Hash-based sharding in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable sharding on the database named 'sales'.

MongoDB

sh.enableSharding('[1]')

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

Ainventory

Bsales

Cusers

Dconfig

Attempts:

3 left

💡 Hint

Common Mistakes

Using a collection name instead of a database name.

Forgetting to put the database name in quotes.

To enable sharding on a database, you use sh.enableSharding() with the database name. Here, 'sales' is the correct database.

2fill in blank

medium

Complete the code to shard the collection 'orders' in the 'sales' database using a hashed shard key on the field 'orderId'.

MongoDB

sh.shardCollection('sales.orders', { [1]: 'hashed' })

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

AcustomerId

Bstatus

Cdate

DorderId

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field that is not the shard key.

Not specifying the shard key field correctly.

The shard key field is 'orderId' and it must be specified as the key with the value 'hashed' to use hash-based sharding.

3fill in blank

hard

Fix the error in the code to shard the collection 'products' in the 'inventory' database using a hashed shard key on 'productId'.

MongoDB

sh.shardCollection('inventory.products', { [1]: 'hashed' })

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

Aproduct_id

BprodId

CproductId

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using a similar but incorrect field name.

Ignoring case sensitivity in field names.

The shard key must match the exact field name used in the documents. Here, 'productId' is the correct field name.

4fill in blank

hard

Fill both blanks to create a hashed shard key on the 'customerId' field for the 'clients' collection in the 'crm' database.

MongoDB

sh.shardCollection('[1].clients', { [2]: 'hashed' })

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

Acrm

Borders

CcustomerId

Dclients

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping database and collection names.

Using the wrong field as shard key.

The database name is 'crm' and the shard key field is 'customerId'. Both must be correctly placed in the command.

5fill in blank

hard

Fill all three blanks to enable sharding on the 'analytics' database, then shard the 'events' collection using a hashed shard key on 'eventId'.

MongoDB

sh.enableSharding('[1]');
sh.shardCollection('[2].[3]', { eventId: 'hashed' })

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

Aanalytics

Cevents

Dlogs

Attempts:

3 left

💡 Hint

Common Mistakes

Using different database names in enableSharding and shardCollection.

Using the wrong collection name.

First, enable sharding on the 'analytics' database. Then shard the 'events' collection in the same database using 'eventId' as the hashed shard key.

Practice

1.

What is the main purpose of hash-based sharding in MongoDB?

easy

A. To store data in sorted order on each shard

B. To compress data before storing it

C. To replicate data for backup purposes

D. To evenly distribute data across shards using a hash function

Solution

1. Step 1: Understand hash-based sharding concept

   Hash-based sharding uses a hash function on the shard key to distribute data evenly.

2. Step 2: Compare options with concept

   Only To evenly distribute data across shards using a hash function describes even distribution using hashing, which matches the concept.

3. Final Answer:

   To evenly distribute data across shards using a hash function -> Option D

4. Quick Check:

   Hash-based sharding = even data distribution [OK]

Hint: Hash-based sharding means even spread by hashing key [OK]

Common Mistakes:

• Confusing hash-based with range-based sharding
• Thinking it sorts data on shards
• Mixing sharding with replication

2.

Which of the following is the correct way to create a hashed shard key on the field userId in MongoDB?

sh.shardCollection("mydb.users", { ??? })

easy

A. { userId: 1 }

B. { userId: "hashed" }

C. { userId: -1 }

D. { userId: true }

Solution

1. Step 1: Recall syntax for hashed shard key

   MongoDB requires the shard key field to be set to "hashed" as a string to enable hash-based sharding.

2. Step 2: Match options with syntax

   Only { userId: "hashed" } uses the correct syntax: { userId: "hashed" }.

3. Final Answer:

   { userId: "hashed" } -> Option B

4. Quick Check:

   Hashed shard key syntax = { field: "hashed" } [OK]

Hint: Hashed keys use string "hashed" in shard key spec [OK]

Common Mistakes:

• Using 1 or -1 instead of "hashed"
• Using boolean true instead of string
• Omitting quotes around hashed

3.

Given a collection sharded by { userId: "hashed" }, which shard will store the document { userId: 42, name: "Alice" }?

Assume the hash function maps 42 to a hash value that falls into shard 2's range.

medium

A. Shard 2

B. Cannot determine without full hash map

C. Shard 3

D. Shard 1

Solution

1. Step 1: Understand hashed shard key routing

   Documents are routed to shards based on the hash of the shard key value.

2. Step 2: Use given hash mapping

   The problem states the hash of 42 falls into shard 2's range, so the document goes to shard 2.

3. Final Answer:

   Shard 2 -> Option A

4. Quick Check:

   Hash value maps to shard 2 [OK]

Hint: Hash value decides shard, not raw key [OK]

Common Mistakes:

• Assuming shard based on userId value directly
• Choosing 'Cannot determine' when info is given
• Confusing shard ranges with key values

4.

Consider this command to shard a collection:

sh.shardCollection("mydb.orders", "orderId: hashed")

After running this, you get an error: "Error: shard key must be an object". What is the likely cause?

medium

A. The shard key argument must be an object, but a string was passed

B. The shard key value should be 1 or -1, not "hashed"

C. The command syntax is correct; the error is from elsewhere

D. The database name is missing in the command

Solution

1. Step 1: Check shardCollection syntax

   The shard key must be an object specifying the field and type, e.g., { orderId: "hashed" }.

2. Step 2: Identify error cause

   The error says shard key must be an object, so likely the argument passed was not an object but a string or other type.

3. Final Answer:

   The shard key argument must be an object, but a string was passed -> Option A

4. Quick Check:

   Shard key must be an object [OK]

Hint: Shard key must be an object, not a string [OK]

Common Mistakes:

• Passing shard key as string instead of object
• Confusing shard key with database name
• Ignoring error message details

5.

You want to shard a collection by email using hash-based sharding. However, you notice some shards have much more data than others. What could be a reason for this uneven distribution?

hard

A. The shard key was set to { email: 1 } instead of hashed

B. Hash-based sharding always causes uneven data distribution

C. The email field has many duplicate values causing hash collisions

D. The collection is too small to distribute evenly

Solution

1. Step 1: Understand hash-based sharding behavior

   Hash-based sharding usually distributes data evenly unless many documents share the same shard key value.

2. Step 2: Analyze cause of uneven data

   If many documents have the same email, their hash is the same, so they go to the same shard, causing imbalance.

3. Final Answer:

   The email field has many duplicate values causing hash collisions -> Option C

4. Quick Check:

   Duplicate keys cause uneven shard data [OK]

Hint: Duplicates in shard key cause uneven shard sizes [OK]

Common Mistakes:

• Assuming hash sharding always balances perfectly
• Confusing range-based with hash-based sharding
• Ignoring data distribution of shard key values