12. Read Concerns (local, majority, linearizable) & Write Concerns (w:majority, j:true) • MongoDB & NoSQL Architecture: CRUD to Sharding & WiredTiger • Rishabh Ki Kaksha

MongoDB & NoSQL Architecture: CRUD to Sharding & WiredTiger › 12. Read Concerns (local, majority, linearizable) & Write Concerns (w:majority, j:true)

45 mins

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Course Syllabus

Module 1: MongoDB Foundations, BSON & CRUD Mechanics

01. NoSQL vs SQL: CAP Theorem & Document-Oriented Architecture 02. BSON Data Types & Document Schema Design (Embedding vs Referencing) 03. High-Performance CRUD Operations, BulkWrite & Write Concerns 04. Schema Validation with JSON Schema ($jsonSchema)

Module 2: Advanced Aggregation Framework & Data Pipelines

05. Aggregation Pipeline Core: $match, $project, $group, $sort & $limit 06. Complex Transformations: $unwind, $lookup (Joins) & $facet (Multi-Facet Analytics) 07. Window Operators ($setWindowFields) & Accumulator Functions

Module 3: Indexing Strategies & Query Optimization

08. B-Tree Indexes in MongoDB: Single Field, Compound & Partial Indexes 09. Special Indexes: TTL (Time-To-Live), Geospatial (2dsphere) & Text Indexes 10. Query Profiling, explain("executionStats") & Index Covering Analysis

Module 4: Storage Engine Internals (WiredTiger) & ACID Transactions

11. WiredTiger Storage Engine Architecture: Cache, Compression & Checkpoints 12. Read Concerns (local, majority, linearizable) & Write Concerns (w:majority, j:true) 13. Multi-Document Distributed ACID Transactions with Sessions

Module 5: High Availability, Replica Sets, Sharding & Backups

14. Replica Sets Architecture: Primary Election, Oplog & Automatic Failover 15. Sharding Architecture: mongos Router, Config Servers & Shard Key Selection 16. Range vs Hash Sharding, Chunk Balancing & Jumbo Chunks Mitigation 17. Change Streams: Building Real-Time Event-Driven Pipelines with MongoDB 18. Production Backup Strategies: mongodump/mongorestore, Point-in-Time & Atlas CLI

Interactive Lesson Studio

# 12. Read Concerns (local, majority, linearizable) & Write Concerns (w:majority, j:true)

> MongoDB Notes by Rishabh - Topic 12

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### 1. Read Concerns: Consistency Levels Explained 📖

*   **`"local"` (Default):** Current node ki memory se turant data read karta hai (Fastest, par Dirty Read ka risk agar primary rollback ho jaye).
*   **`"majority"` (Big Tech Standard):** Sirf wahi data return karta hai jo **Replica Set ke majority voting nodes par commit ho chuka hai (Zero Rollback Risk!)**.
*   **`"linearizable"` (Real-Time Strict):** Primary doosre nodes se heartbeat confirm karta hai ki wo abhi bhi valid primary hai (Prevents Stale Reads during network splits, but slower).

---

### 2. Read Preferences: Routing Reads Across Replicas 🌐

```typescript
// Read from nearest geographical secondary node for low-latency reporting:
db.collection('analytics').find({}, {
    readPreference: 'nearest'
});
```

*   **`primary` (Default):** Saari reads master node par jaati hain.
*   **`secondary`:** Reads strictly replica nodes par jaati hain (Offloads reporting queries).
*   **`primaryPreferred` / `secondaryPreferred`:** Fallback support.

---

### 3. Causal Consistency Sessions (Read Your Own Writes)

```typescript
const session = client.startSession({ causalConsistency: true });
// Guarantee: User ne jo update kiya hai, agle read me use 100% updated data hi dikhega!
```

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### 4. Master Comparison Matrix

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### 5. Interview Ke 3 Sawal Is Topic Se

1. **Dirty Read MongoDB me kab ho sakti hai?** -> Agar `readConcern: "local"` use karein aur primary node par write acknowledge hone ke turant baad primary crash ho jaye bina secondaries ko replicate kiye.
2. **Secondary node se read karte waqt Replication Lag ka kya asar hota hai?** -> Secondary par thoda purana (stale) data read ho sakta hai; critical financial transactions me hamesha `primary` read preference use karni chahiye.
3. **Causal Consistency background me kya track karti hai?** -> Cluster time aur operation time tokens (`OperationTime`) ko session me pass karke Lamport Logical Clocks maintain karti hai.

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**Next Topic:** 13. Multi-Document Distributed ACID Transactions with Sessions

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Rishabh Gehlot

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