Strangler Fig — Incremental Legacy Replacement

Incrementally replace a legacy system by routing new traffic to a new implementation while the old one still runs.

When to use

• Migrating a monolith to services
• Replacing a legacy API too risky to rewrite in one shot
• Need to ship continuously during migration

Tradeoffs

• Two systems in production simultaneously (data sync complexity)
• The facade routing layer becomes critical path during migration

Go

func router(legacyHandler, newHandler http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        if isNewFeature(r) {
            newHandler.ServeHTTP(w, r)
            return
        }
        legacyHandler.ServeHTTP(w, r)
    })
}

func isNewFeature(r *http.Request) bool {
    return strings.HasPrefix(r.URL.Path, "/v2/")
}

Python

from flask import Flask, request, Response
import httpx

app = Flask(__name__)

def is_new_feature(path: str) -> bool:
    return path.startswith("/v2/")

@app.route("/<path:path>", methods=["GET", "POST", "PUT", "DELETE"])
def facade(path: str) -> Response:
    target = "http://new-service" if is_new_feature(request.path) else "http://legacy"
    resp = httpx.request(request.method, f"{target}/{path}")
    return Response(resp.content, status=resp.status_code)

Gotcha: The facade IS the product during migration. Don't treat it as a throwaway — test it like production.