Choosing a good shard key in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why is a shard key important in MongoDB?

Imagine you have a huge collection of documents in MongoDB. You want to split this data across many servers to keep things fast and balanced. Why do you need a shard key for this?

A It stores user passwords securely.

B It decides how data is split and helps queries find data quickly.

C It compresses data to save disk space.

D It backs up data automatically every hour.

💡 Hint

Think about how data is divided and found in many places.

The shard key determines how MongoDB divides data across servers. It helps keep data balanced and makes queries efficient by knowing where to look.

❓ query_result

intermediate

What happens if you choose a shard key with low cardinality?

You have a collection where the shard key is a field with only two possible values. What is the likely result when you shard the collection on this key?

A Queries will run faster because of fewer shard key values.

B Data will be perfectly balanced across all servers.

C MongoDB will reject the shard key due to low cardinality.

D Data will be unevenly distributed, causing some servers to hold most data.

💡 Hint

Think about how many different values the shard key has and how data spreads.

Low cardinality means few distinct values. This causes data to cluster on few shards, leading to imbalance and slower performance.

📝 Syntax

advanced

Which shard key definition is valid and efficient for a collection with user activity logs?

You want to shard a collection storing user activity logs. Each document has a userId and a timestamp. Which shard key is best to balance load and support queries filtering by user and time?

MongoDB

db.activity.createIndex({ userId: 1, timestamp: 1 })

A { timestamp: 1 }

B { userId: 1 }

C { userId: 1, timestamp: 1 }

D { activityType: 1 }

💡 Hint

Consider which key helps distribute data and supports common queries.

Using a compound shard key with userId and timestamp balances data by user and supports queries filtering by user and time range.

❓ optimization

advanced

How to avoid hotspotting with a shard key in MongoDB?

You notice one shard is overloaded because many writes target the same shard key value. What is a good way to avoid this hotspotting?

A Choose a shard key with high cardinality and randomness.

B Use a shard key with only a few distinct values.

C Shard on a field that never changes.

D Disable sharding and use a single server.

💡 Hint

Think about spreading writes evenly across shards.

High cardinality and randomness in the shard key help distribute writes evenly, preventing one shard from becoming a hotspot.

🔧 Debug

expert

Why does this shard key choice cause unbalanced data?

A collection is sharded on the country field. Most documents have country set to 'USA'. What problem arises and why?

A Most data goes to one shard, causing imbalance and slow queries.

B MongoDB automatically balances data evenly regardless of shard key.

C The shard key causes data duplication across shards.

D Queries fail because <code>country</code> is not indexed.

💡 Hint

Think about how many documents share the same shard key value.

When most documents share the same shard key value, data clusters on one shard. This causes imbalance and reduces performance.