The reality is always the same: The rewrite takes 18 months, not 6. The business stalls. The new system launches with critical bugs that the old system solved years ago.

At SpectreDev, we refuse to execute Big Bang rewrites. Instead, we deploy the Strangler Fig Pattern. We slowly suffocate the legacy system by replacing functionality piece by piece.

The Interception Layer

Before writing a single line of microservice code, you must stop clients from talking directly to your monolith. You need an Anti-Corruption Layer (ACL).

We implement this via an API Gateway or a Reverse Proxy. This "Facade" sits between the public internet and your backend.

The Extraction

Identify a distinct domain boundary (e.g., UserAuth or Inventory). Do not pick the hardest module first. Pick the one with the lowest dependency coupling.

The Routing Logic

The Gateway now makes a decision based on the URL path. Here is the Nginx configuration we deploy for Phase 1:

1upstream legacy_backend {
2  server 10.0.1.5:8080;
3}
4
5upstream new_inventory_service {
6  server 10.0.2.10:3000;
7}
8
9server {
10  listen 80;
11
12  # Route 1: The New Service (Canary)
13  location /api/v2/inventory {
14      proxy_pass http://new_inventory_service;
15      proxy_set_header X-Migration-Stage "canary";
16  }
17
18  # Route 2: The Default (Legacy Fallback)
19  location / {
20      proxy_pass http://legacy_backend;
21  }
22}

Data Synchronization

When you move Inventory to a new service, the Monolith still expects to see inventory data in its old database tables. You need a Change Data Capture (CDC) strategy.

We utilise Debezium or a custom worker to listen to the new database transaction log and replay events to the legacy system.

Summary

Migration is not a code problem; it is a traffic control problem. By placing a Facade in front of your legacy system, you buy yourself the time to re-architect without halting the business.