Strangler Fig

What is Strangler Fig?

The Strangler Fig pattern incrementally replaces a legacy system by gradually routing traffic to new components until the old system can be decommissioned. Named after strangler fig trees that grow around host trees.

The Pattern

Phase 1: New component alongside legacy

Phase 2: Gradual migration

Phase 3: Complete migration

How It Works

1. Identify: Find functionality to migrate
2. Build: Create new implementation
3. Route: Proxy routes traffic based on rules
4. Migrate: Gradually increase new system traffic
5. Remove: Decommission legacy when complete

Implementation Strategies

By Feature

Migrate feature by feature:

/users/*     → New service
/orders/*    → Legacy (next)
/products/*  → Legacy

By User Segment

Route specific users to new system:

Beta users    → New system
Regular users → Legacy

By Percentage

Gradual rollout:

5% traffic  → New system (test)
50% traffic → New system (validate)
100% traffic → New system (complete)

Proxy/Router Patterns

URL-Based Routing

location /api/v2/users {
    proxy_pass http://new-service;
}

location /api/users {
    proxy_pass http://legacy;
}

Header-Based Routing

X-Use-New-System: true → New system
(no header)           → Legacy

Benefits

Challenges

Data Migration

Options for handling data during migration:

Option 1: Shared Database

Option 2: Data Sync

Option 3: Event-Based

Real-World Example

E-commerce migration:

Week 1-4:   Product catalog → New system
Week 5-8:   User profiles → New system
Week 9-12:  Shopping cart → New system
Week 13-16: Checkout → New system
Week 17+:   Decommission legacy

Interview Tips

• Explain incremental migration concept
• Discuss routing strategies (URL, header, percentage)
• Mention data synchronization challenges
• Compare with big bang rewrite risks
• Give use cases: monolith to microservices migration