Shard allocation awareness in Elasticsearch

Introduction

Shard allocation awareness helps Elasticsearch keep data copies on different machines or racks. This makes sure your data stays safe even if one machine or rack fails.

You want to keep copies of your data on different physical racks to avoid losing data if a rack goes down.

You run Elasticsearch on multiple data centers and want to spread shards across them for safety.

You want to make sure shards are not all on the same machine to avoid single points of failure.

You want better control over where Elasticsearch places shards based on custom labels like zone or room.

Syntax

Elasticsearch

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

PUT /my_index
{
  "settings": {
    "index.routing.allocation.awareness.include": {
      "rack_id": "rack1,rack2"
    }
  }
}

The cluster.routing.allocation.awareness.attributes setting tells Elasticsearch which node attribute to use for awareness (like rack_id).

The index.routing.allocation.awareness.include setting controls which attribute values shards should be allocated to.

Examples

This sets the awareness attribute to zone, so Elasticsearch will try to spread shards across different zones.

Elasticsearch

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

This tells Elasticsearch to allocate shards only to nodes in zone1 or zone2.

Elasticsearch

PUT /my_index
{
  "settings": {
    "index.routing.allocation.awareness.include": {
      "zone": "zone1,zone2"
    }
  }
}

You can use multiple attributes like rack and zone for more detailed shard spreading.

Elasticsearch

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

Sample Program

This example sets the cluster to use rack_id for shard allocation awareness. Then it creates an index my_index that only allocates shards to nodes with rack1 or rack2. Finally, it shows where shards are placed.

Elasticsearch

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

PUT /my_index
{
  "settings": {
    "index.routing.allocation.awareness.include": {
      "rack_id": "rack1,rack2"
    },
    "number_of_shards": 2,
    "number_of_replicas": 1
  }
}

GET /_cat/shards/my_index?v

Important Notes

Make sure your Elasticsearch nodes have the attribute (like rack_id) set in their elasticsearch.yml file under node.attr.rack_id.

Shard allocation awareness helps prevent data loss by spreading shards, but it does not guarantee perfect balance if nodes are uneven.

Use the GET /_cat/shards API to check where shards are allocated.

Summary

Shard allocation awareness spreads data copies across different physical locations.

Set cluster awareness attributes and index allocation rules to control shard placement.

This improves data safety and availability in case of machine or rack failures.

Practice

1. What is the main purpose of shard allocation awareness in Elasticsearch?

easy

A. To increase the number of shards in an index automatically

B. To compress shard data to save disk space

C. To speed up search queries by caching shards in memory

D. To spread shard copies across different physical locations for better fault tolerance

Solution

1. Step 1: Understand shard allocation awareness concept

   Shard allocation awareness ensures that shard copies are placed on different physical locations like racks or machines.

2. Step 2: Identify the benefit of spreading shards

   This spreading improves fault tolerance by preventing data loss if one location fails.

3. Final Answer:

   To spread shard copies across different physical locations for better fault tolerance -> Option D

4. Quick Check:

   Shard allocation awareness = spreading shards for fault tolerance [OK]

Hint: Think about fault tolerance by spreading data copies [OK]

Common Mistakes:

• Confusing shard allocation awareness with shard count increase
• Thinking it speeds up queries directly
• Assuming it compresses data

2. Which of the following is the correct way to set cluster awareness attributes in Elasticsearch's elasticsearch.yml file?

easy

A. cluster.routing.allocation.awareness.attributes: rack_id

B. cluster.routing.allocation.awareness: rack_id

C. cluster.awareness.routing.allocation.attributes: rack_id

D. routing.allocation.awareness.attributes: rack_id

Solution

1. Step 1: Recall the correct setting syntax

   The correct setting for awareness attributes is cluster.routing.allocation.awareness.attributes.

2. Step 2: Match the option with correct syntax

   cluster.routing.allocation.awareness.attributes: rack_id matches the exact syntax used in Elasticsearch configuration files.

