Replica management in Elasticsearch - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Elasticsearch replica setting query?

Given the following request to update the number of replicas for an index, what will be the number of replicas after the update?

Elasticsearch

PUT /my-index/_settings
{
  "index" : {
    "number_of_replicas" : 2
  }
}

A The request will fail with a syntax error.

B The index will have 0 replicas per shard.

C The index will have 1 replica per shard.

D The index will have 2 replicas per shard.

💡 Hint

The number_of_replicas setting controls how many copies of each shard exist besides the primary.

The PUT request sets the number_of_replicas to 2, so each primary shard will have 2 replica shards.

❓ Predict Output

intermediate

What happens if you set number_of_replicas to 0?

Consider this request to update an index's replica count: PUT /logs/_settings { "index": { "number_of_replicas": 0 } } What is the effect on the index?

Elasticsearch

PUT /logs/_settings
{
  "index": {
    "number_of_replicas": 0
  }
}

A The request will cause an error because replicas cannot be zero.

B The index will have one replica shard per primary shard.

C The index will have no replica shards, only primary shards.

D The index will have two replica shards per primary shard.

💡 Hint

Replicas provide copies of data for fault tolerance. Zero means no copies.

Setting number_of_replicas to 0 means the index will have only primary shards and no replicas.

❓ Predict Output

advanced

What error does this replica setting update cause?

What error will this request produce? PUT /data/_settings { "index": { "number_of_replicas": -1 } }

Elasticsearch

PUT /data/_settings
{
  "index": {
    "number_of_replicas": -1
  }
}

A 400 Bad Request: number_of_replicas must be >= 0

B 200 OK: number_of_replicas set to -1

C 500 Internal Server Error

D 404 Not Found: index does not exist

💡 Hint

Replica count cannot be negative.

Elasticsearch validates that number_of_replicas is zero or positive. Negative values cause a 400 error.

🧠 Conceptual

advanced

How does replica count affect search availability during node failure?

If an Elasticsearch cluster has 1 replica per shard and one data node fails, what happens to search availability for the shards on that node?

A Search fails because primary shards are lost and replicas are not used.

B Search continues without interruption because replicas serve the requests.

C Search is slower but still works because replicas are rebuilt from scratch.

D Search stops until the failed node is back online.

💡 Hint

Replicas provide copies that can serve search requests if primaries fail.

With replicas, if a node holding primary shards fails, the replicas on other nodes serve search requests, keeping availability.

❓ Predict Output

expert

What is the number of shards and replicas after this index creation?

Given this index creation request, how many total shards (primary + replicas) will the index have? PUT /analytics { "settings": { "number_of_shards": 3, "number_of_replicas": 2 } }

Elasticsearch

PUT /analytics
{
  "settings": {
    "number_of_shards": 3,
    "number_of_replicas": 2
  }
}

A 9 total shards: 3 primary + 6 replicas

B 6 total shards: 3 primary + 3 replicas

C 3 total shards: 3 primary only

D 5 total shards: 3 primary + 2 replicas

💡 Hint

Total shards = primary shards + (primary shards × number_of_replicas).

With 3 primary shards and 2 replicas each, total shards = 3 + (3×2) = 9.

Practice

1.

What is the main purpose of setting replicas in an Elasticsearch index?

easy

A. To encrypt data for security

B. To delete old data automatically

C. To compress data for storage savings

D. To create copies of data for faster search and fault tolerance

Solution

1. Step 1: Understand replica role

   Replicas are copies of the original data that help improve search speed and provide backup in case of failure.

2. Step 2: Compare options

   Only To create copies of data for faster search and fault tolerance correctly describes replicas as copies for speed and safety; others describe unrelated features.

3. Final Answer:

   To create copies of data for faster search and fault tolerance -> Option D

4. Quick Check:

   Replicas = copies for speed and safety [OK]

Hint: Replicas are copies that speed up search and protect data [OK]

Common Mistakes:

• Confusing replicas with data deletion
• Thinking replicas compress data
• Assuming replicas encrypt data

2.

Which of the following is the correct syntax to update the number of replicas to 2 for an existing index named my_index using Elasticsearch REST API?

easy

A. POST /my_index/_settings { "number_of_replicas": 2 }

B. PUT /my_index/_settings { "number_of_replicas": 2 }

C. GET /my_index/_settings { "number_of_replicas": 2 }

D. DELETE /my_index/_settings { "number_of_replicas": 2 }

Solution

1. Step 1: Identify correct HTTP method for updating settings

   Elasticsearch uses PUT to update index settings like replicas.

2. Step 2: Match syntax with method

   PUT with the path /my_index/_settings and JSON body setting number_of_replicas to 2 is correct.

3. Final Answer:

   PUT /my_index/_settings { "number_of_replicas": 2 } -> Option B

4. Quick Check:

   Update settings uses PUT method [OK]

Hint: Use PUT to update index settings like replicas [OK]

Common Mistakes:

• Using POST instead of PUT for settings update
• Using GET which only retrieves settings
• Using DELETE which removes resources

3.

Given an index products with number_of_replicas set to 1, what will be the total number of shards (primary + replicas) if the index has 3 primary shards?

medium

A. 6

B. 4

C. 9

D. 3

Solution

1. Step 1: Understand shards and replicas

   Each primary shard has replicas equal to number_of_replicas. Total shards = primary shards + replicas.

2. Step 2: Calculate total shards

   3 primary shards + 1 replica each = 3 + 3 = 6 total shards.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Total shards = primary + replicas = 3 + 3 = 6 [OK]

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

Common Mistakes:

• Counting only primary shards
• Adding replicas as 1 total instead of per shard
• Multiplying incorrectly

4.

What is wrong with this Elasticsearch index settings update request to set replicas to 3?

PUT /store/_settings
{
  "number_of_replicas": "3"
}

medium

A. The index name should be in quotes

B. PUT method cannot be used to update settings

C. The number_of_replicas value should be an integer, not a string

D. The JSON body is missing the settings wrapper

Solution

1. Step 1: Check data type of number_of_replicas

   number_of_replicas must be an integer, but here it is given as a string "3".

2. Step 2: Validate other parts

   PUT is correct method, index name does not require quotes in URL, and settings wrapper is optional in this context.

3. Final Answer:

   The number_of_replicas value should be an integer, not a string -> Option C

4. Quick Check:

   number_of_replicas must be integer [OK]

Hint: Use integer values for number_of_replicas, not strings [OK]

Common Mistakes:

• Putting number_of_replicas as a string
• Thinking PUT is wrong method
• Adding unnecessary JSON wrappers

5.

You want to increase the number of replicas for an index logs from 1 to 2 without downtime. Which approach is correct?

1. Update number_of_replicas setting to 2 using the REST API.
2. Reindex all data into a new index with 2 replicas.
3. Delete the index and recreate it with 2 replicas.
4. Change the number of primary shards to 2.

hard

A. Only step 1 is correct

B. Only step 2 is correct

C. Steps 1 and 2 are correct

D. Steps 3 and 4 are correct

Solution

1. Step 1: Understand replica update without downtime

   Elasticsearch allows changing number_of_replicas dynamically without downtime by updating settings.

2. Step 2: Evaluate other steps

   Reindexing or deleting index causes downtime; changing primary shards is unrelated to replicas.

3. Final Answer:

   Only step 1 is correct -> Option A

4. Quick Check:

   Update replicas via settings without downtime [OK]

Hint: Change replicas via settings update to avoid downtime [OK]

Common Mistakes:

• Thinking reindexing is needed to change replicas
• Deleting index causes data loss and downtime
• Confusing primary shards with replicas