Hash-based sharding in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Effect of Hash-based Sharding on Query Routing

Consider a MongoDB collection sharded by hashing the user_id field. Which statement best describes how queries with a specific user_id value are routed?

A The query is routed to all shards because hashed keys are distributed randomly.

B The query is routed only to the shard that contains the hashed <code>user_id</code> value.

C The query is routed to the primary shard regardless of the <code>user_id</code> value.

D The query is routed to shards based on the range of <code>user_id</code> values.

💡 Hint

Think about how hashing distributes data evenly and how MongoDB uses the hash to locate data.

Hash-based sharding uses a hash function on the shard key to evenly distribute data. Queries with a specific shard key value are routed directly to the shard holding that hashed value, improving efficiency.

🧠 Conceptual

intermediate

Why Use Hash-based Sharding?

What is the main advantage of using hash-based sharding in MongoDB compared to range-based sharding?

A It allows for efficient range queries on the shard key.

B It stores data in sorted order on each shard.

C It evenly distributes data and load across all shards regardless of key distribution.

D It automatically replicates data to all shards.

💡 Hint

Consider how data distribution affects load balancing.

Hash-based sharding evenly distributes documents across shards by hashing the shard key, preventing hotspots caused by uneven data distribution common in range-based sharding.

📝 Syntax

advanced

Correct Syntax to Enable Hash-based Sharding

Which MongoDB command correctly enables sharding on the users collection using a hashed user_id shard key?

A sh.shardCollection('mydb.users', { user_id: 'hashed' })

B sh.shardCollection('mydb.users', { user_id: 'hash' })

C sh.shardCollection('mydb.users', { user_id: 'hashedKey' })

D sh.shardCollection('mydb.users', { user_id: 'hashing' })

💡 Hint

Check the exact keyword MongoDB expects for hashed shard keys.

The correct syntax uses the keyword 'hashed' to specify a hashed shard key in the shardCollection command.

❓ optimization

advanced

Optimizing Query Performance with Hash-based Sharding

You have a MongoDB collection sharded by hashing email. Which practice will improve query performance for lookups by email?

A Avoid indexing the <code>email</code> field to reduce overhead.

B Create a compound shard key with <code>email</code> and <code>signup_date</code>.

C Use range queries on <code>email</code> to leverage shard key ordering.

D Always include the <code>email</code> field in queries to enable targeted shard routing.

💡 Hint

Think about how MongoDB routes queries in hash-based sharding.

Including the shard key field in queries allows MongoDB to route queries directly to the correct shard, improving performance. Range queries and missing indexes reduce efficiency.

🔧 Debug

expert

Diagnosing Uneven Data Distribution in Hash-based Sharding

A MongoDB collection is sharded by hashing customer_id. However, one shard stores 80% of the data, causing performance issues. What is the most likely cause?

A The balancer is disabled, preventing data from moving to balance shards.

B The shard key is not actually hashed, but used as a range shard key by mistake.

C The hash function is not distributing values evenly due to a small shard key value range.

D The <code>customer_id</code> values are not unique, causing data duplication on one shard.

💡 Hint

Consider what controls data movement between shards after initial distribution.

If the balancer is disabled, MongoDB cannot redistribute chunks to balance data across shards, causing uneven data distribution even with hash-based sharding.

Practice

1.

What is the main purpose of hash-based sharding in MongoDB?

easy

A. To store data in sorted order on each shard

B. To compress data before storing it

C. To replicate data for backup purposes

D. To evenly distribute data across shards using a hash function

Solution

1. Step 1: Understand hash-based sharding concept

   Hash-based sharding uses a hash function on the shard key to distribute data evenly.

2. Step 2: Compare options with concept

   Only To evenly distribute data across shards using a hash function describes even distribution using hashing, which matches the concept.

