┌─────────────────────────────────────┐
│           Load Balancer             │
└──────────────┬──────────────────────┘
               │
       ┌───────┴───────┐
       │               │
┌──────▼──────┐ ┌──────▼──────┐
│   Legacy    │ │    New      │
│   System    │ │   Service   │
└─────────────┘ └─────────────┘

1. Route new features to new service
2. Gradually migrate existing features
3. Eventually retire legacy system

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def process(self, amount: float) -> bool:
    return self.legacy.old_process_method(amount)

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// Phase 1: Embed React in jQuery app
const root = document.getElementById("new-component");
ReactDOM.render(<NewComponent />, root);

// Phase 2: Shared state
window.appState = { user: null };
// Both jQuery and React read from appState

// Phase 3: Gradual replacement
// Replace one component at a time

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# Old API signature
def get_user(self, user_id):
    # Translate to new API
    return self.new.fetch_user(id=user_id)

# Deprecation warning
def old_method(self):
    warnings.warn(
        "old_method is deprecated, use new_method instead",
        DeprecationWarning
    )
    return self.new.new_method()

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