Hash-based sharding in MongoDB - Mini Project: Build & Apply

Hash-based Sharding in MongoDB

📖 Scenario: You are managing a large online bookstore database. To handle many users and fast queries, you want to split the book data across multiple servers using hash-based sharding.

🎯 Goal: Set up a MongoDB collection with hash-based sharding on the ISBN field to distribute book documents evenly across shards.

📋 What You'll Learn

Create a MongoDB database called bookstore

Create a collection called books inside bookstore

Enable sharding on the bookstore database

Shard the books collection using a hashed shard key on the ISBN field

Insert sample book documents with ISBN, title, and author

💡 Why This Matters

🌍 Real World

Large-scale applications like online bookstores use sharding to handle huge amounts of data and many users efficiently.

💼 Career

Understanding sharding is important for database administrators and backend engineers working with distributed databases to ensure performance and scalability.

1

Create the bookstore database and books collection

Create a MongoDB database called bookstore and a collection called books. Insert these exact documents into books: { ISBN: "978-0143127741", title: "Sapiens", author: "Yuval Noah Harari" }, { ISBN: "978-0385543767", title: "The Midnight Library", author: "Matt Haig" }, { ISBN: "978-0062316097", title: "The Alchemist", author: "Paulo Coelho" }.

MongoDB

// Use the bookstore database
use bookstore

// Insert the three book documents into books collection
// Your code here

Hint

Use use bookstore to switch to the database. Use db.books.insertMany([...]) to add the documents.

2

Enable sharding on the bookstore database

Enable sharding on the bookstore database by running sh.enableSharding("bookstore").

MongoDB

use bookstore

// Enable sharding on the bookstore database
// Your code here

Hint

Use the sh.enableSharding() command with the database name.

3

Shard the books collection using a hashed shard key on ISBN

Shard the books collection in the bookstore database using a hashed shard key on the ISBN field by running sh.shardCollection("bookstore.books", { ISBN: "hashed" }).

MongoDB

use bookstore

sh.enableSharding("bookstore")

// Shard the books collection on ISBN using hashed shard key
// Your code here

Hint

Use sh.shardCollection() with the full collection name and specify the shard key as { ISBN: "hashed" }.

4

Insert an additional book document after sharding

Insert a new book document into the books collection with these exact values: { ISBN: "978-1501128035", title: "The Power of Habit", author: "Charles Duhigg" }.

MongoDB

use bookstore

sh.enableSharding("bookstore")

sh.shardCollection("bookstore.books", { ISBN: "hashed" })

// Insert the new book document
// Your code here

Hint

Use db.books.insertOne() with the exact document.

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