Modular Monolith: The Architecture Most Startups Should Start With

Source: Dev.to

The Problem With “Just Start With Microservices”

• Microservices solve organizational scale, not early‑stage product chaos.
• Early startups usually have:
  • Limited resources
  • Rapidly changing requirements
  • Small teams

Microservices introduce:

• Complex deployment pipelines
• Increased operational overhead
• Distributed debugging challenges

You trade one hard problem (code structure) for five new ones. At this stage, that trade‑off rarely makes sense.

What a Modular Monolith Actually Is

A modular monolith is a single deployable application with strong internal boundaries.

Key properties

• Clear module boundaries (e.g., /users, /billing, /analytics)
• Each module encapsulates its own domain logic and data access
• Not just a folder structure; boundaries are enforced by code and architecture

Why This Works So Well Early

• Cheap to Run – Deploy once; infrastructure costs stay predictable.
• Fast to Change – Requirements evolve daily; a single codebase is easier to modify.
• Security Is Easier – One application means fewer exposed surfaces.
• Team Clarity – Even with two developers, there’s no “who broke prod?” mystery.

The Most Important Part: Clean Boundaries

A modular monolith fails if boundaries are ignored.

Rules I enforce

1. Modules must own their data and never reach into another module’s tables.
2. Communication between modules occurs through well‑defined interfaces or services.
3. No circular dependencies; each module should be independently testable.

If this discipline isn’t followed, the monolith quickly devolves into a tangled codebase.

The Scaling Story (This Is Where It Shines)

When scale actually arrives:

• The existing modular structure makes it straightforward to extract services.
• You can gradually move high‑traffic modules to independent microservices.
• No full rewrite is required; the architecture evolves, it doesn’t reset.

Why I Recommend This to Early Founders

• Optimizes for speed, cost, and simplicity.
• Allows you to focus on product‑market fit before engineering complexity.
• You’re not betting against microservices; you’re simply delaying them until they’re truly needed.

That discipline is about pragmatic engineering, not fear of change.

Final Thought

Microservices are not a badge of seriousness.
The most expensive architecture is the one you build before you understand your problem.

Start with a modular monolith. Let real scale earn its complexity.