Shard key selection importance in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the shard key when sharding a collection.

MongoDB

sh.shardCollection("mydb.users", [1])

Drag options to blanks, or click blank then click option'

A"userId"

B{ userId: 1 }

C{ age: 1 }

D{ location: 1 }

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a document for the shard key.

Choosing a field that is not suitable as a shard key.

The shard key must be specified as a document indicating the field(s) to shard on. Here, { userId: 1 } is the correct shard key.

2fill in blank

medium

Complete the code to create an index on the shard key field.

MongoDB

db.users.createIndex({ [1]: 1 })

Drag options to blanks, or click blank then click option'

Aage

Blocation

Cemail

DuserId

Attempts:

3 left

💡 Hint

Common Mistakes

Creating an index on a non-shard key field.

Forgetting to create an index on the shard key.

The shard key field must be indexed for efficient query routing. Here, userId is the shard key field.

3fill in blank

hard

Fix the error in the shard key selection to avoid unbalanced data distribution.

MongoDB

sh.shardCollection("mydb.orders", { [1]: 1 })

Drag options to blanks, or click blank then click option'

AcustomerId

BorderDate

Cstatus

DtotalAmount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field with few distinct values as shard key.

Choosing a field not used in queries.

Choosing customerId as the shard key helps distribute data evenly because it has high cardinality and is frequently used in queries.

4fill in blank

hard

Fill both blanks to create a compound shard key for better data distribution.

MongoDB

sh.shardCollection("mydb.logs", { [1]: 1, [2]: 1 })

Drag options to blanks, or click blank then click option'

AuserId

Btimestamp

Cregion

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing fields that do not improve data distribution.

Using only one field when a compound key is better.

A compound shard key with timestamp and region helps distribute data by time and location, improving balance and query performance.

5fill in blank

hard

Fill all three blanks to define a shard key and create an index for efficient queries.

MongoDB

sh.shardCollection("mydb.sales", { [1]: 1, [2]: 1 });
db.sales.createIndex({ [3]: 1 })

Drag options to blanks, or click blank then click option'

Aregion

BproductId

DsaleDate

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same field for all blanks.

Not creating an index on a frequently queried field.

The shard key is a compound key on region and productId. The index on saleDate helps queries filter by date efficiently.

Practice

1. Why is choosing a good shard key important in MongoDB?

easy

A. It reduces the number of collections.

B. It increases the size of each document.

C. It helps distribute data evenly across servers.

D. It makes the database schema simpler.

Solution

1. Step 1: Understand shard key role

   The shard key determines how data is split across multiple servers in a cluster.

2. Step 2: Importance of even distribution

   A good shard key ensures data is balanced, preventing some servers from being overloaded.

3. Final Answer:

   It helps distribute data evenly across servers. -> Option C

4. Quick Check:

   Shard key = data distribution [OK]

Hint: Shard key controls data spread, pick for balance [OK]

Common Mistakes:

• Thinking shard key changes document size
• Confusing shard key with schema design
• Believing shard key reduces collections

2. Which of the following is the correct syntax to create a sharded collection with shard key { userId: 1 } in MongoDB?

easy

A. sh.shardCollection('mydb.users', { userId: 1 })

B. sh.shardCollection('mydb.users', 'userId')

C. sh.shardCollection('mydb.users', { userId: 'asc' })

D. sh.shardCollection('mydb.users', ['userId'])

Solution

1. Step 1: Review shardCollection syntax

   The correct syntax requires the namespace string and a document specifying the shard key fields with direction (1 or -1).

2. Step 2: Match correct option

   sh.shardCollection('mydb.users', { userId: 1 }) uses the correct format: database.collection and shard key as document with 1.

3. Final Answer:

   sh.shardCollection('mydb.users', { userId: 1 }) -> Option A

4. Quick Check:

   shardCollection needs {field: 1} format [OK]

Hint: Shard key must be a document with field and 1 or -1 [OK]

Common Mistakes:

• Passing shard key as string instead of document
• Using array instead of document for shard key
• Using 'asc' instead of 1 for direction

3. Given a collection sharded on { orderId: 1 }, which query will benefit most from the shard key?

medium

A. db.orders.find({ customerName: 'Alice' })

B. db.orders.find({ total: { $gt: 100 } })

C. db.orders.find({ status: 'shipped' })

D. db.orders.find({ orderId: 12345 })

Solution

1. Step 1: Understand shard key query targeting

   Queries that include the shard key field can be routed directly to the relevant shard.

2. Step 2: Identify query with shard key

   Only the query filtering by orderId uses the shard key, so it benefits most.

3. Final Answer:

   db.orders.find({ orderId: 12345 }) -> Option D

4. Quick Check:

   Query with shard key = targeted query [OK]

Hint: Queries with shard key filter hit one shard fast [OK]

Common Mistakes:

• Assuming any query benefits from shard key
• Confusing shard key with indexed fields
• Ignoring shard key presence in query

4. You created a shard key on { country: 1 } but notice one shard stores 90% of data. What is the likely problem?

medium

A. The shard key syntax was incorrect.

B. The shard key has low cardinality causing data imbalance.

C. The collection is not sharded properly.

D. The shard key field is missing in documents.

Solution

1. Step 1: Analyze shard key cardinality

   Shard keys with few unique values (low cardinality) cause uneven data distribution.

2. Step 2: Identify cause of imbalance

   Country likely has few distinct values, so one shard holds most data.

3. Final Answer:

   The shard key has low cardinality causing data imbalance. -> Option B

4. Quick Check:

   Low cardinality shard key = data imbalance [OK]

Hint: Low unique shard keys cause uneven data spread [OK]

Common Mistakes:

• Blaming syntax without checking data
• Assuming collection is unsharded
• Ignoring shard key field presence

5. You want to shard a large user collection. Which shard key choice is best to keep data balanced and queries efficient?

hard

A. Use { userId: 1 } because it has many unique values and is in queries.

B. Use { country: 1 } because it groups users by location.

C. Use { signupDate: 1 } because dates are sequential.

D. Use { status: 1 } because it has few distinct values.

Solution

1. Step 1: Evaluate shard key uniqueness

   A good shard key has many unique values to distribute data evenly.

2. Step 2: Match shard key to query patterns

   UserId is unique and commonly used in queries, improving efficiency.

3. Step 3: Avoid low cardinality keys

   Country and status have few distinct values, causing imbalance; signupDate is sequential, causing chunk hotspots.

4. Final Answer:

   Use { userId: 1 } because it has many unique values and is in queries. -> Option A

5. Quick Check:

   High cardinality + query use = best shard key [OK]

Hint: Pick shard key with many unique values and query use [OK]

Common Mistakes:

• Choosing sequential or low cardinality fields
• Ignoring query patterns when selecting shard key
• Assuming grouping fields balance data well