Index settings (shards, replicas) in Elasticsearch

Introduction

Index settings control how data is stored and copied in Elasticsearch. Shards split data for speed, replicas copy data for safety.

When you want to make your search faster by splitting data into parts.

When you want to keep extra copies of your data to avoid losing it.

When you need to balance storage and performance on your Elasticsearch cluster.

When setting up a new index and deciding how many pieces and copies it should have.

When you want to improve search availability during server failures.

Syntax

Elasticsearch

{
  "settings": {
    "number_of_shards": <number>,
    "number_of_replicas": <number>
  }
}

number_of_shards is how many parts your data is split into.

number_of_replicas is how many copies of each shard exist besides the original.

Examples

This creates an index with 3 shards and 1 replica for each shard.

Elasticsearch

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

This creates an index with 1 shard and no replicas, meaning no extra copies.

Elasticsearch

{
  "settings": {
    "number_of_shards": 1,
    "number_of_replicas": 0
  }
}

Sample Program

This command creates an index named my_sample_index with 2 shards and 1 replica each. This means data is split into 2 parts, and each part has one extra copy for safety.

Elasticsearch

PUT /my_sample_index
{
  "settings": {
    "number_of_shards": 2,
    "number_of_replicas": 1
  }
}

Important Notes

You cannot change the number of shards after creating the index.

You can change the number of replicas anytime to add or remove copies.

Summary

Shards split your data to make searching faster.

Replicas keep extra copies to protect your data.

Set shards and replicas when creating an index to balance speed and safety.

Practice

1. What is the main purpose of setting number_of_shards in an Elasticsearch index?

easy

A. To define the size of the index

B. To create backup copies of data

C. To split data into parts for faster searching

D. To set the maximum number of documents

Solution

1. Step 1: Understand shards role

   Shards divide the index data into smaller parts to allow parallel searching.

2. Step 2: Differentiate shards from replicas

   Replicas are copies for safety, not for splitting data.

3. Final Answer:

   To split data into parts for faster searching -> Option C

4. Quick Check:

   Shards = split data for speed [OK]

Hint: Shards split data, replicas copy data [OK]

Common Mistakes:

• Confusing shards with replicas
• Thinking shards create backups
• Assuming shards set index size

2. Which of the following is the correct way to set 3 shards and 2 replicas in an Elasticsearch index creation JSON?

easy

A. { "settings": { "number_of_shards": 3, "number_of_replicas": 2 } }

B. { "settings": { "shards": 3, "replicas": 2 } }

C. { "settings": { "shard_count": 3, "replica_count": 2 } }

D. { "settings": { "num_shards": 3, "num_replicas": 2 } }

Solution

1. Step 1: Recall correct setting keys

   The official keys are number_of_shards and number_of_replicas.

2. Step 2: Match keys with options

   Only { "settings": { "number_of_shards": 3, "number_of_replicas": 2 } } uses the exact correct keys and valid JSON format.

3. Final Answer:

   { "settings": { "number_of_shards": 3, "number_of_replicas": 2 } } -> Option A

4. Quick Check:

   Correct keys = number_of_shards, number_of_replicas [OK]

Hint: Use full keys: number_of_shards and number_of_replicas [OK]

Common Mistakes:

• Using shorthand keys like shards or replicas
• Wrong JSON syntax
• Mixing shard and replica counts

3. Given an index with number_of_shards set to 4 and number_of_replicas set to 1, how many total shard copies exist in the cluster?

medium

A. 4

B. 6

C. 5

D. 8

Solution

1. Step 1: Calculate primary shards

   There are 4 primary shards as set.

2. Step 2: Calculate replica shards

   Each primary shard has 1 replica, so 4 replicas total.

3. Step 3: Add primary and replicas

   Total shards = 4 primary + 4 replicas = 8 shards.

4. Final Answer:

   8 -> Option D

5. Quick Check:

   Total shards = primary + replicas = 8 [OK]

Hint: Total shards = shards x (1 + replicas) [OK]

Common Mistakes:

• Counting only primary shards
• Adding shards and replicas incorrectly
• Confusing replicas with shards

4. You tried to create an index with this setting: { "settings": { "number_of_shards": 2, "number_of_replicas": -1 } }. What is the problem?

medium

A. Negative replicas value is invalid

B. Number of shards cannot be 2

C. Missing required field for replicas

D. JSON syntax error

Solution

1. Step 1: Check shards value

   Number of shards is 2, which is valid.

2. Step 2: Check replicas value

   Replicas cannot be negative; -1 is invalid.

3. Final Answer:

   Negative replicas value is invalid -> Option A

4. Quick Check:

   Replicas must be zero or positive [OK]

Hint: Replicas cannot be negative numbers [OK]

Common Mistakes:

• Assuming negative replicas mean no replicas
• Thinking shards must be > 3
• Ignoring JSON correctness

5. You want to create an index optimized for high availability with 5 shards. Which setting combination is best to ensure data safety and fast search?

hard

A. { "number_of_shards": 5, "number_of_replicas": 0 }

B. { "number_of_shards": 5, "number_of_replicas": 1 }

C. { "number_of_shards": 1, "number_of_replicas": 5 }

D. { "number_of_shards": 10, "number_of_replicas": 1 }

Solution

1. Step 1: Understand shards and replicas roles

   Shards split data for speed; replicas provide copies for safety.

2. Step 2: Evaluate options for availability and speed

   { "number_of_shards": 5, "number_of_replicas": 1 } keeps 5 shards for speed and 1 replica for safety, balancing both.

3. Step 3: Reject other options

   The option with zero replicas has no safety, the option with 1 shard is slow for search, the option with 10 shards has overhead.

4. Final Answer:

   { "number_of_shards": 5, "number_of_replicas": 1 } -> Option B

5. Quick Check:

   Balance shards and replicas for speed and safety [OK]

Hint: Balance shards for speed and replicas for safety [OK]

Common Mistakes:

• Setting zero replicas for safety
• Using too few shards for speed
• Over-sharding causing overhead