Read Preferences: Distributing Read Load
Learners will configure primary, primaryPreferred, secondary, nearest, and tagged read preferences to route reads across the replica set.
Read Preferences: Distributing Read Load is a free MongoDB Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the MongoDB Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What Is a Read Preference?
A read preference controls which replica set member the driver sends read operations to. By default, all reads go to the primary to ensure you always see the latest data. However, redirecting reads to secondaries can reduce load on the primary and bring reads geographically closer to users — at the cost of potentially reading slightly stale data.
primary: Always Read From Primary
primary (the default) routes every read to the primary. This guarantees strong consistency — you will always read your own writes and see the most up-to-date data. The downside is that the primary handles all read and write traffic. Use this mode whenever stale data would be unacceptable, such as in financial applications or user-facing dashboards.
// Explicit primary read preference (this is the default)
const col = db.collection('accounts', {
readPreference: 'primary'
})
await col.findOne({ _id: userId })primaryPreferred: Fallback to Secondary
primaryPreferred reads from the primary when it is available but falls back to a secondary if the primary is unreachable. This provides a balance: you normally get fresh data, but the application degrades gracefully during primary elections. Be aware that during the fallback, you may read data that is a few seconds behind.
const col = db.collection('products', {
readPreference: 'primaryPreferred'
})
await col.find({}).toArray()secondary: Read Only From Secondaries
secondary sends all reads to available secondaries, never touching the primary. This is ideal for analytics queries, batch jobs, or reporting that can tolerate eventual consistency. It offloads significant read pressure from the primary. If no secondary is available, the operation fails — it will NOT fall back to the primary.
// Route analytics query to secondary
const col = db.collection('events', {
readPreference: 'secondary'
})
const count = await col.countDocuments({ type: 'click' })secondaryPreferred: Secondary With Fallback
secondaryPreferred prefers secondaries but falls back to the primary if no secondary is available. This is the most commonly used non-default mode — it spreads read load across secondaries under normal conditions while remaining available when a secondary goes down. The caveat is the same as secondary: reads may be slightly stale.
const col = db.collection('catalog', {
readPreference: 'secondaryPreferred'
})
await col.find({ category: 'books' }).toArray()nearest: Lowest Network Latency
nearest sends reads to the member with the lowest measured network round-trip time, regardless of whether it is primary or secondary. This is perfect for globally distributed applications where users in different regions should read from the closest data center. Note that different users may read from different members, so they may temporarily see different data states.
const col = db.collection('content', {
readPreference: 'nearest'
})
await col.findOne({ slug: 'hello-world' })Tag Sets: Targeting Specific Members
Tag sets let you label replica set members (e.g., { region: 'us-east' }, { purpose: 'analytics' }) and then write read preferences that target only matching members. This enables sophisticated routing: send application reads to the nearest region and analytics queries to a dedicated secondary, all within the same replica set.
// Tag a member in replica set config
let cfg = rs.conf()
cfg.members[2].tags = { region: 'eu-west', purpose: 'analytics' }
rs.reconfig(cfg)
// Read from eu-west analytics member
const col = db.collection('reports', {
readPreference: new ReadPreference('secondary', [{ region: 'eu-west' }])
})Stale Reads and Replication Lag
When reading from secondaries, you may encounter stale reads — the secondary hasn't replicated the latest writes from the primary yet. The staleness depends on replication lag. MongoDB 3.6 introduced maxStalenessSeconds: the driver refuses to use a secondary lagging more than the specified number of seconds, preventing very stale reads.
// Reject secondaries more than 90 seconds behind
const col = db.collection('products', {
readPreference: new ReadPreference(
'secondaryPreferred',
[],
{ maxStalenessSeconds: 90 }
)
})Read Preference in the Connection String
You can set the default read preference for all operations in the connection string using the readPreference parameter. This is the easiest way to configure the default without changing every call site. Individual operations can still override it by passing a readPreference option explicitly.
// Connection string with secondaryPreferred default
const uri = 'mongodb+srv://host/db?readPreference=secondaryPreferred&maxStalenessSeconds=120'
const client = new MongoClient(uri)Read Preference and Transactions
Multi-document transactions must always use primary read preference. Operations inside a transaction that specify a different read preference will throw an error. This ensures that all reads within the same transaction see a consistent snapshot as of the transaction's start time on the primary.
// Inside a transaction, reads always go to primary
const session = client.startSession()
await session.withTransaction(async () => {
// This implicitly uses primary read preference
const order = await db.orders.findOne({ _id: orderId }, { session })
await db.inventory.updateOne({ sku: order.sku }, { $inc: { qty: -1 } }, { session })
})Choosing Read Preference in Practice
Use this guide: Real-time user data (cart, profile, notifications) → primary. Product catalog, content → secondaryPreferred. Reports and analytics → secondary with a dedicated analytics member. Global CDN-like reads → nearest. Always measure replication lag before routing critical reads to secondaries.
Quick Check
Test your understanding of MongoDB & NoSQL Databases concepts from this lesson.
Lesson Recap
In this lesson you learned: read preferences control which replica set member handles read operations, secondary and secondaryPreferred offload read traffic at the cost of eventual consistency, and nearest minimises latency in geo-distributed deployments. Next up we explore sharding — MongoDB's strategy for scaling beyond a single replica set.
Frequently asked questions
Is the “Read Preferences: Distributing Read Load” lesson free?
Yes — the full text of “Read Preferences: Distributing Read Load” is free to read here on the web, and the MongoDB Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the MongoDB Academy course, upgrade to CoddyKit PRO.
What will I learn in “Read Preferences: Distributing Read Load”?
Learners will configure primary, primaryPreferred, secondary, nearest, and tagged read preferences to route reads across the replica set. You practise MongoDB Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start MongoDB Academy?
No prior experience is required. MongoDB Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Read Preferences: Distributing Read Load” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this MongoDB Academy lesson?
Yes. Every MongoDB Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Replica Set Members: Primary, Secondary, Arbiter
- Elections and Automatic Failover
- Write Concerns and Acknowledged Durability
- Read Preferences: Distributing Read Load