Migrating a monolithic application to microservices is one of the most common — and most misunderstood — undertakings in modern software engineering. After leading three such migrations, here is what I wish I had known from the start.
Why Migrate at All?
Not every monolith needs to be broken apart. Before reaching for the microservices hammer, ask yourself:
- Is the team struggling to deploy independently?
- Are build times exceeding 15-20 minutes?
- Is scaling one component impossible without scaling everything?
If the answer to at least two of these is yes, migration might be justified.
The Strangler Fig Pattern
The safest approach is the Strangler Fig Pattern. Instead of rewriting everything from scratch, you gradually replace pieces of the monolith with new services:
- Identify a bounded context with clear boundaries
- Build the new service alongside the monolith
- Route traffic to the new service via an API gateway
- Decommission the old code once the new service is stable
// Example: Routing via Spring Cloud Gateway
@Bean
public RouteLocator customRoutes(RouteLocatorBuilder builder) {
return builder.routes()
.route("orders-service", r -> r
.path("/api/orders/**")
.uri("lb://orders-service"))
.route("legacy-fallback", r -> r
.path("/api/**")
.uri("http://monolith:8080"))
.build();
}
Data Decomposition Is the Hard Part
Splitting the code is straightforward. Splitting the database is where the real complexity lies. Each service should own its data, but achieving that requires:
- Event sourcing for cross-service data synchronization
- Saga patterns for distributed transactions
- Temporary dual-writes during the transition period
Key Takeaways
- Start with the least coupled domain
- Invest in observability before you split anything
- Accept that the migration will take 2-3x longer than estimated
- Keep the monolith running until each new service proves itself in production
The goal is not microservices for their own sake — it is sustainable, independently deployable software that lets your team move faster.