Choosing a good shard key in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a shard key in MongoDB?

A shard key is a field or set of fields that determines how data is distributed across different shards in a MongoDB cluster.

beginner

Why should a shard key have high cardinality?

High cardinality means many unique values, which helps distribute data evenly across shards and avoids hotspots.

beginner

What happens if a shard key has low cardinality?

Data may cluster on a few shards causing uneven load and poor performance.

intermediate

How does a shard key affect query performance?

A good shard key allows queries to target specific shards, reducing the amount of data scanned and improving speed.

intermediate

What is a common mistake when choosing a shard key?

Choosing a shard key that is monotonically increasing, like a timestamp or ObjectId, can cause all writes to go to one shard, creating a bottleneck.

Which property is important for a good shard key?

A High cardinality

B Low cardinality

C Always the _id field

D A field with only a few distinct values

A good shard key should have high cardinality to distribute data evenly.

What is a risk of using a monotonically increasing shard key?

A Shards automatically balance

B Data is evenly spread

C Queries become faster

D All writes go to one shard causing a hotspot

Monotonically increasing keys cause writes to target one shard, creating a bottleneck.

How does a good shard key improve query performance?

A By targeting queries to specific shards

B By duplicating data on all shards

C By ignoring the shard key in queries

D By making queries scan all shards

A good shard key lets queries target only relevant shards, speeding up queries.

Which shard key choice can cause uneven data distribution?

A A field with many unique values

B A field with few unique values

C A hashed shard key

D A compound shard key

Few unique values cause data to cluster on few shards, leading to imbalance.

What is a compound shard key?

A A shard key that is always _id

B A shard key with only one field

C A shard key made of multiple fields

D A shard key that changes over time

A compound shard key uses multiple fields to improve data distribution.

Explain what makes a shard key good for MongoDB sharding.

Think about how data spreads and how queries use the shard key.

Describe the problems caused by choosing a monotonically increasing shard key.

Consider how data is inserted over time.