3. Final Answer:

   To evenly distribute data across shards using a hash function -> Option D

4. Quick Check:

   Hash-based sharding = even data distribution [OK]

Hint: Hash-based sharding means even spread by hashing key [OK]

Common Mistakes:

• Confusing hash-based with range-based sharding
• Thinking it sorts data on shards
• Mixing sharding with replication

2.

Which of the following is the correct way to create a hashed shard key on the field userId in MongoDB?

sh.shardCollection("mydb.users", { ??? })

easy

A. { userId: 1 }

B. { userId: "hashed" }

C. { userId: -1 }

D. { userId: true }

Solution

1. Step 1: Recall syntax for hashed shard key

   MongoDB requires the shard key field to be set to "hashed" as a string to enable hash-based sharding.

2. Step 2: Match options with syntax

   Only { userId: "hashed" } uses the correct syntax: { userId: "hashed" }.

3. Final Answer:

   { userId: "hashed" } -> Option B

4. Quick Check:

   Hashed shard key syntax = { field: "hashed" } [OK]

Hint: Hashed keys use string "hashed" in shard key spec [OK]

Common Mistakes:

• Using 1 or -1 instead of "hashed"
• Using boolean true instead of string
• Omitting quotes around hashed

3.

Given a collection sharded by { userId: "hashed" }, which shard will store the document { userId: 42, name: "Alice" }?

Assume the hash function maps 42 to a hash value that falls into shard 2's range.

medium

A. Shard 2

B. Cannot determine without full hash map

C. Shard 3

D. Shard 1

Solution

1. Step 1: Understand hashed shard key routing

   Documents are routed to shards based on the hash of the shard key value.

2. Step 2: Use given hash mapping

   The problem states the hash of 42 falls into shard 2's range, so the document goes to shard 2.

3. Final Answer:

   Shard 2 -> Option A

4. Quick Check:

   Hash value maps to shard 2 [OK]

Hint: Hash value decides shard, not raw key [OK]

Common Mistakes:

• Assuming shard based on userId value directly
• Choosing 'Cannot determine' when info is given
• Confusing shard ranges with key values

4.

Consider this command to shard a collection:

sh.shardCollection("mydb.orders", "orderId: hashed")

After running this, you get an error: "Error: shard key must be an object". What is the likely cause?

medium

A. The shard key argument must be an object, but a string was passed

B. The shard key value should be 1 or -1, not "hashed"

C. The command syntax is correct; the error is from elsewhere

D. The database name is missing in the command

Solution

1. Step 1: Check shardCollection syntax

   The shard key must be an object specifying the field and type, e.g., { orderId: "hashed" }.

2. Step 2: Identify error cause

   The error says shard key must be an object, so likely the argument passed was not an object but a string or other type.

3. Final Answer:

   The shard key argument must be an object, but a string was passed -> Option A

4. Quick Check:

   Shard key must be an object [OK]

Hint: Shard key must be an object, not a string [OK]

Common Mistakes:

• Passing shard key as string instead of object
• Confusing shard key with database name
• Ignoring error message details

5.

You want to shard a collection by email using hash-based sharding. However, you notice some shards have much more data than others. What could be a reason for this uneven distribution?

hard

A. The shard key was set to { email: 1 } instead of hashed

B. Hash-based sharding always causes uneven data distribution

C. The email field has many duplicate values causing hash collisions

D. The collection is too small to distribute evenly

Solution

1. Step 1: Understand hash-based sharding behavior

   Hash-based sharding usually distributes data evenly unless many documents share the same shard key value.

2. Step 2: Analyze cause of uneven data

   If many documents have the same email, their hash is the same, so they go to the same shard, causing imbalance.

3. Final Answer:

   The email field has many duplicate values causing hash collisions -> Option C

4. Quick Check:

   Duplicate keys cause uneven shard data [OK]

Hint: Duplicates in shard key cause uneven shard sizes [OK]

Common Mistakes:

• Assuming hash sharding always balances perfectly
• Confusing range-based with hash-based sharding
• Ignoring data distribution of shard key values