Read preference for replica sets in MongoDB - Mini Project: Build & Apply

Read Preference for Replica Sets in MongoDB

📖 Scenario: You are managing a MongoDB database with a replica set. You want to control how your application reads data from the primary and secondary members of the replica set to balance load and ensure data freshness.

🎯 Goal: Build a MongoDB query setup that uses different read preferences to read data from the replica set members.

📋 What You'll Learn

Create a MongoDB collection named products with sample documents.

Set a read preference variable to secondaryPreferred.

Write a query that uses the read preference variable to read documents from the products collection.

Add a final configuration to confirm the read preference is applied.

💡 Why This Matters

🌍 Real World

In real applications, controlling read preference helps balance load between primary and secondary database servers, improving performance and availability.

💼 Career

Database administrators and backend developers often configure read preferences to optimize data reads in distributed MongoDB setups.

1

Create the products collection with sample data

Create a MongoDB collection called products and insert these exact documents: { _id: 1, name: "Pen", price: 1.5 }, { _id: 2, name: "Notebook", price: 3 }, and { _id: 3, name: "Eraser", price: 0.5 }.

MongoDB

// Insert the sample documents into the products collection
// Your code here

Hint

Use db.products.insertMany() with an array of documents.

2

Set the read preference variable

Create a variable called readPref and set it to the string "secondaryPreferred" to specify the read preference.

MongoDB

db.products.insertMany([
  { _id: 1, name: "Pen", price: 1.5 },
  { _id: 2, name: "Notebook", price: 3 },
  { _id: 3, name: "Eraser", price: 0.5 }
])

// Create the readPref variable and set it to "secondaryPreferred"
// Your code here

Hint

Use const readPref = "secondaryPreferred"; to set the variable.

3

Query the products collection using the read preference

Write a query that finds all documents in the products collection using the readPref variable as the read preference option.

MongoDB

db.products.insertMany([
  { _id: 1, name: "Pen", price: 1.5 },
  { _id: 2, name: "Notebook", price: 3 },
  { _id: 3, name: "Eraser", price: 0.5 }
])

const readPref = "secondaryPreferred";

// Use the readPref variable to read all documents from products
// Your code here

Hint

Use db.products.find().readPref(readPref) to apply the read preference.

4

Confirm the read preference is applied

Add a line to assign the read preference readPref to the cursor variable explicitly to confirm the read preference is set.

MongoDB

db.products.insertMany([
  { _id: 1, name: "Pen", price: 1.5 },
  { _id: 2, name: "Notebook", price: 3 },
  { _id: 3, name: "Eraser", price: 0.5 }
])

const readPref = "secondaryPreferred";

const cursor = db.products.find().readPref(readPref);

// Confirm the read preference is set on cursor
// Your code here

Hint

Call cursor.readPref() to confirm the read preference is applied.

Practice

1. What does the readPreference setting control in a MongoDB replica set?

easy

A. The size of the replica set

B. Which member to write data to

C. How often data is backed up

D. Which replica set member to read data from

Solution

1. Step 1: Understand the role of readPreference

   The readPreference setting tells MongoDB which replica set member to use for read operations.

2. Step 2: Differentiate read and write roles

   Writes always go to the primary, but reads can be directed to primary or secondary members based on readPreference.

3. Final Answer:

   Which replica set member to read data from -> Option D

4. Quick Check:

   Read preference = controls read source [OK]

Hint: Read preference decides read source, not write or backup [OK]

Common Mistakes:

• Confusing readPreference with write concern
• Thinking it controls backups
• Assuming it changes replica set size

2. Which of the following is the correct syntax to set read preference to secondaryPreferred in a MongoDB connection string?

easy

A. mongodb://host1,host2/?readPreference=secondaryPreferred

B. mongodb://host1,host2/?readPreference=primaryOnly

C. mongodb://host1,host2/?readPref=secondaryPreferred

D. mongodb://host1,host2/?readPreference=secondaryOnly

Solution

1. Step 1: Identify correct parameter name

   The correct parameter to set read preference in the connection string is readPreference.

2. Step 2: Match correct value

   secondaryPreferred is a valid read preference mode; others like primaryOnly or secondaryOnly are invalid or incorrect.

3. Final Answer:

   mongodb://host1,host2/?readPreference=secondaryPreferred -> Option A

4. Quick Check:

   Use readPreference with valid mode [OK]

Hint: Use exact parameter name 'readPreference' and valid mode [OK]

Common Mistakes:

• Using 'readPref' instead of 'readPreference'
• Using invalid modes like 'primaryOnly'
• Misspelling 'secondaryPreferred'

3. Given the following MongoDB query with read preference set to secondary:

db.collection.find({ status: "active" }).readPref("secondary")

What will happen if all secondary members are down?

medium

A. The query will read from the primary member

B. The query will fail with an error

C. The query will wait until a secondary is available

D. The query will return empty results

Solution

1. Step 1: Understand 'secondary' read preference behavior

   When read preference is set to secondary, reads must come from secondary members only.

2. Step 2: Consider all secondaries down scenario

   If no secondary is available, the query cannot read from primary or fallback; it results in an error.

3. Final Answer:

   The query will fail with an error -> Option B

4. Quick Check:

   Strict 'secondary' fails if no secondary [OK]

Hint: Strict 'secondary' read fails if no secondary available [OK]

Common Mistakes:

• Assuming fallback to primary happens automatically
• Thinking query waits indefinitely
• Expecting empty results instead of error

4. You wrote this code to read from a secondary member:

db.collection.find().readPref('secondaryPreferred')

But it still reads from the primary. What is the likely cause?

medium

A. The query must specify a filter to use read preference

B. The syntax for setting read preference is incorrect

C. No secondary members are available, so it falls back to primary

D. Read preference only works for writes

Solution

1. Step 1: Understand 'secondaryPreferred' behavior

   This mode reads from secondary if available; otherwise, it falls back to primary.

2. Step 2: Reason why primary is used

   If no secondary is available or reachable, the driver reads from primary as fallback.

3. Final Answer:

   No secondary members are available, so it falls back to primary -> Option C

4. Quick Check:

   'secondaryPreferred' falls back to primary if no secondary [OK]

Hint: 'secondaryPreferred' falls back to primary if no secondary [OK]

Common Mistakes:

• Thinking syntax is wrong when it is correct
• Believing read preference needs a filter
• Confusing read preference with write behavior

5. You want to optimize read performance by reading from secondaries but ensure data is fresh enough. Which read preference mode should you choose?

hard

A. secondaryPreferred

B. nearest

C. primary

D. secondary

Solution

1. Step 1: Understand read preference modes

   primary reads only from primary (fresh but may be slower), secondary reads only from secondaries (may be stale), secondaryPreferred reads secondaries or falls back to primary, nearest reads from the closest member regardless of type.

2. Step 2: Balance freshness and performance

   secondaryPreferred prefers secondaries for read performance optimization but falls back to the primary to ensure data freshness and availability.

3. Final Answer:

   secondaryPreferred -> Option A

4. Quick Check:

   secondaryPreferred prefers secondary perf + primary freshness fallback [OK]

Hint: 'secondaryPreferred' for secondary perf with primary fallback [OK]

Common Mistakes:

• Choosing 'secondary' which may return stale data
• Choosing 'primary' which may be slower
• Confusing 'secondaryPreferred' with 'nearest'