Shard key selection importance in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why is shard key selection important in MongoDB?

In MongoDB, choosing the right shard key affects how data is distributed and accessed. Which of the following best explains why selecting an appropriate shard key is crucial?

A It controls the encryption level of the data stored in the cluster.

B It sets the backup frequency for each shard automatically.

C It determines how data is partitioned across shards, impacting query speed and cluster balance.

D It decides the maximum size of each document in the database.

💡 Hint

Think about how data is split and accessed in a distributed system.

The shard key defines how MongoDB distributes data across shards. A good shard key ensures even data distribution and efficient queries. Other options relate to different database features.

❓ query_result

intermediate

What is the output of this shard key usage scenario?

Consider a MongoDB collection sharded on the field userId. If queries mostly filter by userId, what is the expected effect on query performance?

A Queries will be efficient because they target specific shards based on <code>userId</code>.

B Queries will be slow because all shards must be scanned for <code>userId</code>.

C Queries will fail because <code>userId</code> is not a valid shard key.

D Queries will be efficient only if <code>userId</code> is indexed but not sharded.

💡 Hint

Think about how shard keys help route queries.

When queries filter by the shard key, MongoDB routes the query to the relevant shard(s), improving performance. Scanning all shards happens if the shard key is not used in the query filter.

📝 Syntax

advanced

Identify the correct shard key creation command

Which MongoDB command correctly creates a sharded collection with orderId as the shard key?

MongoDB

sh.enableSharding("salesDB");
sh.shardCollection("salesDB.orders", {orderId: 1});

A sh.shardCollection("salesDB.orders", {orderId: "asc"})

B sh.shardCollection("salesDB.orders", "orderId")

C sh.shardCollection("salesDB.orders", {orderId})

D sh.shardCollection("salesDB.orders", {orderId: 1})

💡 Hint

Look for the correct syntax for specifying shard keys as a document with field and direction.

The shard key must be specified as a document with the field name and sort order (1 for ascending). Option D uses the correct syntax. Option D uses a string instead of a document. Option D is invalid syntax. Option D uses a string instead of 1 or -1.

❓ optimization

advanced

Which shard key choice leads to better cluster balance?

You have a collection with a status field that has only three possible values: 'new', 'processing', and 'done'. You want to shard this collection. Which shard key choice will likely lead to better data distribution?

A Shard on a compound key: <code>{status: 1, createdAt: 1}</code>.

B Shard on <code>status</code> field alone.

C Shard on <code>createdAt</code> field alone.

D Shard on a constant value field.

💡 Hint

Think about how many unique values the shard key has and how that affects distribution.

Sharding on status alone causes only three chunks, leading to uneven distribution. Adding createdAt creates many unique shard key values, improving balance. Sharding on createdAt alone can cause hotspotting if data is inserted in order. Sharding on a constant value is invalid.

🔧 Debug

expert

Why does this shard key choice cause a performance problem?

A collection is sharded on the timestamp field, which is always increasing. Over time, queries slow down and one shard becomes overloaded. What is the main reason for this problem?

A The shard key has too many unique values, causing overhead.

B The shard key is monotonically increasing, causing all inserts to go to one shard, creating a hotspot.

C The shard key is not indexed, causing slow queries.

D The shard key is a compound key, which MongoDB does not support.

💡 Hint

Consider how data is distributed when the shard key values increase over time.

Using a monotonically increasing shard key like timestamp causes all new data to be routed to the same shard, creating a hotspot and unbalanced load. This slows down queries and affects cluster performance. MongoDB supports compound keys and indexing is automatic on shard keys.