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

Challenge - 5 Problems

❓ query_result

intermediate

Find the shard key range for a given document

Given a MongoDB sharded collection with a range-based shard key on the field age, which shard will store the document { "name": "Alice", "age": 25 } if the shard ranges are:

- Shard1: age < 20
- Shard2: 20 ≤ age < 40
- Shard3: age ≥ 40

Choose the correct shard that will contain Alice's document.

A Shard2

B Shard1

C Shard3

D None of the above

💡 Hint

Check which range the age 25 falls into.

Since Alice's age is 25, it falls within the range 20 ≤ age < 40, which belongs to Shard2.

🧠 Conceptual

intermediate

Understanding range-based shard key distribution

Why is range-based sharding potentially problematic if the incoming data is mostly increasing in the shard key value?

Choose the best explanation.

A It evenly distributes writes across all shards.

B It causes all writes to go to a single shard, leading to unbalanced load.

C It causes queries to be slower because all shards must be scanned.

D It prevents any shard from storing data.

💡 Hint

Think about what happens if new data always has a higher shard key value.

If data mostly increases in shard key value, all new writes go to the shard holding the highest range, causing that shard to be overloaded.

📝 Syntax

advanced

Identify the correct MongoDB command to enable range-based sharding

Which of the following MongoDB shell commands correctly enables sharding on the users collection using age as the shard key with range-based sharding?

A sh.shardCollection("mydb.users", "age")

B sh.enableSharding("mydb.users", { age: 1 })

C sh.shardCollection("mydb", { users: { age: 1 } })

D sh.shardCollection("mydb.users", { age: 1 })

💡 Hint

The command to shard a collection requires the namespace and shard key as an object.

The correct syntax is sh.shardCollection("database.collection", { shardKey: 1 }) to enable range-based sharding on the shard key.

❓ optimization

advanced

Optimizing range-based shard key selection

You have a collection where most queries filter by region and timestamp. Which shard key choice will best optimize query performance and data distribution using range-based sharding?

A { region: 1, timestamp: 1 }

B { timestamp: 1 }

C { region: 1 }

D { _id: 1 }

💡 Hint

Consider which fields are used in queries and how combining them affects shard distribution.

Using a compound shard key on region and timestamp allows queries filtering on both fields to target specific shards and balances data by region and time.

🔧 Debug

expert

Diagnose the cause of unbalanced shards in range-based sharding

A MongoDB cluster uses range-based sharding on the orderDate field. Over time, one shard stores 90% of the data, causing performance issues. What is the most likely cause?

A The shard key is not indexed.

B The cluster has too many shards configured.

C The shard key <code>orderDate</code> is monotonically increasing, causing all new inserts to go to one shard.

D The data is evenly distributed but queries are slow.

💡 Hint

Think about how range-based sharding handles sequential values.

A monotonically increasing shard key causes all new data to be routed to the last shard range, leading to unbalanced data and load.