Index settings (shards, replicas) in Elasticsearch - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the default number of primary shards for a new Elasticsearch index?

By default, when you create a new index in Elasticsearch without specifying shard settings, how many primary shards does it have?

A 5

B 0

C 3

D 1

💡 Hint

Think about the default shard count Elasticsearch uses for balancing data.

Elasticsearch creates 5 primary shards by default for each new index unless specified otherwise.

❓ query_result

intermediate

How many replicas are created by default for an Elasticsearch index?

When you create an index in Elasticsearch without specifying replica settings, how many replica shards are created by default?

A 0

B 1

C 2

D 5

💡 Hint

Replicas provide fault tolerance and improve search performance.

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

📝 Syntax

advanced

Which JSON snippet correctly sets an index with 3 primary shards and 2 replicas?

Select the valid JSON configuration to create an Elasticsearch index with 3 primary shards and 2 replicas.

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

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

C { "settings": { "number_of_shards": "3", "number_of_replicas": "2" } }

D { "settings": { "primary_shards": 3, "replica_shards": 2 } }

💡 Hint

Look for the exact keys Elasticsearch expects for shard settings.

Elasticsearch expects the keys 'number_of_shards' and 'number_of_replicas' with integer values.

🧠 Conceptual

advanced

Why should you avoid setting the number of primary shards too high for a small index?

What is the main reason to avoid configuring too many primary shards for a small Elasticsearch index?

A It automatically disables replicas for the index.

B It causes Elasticsearch to reject writes to the index.

C It merges all shards into one, reducing parallelism.

D It increases cluster overhead and wastes resources without improving performance.

💡 Hint

Think about resource use and efficiency.

Too many shards for small data cause unnecessary resource use and slow cluster operations.

🔧 Debug

expert

What error occurs if you try to set number_of_shards to 0 when creating an index?

You run this command to create an index with zero primary shards: { "settings": { "number_of_shards": 0, "number_of_replicas": 1 } } What error will Elasticsearch return?

A No error, index created with zero shards

B IndexCreationException: replicas cannot be set without shards

C ValidationException: number_of_shards must be greater than 0

D ClusterBlockException: cluster is blocked due to invalid shard count

💡 Hint

Primary shards must be at least one for an index to exist.

Elasticsearch requires at least one primary shard; zero is invalid and triggers a validation error.

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