3. Final Answer:

   cluster.routing.allocation.awareness.attributes: rack_id -> Option A

4. Quick Check:

   Correct config key = cluster.routing.allocation.awareness.attributes [OK]

Hint: Look for full correct config key with 'cluster.routing.allocation' [OK]

Common Mistakes:

• Omitting 'cluster.routing' prefix
• Swapping order of words in the key
• Using incomplete or wrong keys

3. Given the following index setting, what will happen when Elasticsearch allocates shards?

{
  "settings": {
    "index.routing.allocation.awareness.include": {
      "rack_id": "rack1,rack2"
    }
  }
}

medium

A. Shards will be allocated only on rack3 nodes

B. Shards will be allocated on any node regardless of rack_id

C. Shards will only be allocated on nodes with rack_id rack1 or rack2

D. Allocation will fail because of invalid syntax

Solution

1. Step 1: Understand the setting meaning

   The setting index.routing.allocation.awareness.include with rack_id values means shards should only go to nodes with those rack_ids.

2. Step 2: Apply to given values

   Since rack1 and rack2 are included, shards will only be allocated on nodes labeled with rack1 or rack2.

3. Final Answer:

   Shards will only be allocated on nodes with rack_id rack1 or rack2 -> Option C

4. Quick Check:

   Allocation include rack1,rack2 = shards on rack1 or rack2 only [OK]

Hint: Include means restrict allocation to listed racks [OK]

Common Mistakes:

• Thinking shards can go to any rack
• Confusing include with exclude
• Assuming syntax error due to JSON format

4. You configured cluster awareness with cluster.routing.allocation.awareness.attributes: rack_id but shards are still allocated on the same rack. What is the likely cause?

medium

A. The index has no replicas configured

B. Nodes do not have the node.attr.rack_id setting defined

C. The cluster is in read-only mode

D. The shards are too large to move

Solution

1. Step 1: Check cluster awareness prerequisites

   For awareness to work, each node must have node.attr.rack_id set to identify its rack.

2. Step 2: Identify missing node attribute effect

   If nodes lack this attribute, Elasticsearch cannot distinguish racks and may place shards on the same rack.

3. Final Answer:

   Nodes do not have the node.attr.rack_id setting defined -> Option B

4. Quick Check:

   Missing node.attr.rack_id = shards not spread by rack [OK]

Hint: Check node attributes match cluster awareness keys [OK]

Common Mistakes:

• Assuming replicas count affects awareness
• Thinking cluster read-only blocks allocation
• Blaming shard size for allocation issues

5. You want to ensure that primary and replica shards are never allocated on the same rack to improve fault tolerance. Which combination of settings achieves this?

hard

A. Set cluster.routing.allocation.awareness.attributes: rack_id and index.routing.allocation.awareness.force.rack_id: true

B. Set cluster.routing.allocation.awareness.attributes: rack_id and index.routing.allocation.awareness.force.rack_id: false

C. Set cluster.routing.allocation.awareness.attributes: rack_id and index.routing.allocation.include.rack_id: rack1,rack2

D. Set cluster.routing.allocation.awareness.attributes: rack_id only

Solution

1. Step 1: Identify setting to enforce shard separation

   The index.routing.allocation.awareness.force.rack_id: true setting forces Elasticsearch to allocate primary and replica shards on different racks.

2. Step 2: Combine with cluster awareness attribute

   Setting cluster.routing.allocation.awareness.attributes: rack_id enables awareness based on rack_id attribute.

3. Step 3: Confirm other options do not enforce separation

   Simply setting the awareness attribute does not force separation. Setting force to false prevents enforcement. Using include settings restricts available racks but does not ensure primary and replica are on different ones.

4. Final Answer:

   Set cluster.routing.allocation.awareness.attributes: rack_id and index.routing.allocation.awareness.force.rack_id: true -> Option A

5. Quick Check:

   Force awareness true + rack_id attribute = shards separated by rack [OK]

Hint: Use force awareness true to separate primary and replica shards [OK]

Common Mistakes:

• Forgetting to set force awareness to true
• Only setting awareness attribute without force
• Confusing include with force settings