14. Replica Sets Architecture: Primary Election, Oplog & Automatic Failover • MongoDB & NoSQL Architecture: CRUD to Sharding & WiredTiger • Rishabh Ki Kaksha

MongoDB & NoSQL Architecture: CRUD to Sharding & WiredTiger › 14. Replica Sets Architecture: Primary Election, Oplog & Automatic Failover

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

# 14. Replica Sets Architecture: Primary Election, Oplog & Automatic Failover

> MongoDB Notes by Rishabh - Topic 14

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### 1. Replica Set Topology (High Availability 99.999% 🛡️)

Production MongoDB hamesha minimum **3-Node Replica Set (1 Primary + 2 Secondaries)** me run karta hai:

```
                  [ Primary (Master - Port 27017) ] (Accepts all Writes)
                                ⬇ (Async Oplog Replication)
        ┌───────────────────────┴───────────────────────┐
        ▼                                               ▼
[ Secondary 1 (Port 27018) ]                [ Secondary 2 (Port 27019) ]
```

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### 2. The Oplog (`local.oplog.rs` Capped Collection)

Primary par hone wala har write operation ek special circular buffer collection **Oplog** me idempotent format me record hota hai:
*   Secondaries lagatar Primary ke Oplog ko tail karke exact same operations apne local storage engine par apply karti hain.

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### 3. Raft-Based Automatic Election & Failover ⚡

Agar Primary crash ho jaye:
1. Secondaries **2-second heartbeats** miss hone par detect karti hain.
2. 10-second election timeout trigger hota hai.
3. Jis Secondary ke paas **sabse latest Oplog timestamp** hota hai, baki nodes use vote karti hain.
4. **Majority consensus (2/3 votes)** aate hi wo Secondary naya **Primary** ban jaata hai (Failover in `< 3 seconds`!).

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### 4. Master Comparison Matrix

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

1. **Arbiter node production me kyu discourage kiya jata hai?** -> Arbiter data store nahi karta isliye redundancy provide nahi karta; modern architecture me 3 full data-bearing nodes prefer kiye jaate hain.
2. **Oplog window size kyu monitor karni chahiye?** -> Agar heavy bulk writes ke dauran secondary node network se disconnect ho jaye aur Oplog overwrite ho jaye, toh secondary stale ho jayegi aur use full initial sync (`resync`) karna padega.
3. **Hidden Secondary node ka use case kya hota hai?** -> Analytics / ETL pipelines chalane ke liye aisi dedicated secondary node banana jo client read traffic se invisible rahe aur primary election me vote na kare.

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**Next Topic:** 15. Sharding Architecture: mongos Router, Config Servers & Shard Key Selection

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