Read from secondaries trade-offs in MongoDB

Introduction

Reading from secondary servers helps spread out the work and can make your app faster, but it might show data that is a little old.

When you want to reduce the load on the main database server.

When your app can handle slightly older data without problems.

When you want faster read responses by using nearby secondary servers.

When you need to keep your main server focused on writing data.

When you want to improve availability by reading from multiple servers.

Syntax

MongoDB

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

Use readPref('secondary') to tell MongoDB to read from secondary servers.

Secondary servers might not have the very latest data because of replication delay.

Examples

Reads user data from a secondary server to reduce load on the main server.

MongoDB

db.users.find().readPref('secondary')

Reads from secondary if available, otherwise reads from primary.

MongoDB

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

Always reads from the primary server to get the most up-to-date data.

MongoDB

db.products.find().readPref('primary')

Sample Program

This query reads the list of products from a secondary server to avoid putting extra work on the primary server.

MongoDB

use shopDB
// Read product list from secondary to reduce load
const products = db.products.find().readPref('secondary').toArray()
printjson(products)

Important Notes

Secondary reads may return data that is slightly out of date due to replication lag.

If your application needs the latest data, always read from the primary server.

Using 'secondaryPreferred' is a good balance if you want to read from secondaries but fall back to primary if needed.

Summary

Reading from secondaries helps balance load and improve read speed.

Data from secondaries might be a bit old because of replication delay.

Choose read preference based on how fresh your data needs to be.

Practice

1. What is a main advantage of reading data from secondary nodes in a MongoDB replica set?

easy

A. It helps balance the read load and can improve read speed.

B. It guarantees the freshest data with zero delay.

C. It prevents any replication lag from occurring.

D. It disables writes to the primary node.

Solution

1. Step 1: Understand the role of secondaries in MongoDB

   Secondary nodes replicate data from the primary and can serve read requests to reduce load on the primary.

2. Step 2: Identify the benefit of reading from secondaries

   Reading from secondaries balances read load and can improve read speed, but data might be slightly stale.

3. Final Answer:

   It helps balance the read load and can improve read speed. -> Option A

4. Quick Check:

   Reading from secondaries = load balancing and faster reads [OK]

Hint: Secondaries help with read speed but may lag [OK]

Common Mistakes:

• Thinking secondaries always have the freshest data
• Assuming secondaries prevent replication lag
• Believing reading from secondaries disables writes

2. Which of the following is the correct way to set a read preference to read from secondary nodes in MongoDB using the shell?

easy

A. db.collection.find().readPreference('secondary')

B. db.collection.find().setReadPref('primary')

C. db.collection.find().readPreference('primaryPreferred')

D. db.collection.find().readPreference('secondaryPreferred')

Solution

1. Step 1: Recall MongoDB read preference options

   MongoDB supports 'primary', 'primaryPreferred', 'secondary', and 'secondaryPreferred' as read preferences.

2. Step 2: Identify correct syntax for reading from secondaries

   To read from secondaries but allow fallback to primary, use 'secondaryPreferred'. The method is .readPreference('secondaryPreferred').

3. Final Answer:

   db.collection.find().readPreference('secondaryPreferred') -> Option D

4. Quick Check:

   Use .readPreference('secondaryPreferred') to read from secondaries [OK]

Hint: Use 'secondaryPreferred' to read from secondaries safely [OK]

Common Mistakes:

• Using 'secondary' which fails if no secondary is available
• Confusing 'primaryPreferred' with reading from secondaries
• Using incorrect method names like setReadPref

3. Consider a MongoDB replica set with some replication delay. If you run a query with read preference set to secondary, what is the most likely outcome?

medium

A. You get the freshest data always from the primary.

B. You might get slightly stale data due to replication lag.

C. The query will fail if the primary is down.

D. The query will always return an error if secondaries exist.

Solution

1. Step 1: Understand replication lag in secondaries

   Secondaries replicate data asynchronously, so there can be a delay causing stale data.

2. Step 2: Analyze read preference 'secondary'

   Reading from secondaries means data may not be the freshest due to replication lag.

3. Final Answer:

   You might get slightly stale data due to replication lag. -> Option B

4. Quick Check:

   Reading from secondaries = possible stale data [OK]

Hint: Secondary reads can be stale due to lag [OK]

Common Mistakes:

• Assuming secondary reads always return fresh data
• Thinking queries fail if primary is down when reading from secondary
• Believing reading from secondaries causes errors if secondaries exist

4. You set your MongoDB client to read from secondaries using readPreference: 'secondary', but your application sometimes gets errors when no secondaries are available. How can you fix this?

medium

A. Remove read preference to always read from primary.

B. Set read preference to 'primary' to avoid errors.

C. Change read preference to 'secondaryPreferred' to allow fallback to primary.

D. Restart the MongoDB server to fix the errors.

Solution

1. Step 1: Identify cause of errors

   Using 'secondary' read preference causes errors if no secondary nodes are available.

2. Step 2: Choose a read preference that allows fallback

   'secondaryPreferred' reads from secondaries if available, else falls back to primary, preventing errors.

3. Final Answer:

   Change read preference to 'secondaryPreferred' to allow fallback to primary. -> Option C

4. Quick Check:

   Use 'secondaryPreferred' to avoid errors when no secondaries [OK]

Hint: Use 'secondaryPreferred' to avoid errors if no secondaries [OK]

Common Mistakes:

• Switching to 'primary' loses read load balancing benefits
• Removing read preference may not fix fallback issues
• Restarting server does not solve read preference errors

5. Your application requires fast reads but can tolerate slightly stale data. You want to minimize load on the primary node. Which read preference should you choose and why?

hard

A. Use 'secondaryPreferred' to read from secondaries when available, else primary, balancing load and freshness.

B. Use 'secondary' to read only from secondaries, balancing load but risking stale data.

C. Use 'primaryPreferred' to always read from primary regardless of load.

D. Use 'primary' to ensure freshest data and reduce load on secondaries.

Solution

1. Step 1: Analyze application needs

   The app wants fast reads, can accept some stale data, and wants to reduce primary load.

2. Step 2: Evaluate read preference options

   'secondary' risks errors if no secondaries; 'secondaryPreferred' reads from secondaries if available, else primary, balancing load and freshness.

3. Step 3: Choose best option

   'secondaryPreferred' fits best as it balances load and freshness with fallback.

4. Final Answer:

   Use 'secondaryPreferred' to read from secondaries when available, else primary, balancing load and freshness. -> Option A

5. Quick Check:

   'secondaryPreferred' balances load and freshness [OK]

Hint: Choose 'secondaryPreferred' for load balance and fallback [OK]

Common Mistakes:

• Choosing 'primary' ignores load balancing
• Choosing 'secondary' risks errors if no secondaries
• Choosing 'primaryPreferred' ignores secondary reads