Read preference for replica sets in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Which read preference returns data only from the primary?

In MongoDB replica sets, which read preference ensures that all read operations return data only from the primary node?

A secondary

B secondaryPreferred

C primary

D nearest

💡 Hint

Think about which node is the main source of truth in a replica set.

The 'primary' read preference directs all reads to the primary node, which is the main writable node in a replica set.

❓ query_result

intermediate

What is the output of this read preference setting?

Given the MongoDB read preference set to 'secondaryPreferred', where will the read operations be directed if a secondary is available?

A Reads go to a secondary node if available, otherwise to the primary.

B Reads go only to the nearest node regardless of type.

C Reads go only to the primary node.

D Reads go only to secondary nodes and never to primary.

💡 Hint

Consider the fallback behavior of 'secondaryPreferred'.

'secondaryPreferred' tries to read from a secondary but falls back to primary if no secondary is available.

🧠 Conceptual

advanced

Which read preference minimizes read latency by choosing the closest node?

In a MongoDB replica set, which read preference directs read operations to the node with the lowest network latency, regardless of whether it is primary or secondary?

A nearest

B primary

C primaryPreferred

D secondary

💡 Hint

Think about the read preference that considers network distance.

The 'nearest' read preference sends reads to the node with the lowest network latency, whether primary or secondary.

📝 Syntax

advanced

Identify the correct syntax to set read preference to 'secondary' in a MongoDB connection string

Which of the following MongoDB connection strings correctly sets the read preference to 'secondary'?

A mongodb://host1,host2/?read_preference=secondary

B mongodb://host1,host2/?readPreference=secondary

C mongodb://host1,host2/?readPreference=Secondary

D mongodb://host1,host2/?readPreferenceSecondary

💡 Hint

Check the exact parameter name and case sensitivity.

The correct parameter is 'readPreference' with exact casing and the value 'secondary' in lowercase.

❓ optimization

expert

Optimizing read preference for a geographically distributed replica set

You have a MongoDB replica set with nodes in three different continents. You want to minimize read latency for users worldwide while ensuring strong consistency when necessary. Which read preference strategy should you use?

A Use 'secondary' only to avoid primary node load, ignoring consistency.

B Use 'secondaryPreferred' for all operations to balance load evenly.

C Always use 'primary' to ensure strong consistency, ignoring latency.

D Use 'nearest' for reads to minimize latency, and 'primary' for writes to ensure consistency.

💡 Hint

Consider balancing latency and consistency requirements.

Using 'nearest' for reads reduces latency by reading from the closest node, while writes go to the primary to maintain strong consistency.

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'