    "cluster.routing.allocation.awareness.attributes": "rack"
  }
}

PUT my_index
{
  "settings": {
    "number_of_shards": 3,
    "number_of_replicas": 1
  }
}

A 0 shards

B 1 shard

C 2 shards

D 3 shards

💡 Hint

Consider how replicas are allocated across different rack values.

Since node1 and node3 share the same rack 'a', replicas cannot be allocated on the same rack as primaries. Node3 will only get replicas for shards whose primaries are on rack 'b'.