What is the strangler fig pattern for migrating Flask to FastAPI?

The strangler fig pattern migrates a Flask API to FastAPI incrementally without a big-bang rewrite. Strategy: 1. Set up a reverse proxy (Nginx) in front of both Flask and FastAPI apps running simultaneously. 2. New endpoints: implement in FastAPI. 3. Migrate existing endpoints one by one from Flask to FastAPI — route each migrated path to FastAPI in Nginx. 4. Gradually strangle Flask as more routes move to FastAPI. Mounting apps: FastAPI can mount a WSGI app: from a2wsgi import WSGIMiddleware; app.mount('/legacy', WSGIMiddleware(flask_app)). This lets FastAPI and Flask share the same process — FastAPI handles new routes, Flask handles legacy routes. Share a database connection pool and authentication logic. Eventually, all routes are in FastAPI and the Flask app is removed. This approach minimizes risk by never having a complete application rewrite that must be deployed atomically.