Index settings (shards, replicas) in Elasticsearch - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a shard in Elasticsearch?

A shard is a basic unit of storage in Elasticsearch. It holds a subset of the data in an index, allowing the data to be split across multiple nodes for better performance and scalability.

beginner

What does the number_of_shards setting control?

It controls how many primary shards an index is divided into. This number cannot be changed after the index is created.

beginner

What is a replica in Elasticsearch?

A replica is a copy of a primary shard. It provides fault tolerance and increases search capacity by allowing queries to run on multiple copies.

intermediate

How does the number_of_replicas setting affect an index?

It sets how many copies of each primary shard are created. More replicas mean better fault tolerance and faster searches but use more storage.

intermediate

Can you change the number_of_replicas after creating an index?

Yes, you can change the number_of_replicas at any time to add or remove replica shards without downtime.

What happens if you increase the number_of_replicas for an index?

A More copies of shards are created, improving fault tolerance and search speed.

B The index data is deleted and recreated.

C The number of primary shards increases.

D The index becomes read-only.

Increasing number_of_replicas creates more copies of primary shards, which helps with fault tolerance and allows more nodes to serve search requests.

Which setting cannot be changed after index creation?

A refresh_interval

B number_of_shards

C number_of_replicas

D index.mapping.total_fields.limit

The number_of_shards is fixed when the index is created and cannot be changed later.

Why do we use shards in Elasticsearch?

A To encrypt data.

B To compress data.

C To backup data to external storage.

D To split data across nodes for scalability and performance.

Shards split the index data so Elasticsearch can store and search data efficiently across multiple nodes.

What is the default number_of_replicas in Elasticsearch?

A 1

B 2

C 0

D 3

By default, Elasticsearch creates 1 replica for each primary shard.

If an Elasticsearch node fails, what helps keep the data available?

A Index templates

B Primary shards only

C Replica shards

D Cluster settings

Replica shards provide copies of data so if a node with primary shards fails, replicas can serve the data.

Explain the roles of shards and replicas in Elasticsearch index settings.

Think about how data is stored and protected in Elasticsearch.

Describe how changing the number_of_replicas affects an Elasticsearch cluster.

Consider the benefits and costs of having more copies of data.

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