How to migrate monoliths to secure microservices

Why escape the monolith without breaking your application

Many companies are born on top of a large unified block of code, known in the industry as a monolith. At the beginning, this simplicity accelerates software delivery. However, as the business grows, any simple change requires recompiling the entire system and the risk of taking everything down increases dramatically. In practice, migrating to microservices — which are small independent programs talking to each other — should never be done overnight. Rushing this process usually turns a large monolith into a distributed, hard-to-debug chaos.

To avoid this disaster, modern engineering uses gradual strategies. Instead of rewriting the system from scratch over a weekend, the core idea is to slice the monolith bit by bit, isolating specific business domains. This approach requires architectural discipline and careful attention to the contact points between legacy systems and new services. The network and traffic security flowing between these new blocks becomes a primary pillar to prevent unwanted vulnerabilities.

The strangler strategy to carve up systems

The strangler pattern, known in literature as the Strangler Fig Application, relies on the behavior of vines wrapping around trees until they completely replace them. In software engineering, we apply this concept by building a new modern feature alongside the old monolith. When users access the system, an intermediary layer decides whether the request should go to the legacy code or the new microservice. Thus, very little changes for the end user, while the monolith gradually loses responsibilities month after month.

To illustrate how this process works, imagine we need to extract the customer registration module from a giant legacy system. We create an isolated microservice just for that and redirect registration traffic in a controlled manner. The rest of the application keeps running on the monolith without noticing the change. Real code to manage this transition in modern services frequently uses reverse proxy routines and integrated load balancers.

// Conceptual reverse proxy snippet in Node.js for strangulation
const express = require('express');
const { createProxyMiddleware } = require('http-proxy-middleware');
const app = express();

// Routes new user domain traffic to the microservice
app.use('/api/v2/users', createProxyMiddleware({ target: 'http://users-service:4000', changeOrigin: true }));

// All remaining traffic still goes to the legacy monolith
app.use('/', createProxyMiddleware({ target: 'http://legacy-monolith:8080', changeOrigin: true }));

app.listen(3000, () => console.log('Strangulation API Gateway running on port 3000'));

The vital role of the API Gateway in security

As we split the monolith into dozens of smaller pieces, a new problem arises: how does the external world talk to this fleet of services without exposing unprotected ports? This is exactly where the API Gateway comes in, acting like the central front desk of a gated community. No external request reaches internal microservices directly; everything goes through the Gateway first, which validates access tokens, checks rate limits, and enforces strict security rules.

In addition to protecting internal services against denial-of-service attacks and data injection, the Gateway centralizes authentication. Instead of every microservice having to validate passwords and JWT tokens (JSON Web Tokens, an open standard for securely transmitting compact information), the Gateway does this heavy lifting and passes down a secure header with the already validated user identity. This centralization drastically simplifies application code and reduces attack surfaces.

Database isolation and the end of foreign keys

One of the most serious and common mistakes when migrating to microservices is keeping all new services connected to a single gigantic relational database. If two microservices access the same table directly, they remain coupled under the hood, neutralizing any advantage of the new architecture. In practice, each microservice must have its own exclusive database, whether SQL or NoSQL, accessible strictly by its own code.

When a microservice needs data belonging to another domain, it does not execute a direct SQL JOIN on another team's database; instead, it consumes an internal API or listens to an event published on a message broker. This shift requires rethinking how we handle data consistency. What used to be solved with a simple atomic database transaction now requires patterns like the Saga Pattern, which manages eventual consistency across different services through compensating actions in case of failure. To complement this implementation in production environments, see our related technical analyses on How to repair broken printed circuit board traces and How to choose the ideal power supply for high-end PCs.

Final considerations on the migration journey

Migrating from a monolith to secure microservices is not just a technical decision about languages or frameworks, but a transformation in how engineering delivers value to the business. Adopting strategies such as gradual strangulation, rigorous use of an API Gateway, and absolute database isolation ensures that the transition happens predictably and without downtime. Carefully evaluating each step and securing communication boundaries is the secret to building resilient distributed systems prepared for sustainable growth.