Modern Backend Architecture: From Monolith to Event-Driven

I've shipped backends at every stage of this evolution — from a single Spring Boot JAR handling everything, to a distributed system with 40+ microservices communicating through event streams. Here's what each stage looks like and when to move between them.

Stage 1: The Well-Structured Monolith

Every system should start here. Not because monoliths are inferior — but because they're the fastest path to validating your domain model.

The key insight: modules talk through service interfaces, never directly to each other's repositories. This discipline makes extraction possible later.

Stage 2: API Gateway + Service Extraction

When your monolith hits scaling limits — usually one module needs to scale differently than others — you extract your first service.

The golden rule: database per service. The moment two services share a database, you've created a distributed monolith — all the complexity of microservices with none of the benefits.

Stage 3: Event-Driven Architecture

Synchronous REST calls between services create tight coupling. Service A calling Service B which calls Service C means all three must be up simultaneously. Events break this chain.

The Order Service doesn't know (or care) who consumes its events. Adding a new analytics service? Just subscribe to the topic. Zero changes to existing services.

The Saga Pattern: Distributed Transactions

In a monolith, a database transaction guarantees atomicity. With microservices, you need Sagas — a sequence of local transactions coordinated through events.

Each service either continues the saga or triggers a compensating action to rollback. No distributed locks, no two-phase commit.

Observability: The Non-Negotiable Layer

You cannot debug a distributed system with System.out.println. The three pillars — metrics (Prometheus), logs (Loki/ELK), and distributed traces (Tempo/Jaeger) — are non-negotiable from day one.

Choosing Your Stage

The worst architecture is the one that's more complex than your team can operate. Scale your architecture with your team, not ahead of it.