The Strangler Fig Pattern

The strangler fig pattern places a routing layer (Nginx, a front controller, or a modern framework) in front of the legacy app. New or rewritten features are served by new code; everything else falls through to the old app. Over time, routes migrate until the legacy code can be removed. Share sessions/auth between both during the transition.

# Nginx: new routes go to the Symfony/Laravel app, everything else to legacy
location ^~ /api/v2/ {
    try_files $uri /new/index.php$is_args$args;
}
location ^~ /checkout/ {
    try_files $uri /new/index.php$is_args$args;
}
location / {
    try_files $uri $uri/ /legacy/index.php$is_args$args;
}

Alternatively, a modern framework can be the front controller and forward unknown routes to legacy scripts via a fallback controller (e.g., a "legacy bridge" that requires the old file inside output buffering).

Common pitfalls

• Two apps with different session formats, forcing users to log in twice.
• No plan to delete legacy routes after migration.
• Duplicating business rules in old and new code that drift apart.

Key points

• Route new features to new code
• Migrate route by route
• Share auth/session; delete old code when done

Further reading: Martin Fowler: Strangler Fig