MongoDB Academy · レッスン

読み取り設定：読み取り負荷の分散

primary、primaryPreferred、secondary、nearest、およびタグ付きの読み取り設定を構成し、レプリカセット全体に読み取りを振り分けます。

「読み取り設定：読み取り負荷の分散」はCoddyKit上の無料MongoDB Academyレッスンです。これはレッスン4/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはMongoDB Academy学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 MongoDB Academyコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

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.

よくある質問

「読み取り設定：読み取り負荷の分散」レッスンは無料ですか？

はい。「読み取り設定：読み取り負荷の分散」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、MongoDB Academyコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 MongoDB Academyコースには全4レッスンが含まれています。

「読み取り設定：読み取り負荷の分散」で何を学びますか？

primary、primaryPreferred、secondary、nearest、およびタグ付きの読み取り設定を構成し、レプリカセット全体に読み取りを振り分けます。ブラウザで直接実行するハンズオンコードでMongoDB Academyを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

MongoDB Academyを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのMongoDB Academyは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン4/4です。

「読み取り設定：読み取り負荷の分散」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このMongoDB Academyレッスンでコードを書いて実行できますか？

はい。すべてのMongoDB Academyレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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