Database Replication Explained (Replication Lag, Sync vs Async) | System Design with Microsoft SWE
Database replication is one of those topics that many engineers use every day, but few truly understand end-to-end.
In this video, I explain how database replication actually works in real systems, starting from the basics and gradually going deeper into the internal mechanics that matter in production and system design interviews.
We’ll cover:
What primary–replica replication is and why it exists
How write-ahead logs (WAL / binlog) power replication
Synchronous vs asynchronous replication and real-world trade-offs
What replication lag really means and why it happens
How systems handle stale reads using read-your-own-write and session stickiness
Why large transactions increase replication lag
How replication works in geo-distributed systems and where bottlenecks appear
The goal of this video is not to give definitions, but to build clear intuition around replication, consistency, durability, and performance trade-offs — the same way they appear in real backend systems.
Timestamps
00:00 Intro
00:10 What is Database Replication?
05:12 How replication works.
07:06 Synchronous Replication
09:28 Asynchronous Replication
10:20 Semi-Synchronous Replication
11:59 Replication Lag
14:30 Read Routing Based on Lag
16:24 Read-Your-Own-Writes
17:14 Session Stickiness
19:26 Outro
#database #systemdesign #coding #distributedsystems #sql #softwarearchitecture
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