Three Modules, One Process
DevMentors' ModularMonolith sample packs three conference-system modules, a mini-framework and one deliberately broken contract into eighty-four C# files - opening a series that reads all of it, defects included.
Series
One Process, Three Modules
In reading order.
- 14 parts
- ≈ 66 min total
- 14 published
The Compiler Guards the Module Border
ModularMonolith makes every controller internal and teaches MVC to find them anyway - compile-time module isolation from one feature provider, an InternalsVisibleTo chain, and the one place the wall has a hole.
Add, Use, Repeat — The Module Recipe
Every module in ModularMonolith plugs in through the same Add/Use extension pair - a recipe small enough to memorise, complete enough to be a mini-framework, and visible even where a module registers nothing at all.
Startup Order Is Load-Bearing
The last commit of ModularMonolith moves one line of Startup from first to last - and the whole cross-module event system depends on it. Assembly scanning, throwaway service providers and migrations inside ConfigureServices.
An Event Bus in Seven Files
ModularMonolith builds its message broker from System.Threading.Channels and one BackgroundService - a textbook Message Channel and Event-Driven Consumer in miniature, with every missing enterprise-integration pattern visible by its absence.
Serialize, Deserialize, Deliver
ModularMonolith routes events between modules by class name and converts them by JSON round-trip - a one-line Message Translator that lets modules share contracts without sharing types, at a price the next part collects.
One Rename Broke the Only Contract
The last commit of ModularMonolith renames Name to ConferenceName in the subscriber's copy of its one integration event - and the JSON translator has delivered null into that field ever since. Anatomy of a silent contract break.
One Flag, Two Failure Modes
A single boolean in appsettings decides whether ModularMonolith's events run inside your HTTP request or on a background pump - and each setting fails differently. Delivery semantics, read honestly from a ternary.
The Database Splits Before the App Does
One Postgres database, one schema per module, and entity configurations that are all empty bodies - what ModularMonolith's data layer declares about future service boundaries, and what its migrations quietly gave away.
Two Repositories, One Winner
Conferences registers in-memory and EF repositories for the same interfaces and lets last-registration-wins pick - a live lesson in DI as strategy selector, dead registrations as fossils, and a no-op UpdateAsync nobody deleted.
Exception Names Become Error Codes
One middleware turns ConferenceNotFoundException into the wire code conference_not_found via Humanizer - a naming convention promoted to API contract, with a 400-versus-404 tension and Error-level logs it never resolved.
Invariants Enforced by Comment
ModularMonolith's entities are public-setter data bags, its one immutability rule is a fixup with a comment, and its hardest domain question is asked in a comment and answered by nothing - a reading of domain modelling by annotation.
Two Modules, