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.