Database Replication

Replication means keeping copies of your data on multiple database servers. It’s essential for availability and read scaling.

Visual: Primary-Replica Replication

flowchart LR
    App["📱 Application"] --> Primary["Primary (Master)<br/>Writes + Reads"]
    Primary -->|"Sync Replication"| Replica1["Replica 1<br/>Read-only"]
    Primary -->|"Async Replication"| Replica2["Replica 2<br/>Read-only"]
    Primary -->|"Async Replication"| Replica3["Replica 3<br/>Read-only<br/>🌍 Different region"]

    Users["👥 Users"] --> App

    style App fill:#7c3aed,color:#fff
    style Primary fill:#059669,color:#fff
    style Replica1 fill:#4f46e5,color:#fff
    style Replica2 fill:#6366f1,color:#fff
    style Replica3 fill:#8b5cf6,color:#fff

Sync vs Async Replication

Aspect	Synchronous	Asynchronous
Write completes when…	All replicas confirm	Primary alone confirms
Data loss on failure?	❌ None	✅ Some (not-yet-replicated writes)
Write latency	Higher (wait for replicas)	Lower (primary only)
Read consistency	Always up-to-date	Potentially stale replicas
Best for	Financial transactions	Social media, analytics

Replication Topologies

Topology	How It Works	Trade-off
Single Primary	One leader writes, many replicas read	Most common. Leader is SPOF
Multi-Primary	Multiple nodes accept writes	Complex conflict resolution
Synchronous Multi-Primary	Write to all before confirming	High latency, very safe
Cascading Replica	Replica replicates to another replica	Reduces load on primary

Why Replicate?

Reason	Explanation
High availability	If primary fails, a replica can become the new primary
Read scaling	Distribute read queries across replicas (most apps are read-heavy)
Disaster recovery	Replicas in different regions survive regional outages
Analytics	Run heavy queries on replicas without impacting production

Trade-offs

Sync replication is safer but slower (every write waits for replicas).
Async replication is faster but risks losing recent writes if the primary crashes.
Replicas add operational cost — more servers to manage.

In Simple Words

Replication = making copies of your database so you don’t lose data if one server dies.
Primary handles writes. Replicas handle reads (most apps read way more than they write).
Sync is safe but slow. Async is fast but might lose a few writes during a crash.