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Replication Lag Explained for System Design Interviews
Last Updated: January 5, 2026
Ashish Pratap Singh
8 min read
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Replication Lag Explained for System Design Interv...
1. What Is Replication Lag?
1.1 The Basic Setup
1.2 Lag in Time vs Lag in Bytes
1.3 Synchronous vs Asynchronous Replication
2. Why Replication Lag Occurs
2.1 The Replication Pipeline
2.2 Common Causes of Lag
2.3 Lag Characteristics
3. Problems Caused by Replication Lag
3.1 Reading Your Own Writes
3.2 Monotonic Reads Violation
3.3 Consistent Prefix Reads Violation
3.4 Lost Updates Illusion
3.5 Summary of Anomalies
4. Measuring and Monitoring Lag
4.1 PostgreSQL Replication Lag
4.2 MySQL Replication Lag
4.3 MongoDB Replica Set Lag
4.4 Key Metrics to Monitor
4.5 Alerting Strategy
5. Strategies to Handle Replication Lag
5.1 Read-Your-Writes Consistency
5.2 Monotonic Reads Consistency
5.3 Causal Consistency
5.4 Bounded Staleness
5.5 Summary of Strategies
6. Reducing Replication Lag
6.1 Optimize the Replica
6.2 Parallel Replication
6.3 Reduce Read Load on Replica
6.4 Break Up Large Transactions
6.5 Geographic Optimization
6.6 Replication Compression
7. Replication Lag in Different Databases
7.1 PostgreSQL
7.2 MySQL
7.3 MongoDB
7.4 Cassandra
7.5 Comparison
8. System Design Scenarios
8.1 Scenario: E-Commerce Order System
8.2 Scenario: Social Media Feed
8.3 Scenario: Analytics Dashboard
8.4 Scenario: Inventory System
9. Common Interview Questions
9.1 Conceptual Questions
9.2 Design Questions
9.3 Quick Reference
Summary
References
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