Strangler Fig

A full rewrite is a multi-year bet with no working software until it ships

Rewriting a large legacy system from scratch means running both the old and new systems' full feature sets in parallel for a long stretch, with a risky cutover at the end — and the business logic buried in the old system tends to be poorly documented, so scope creeps as "just one more thing" is discovered.

A façade routes traffic, piece by piece, to the new system

Named after a fig vine that grows around a host tree and gradually replaces it: a routing façade sits in front of the legacy system. New functionality is built and deployed behind the façade, one route at a time; the façade sends matching requests to the new service and everything else to the legacy system — until nothing routes to the legacy system anymore.

A façade routing incrementally to a new service

Only `/orders/*` has been migrated so far; every other path still falls through to the legacy system unchanged.

const legacyMonolith = {
  handle: (path) => `legacy: handled ${path}`,
};

const newOrdersService = {
  handle: (path) => `new-orders-service: handled ${path}`,
};

function facade(path) {
  if (path.startsWith("/orders")) {
    return newOrdersService.handle(path); // migrated route
  }
  return legacyMonolith.handle(path); // not migrated yet
}

console.log(facade("/orders/42"));   // routed to the new service
console.log(facade("/inventory/9")); // still routed to legacy

Trade-offs

• Always shippable, low-risk steps — Each migrated route can go live independently, with the rest of the system unaffected.
• Rollback is one routing rule — If a newly migrated piece has a problem, the façade can route that path back to legacy immediately.
• The façade runs for a long time — Migrations like this can take months or years — the façade itself needs to be treated as production infrastructure, not a temporary hack.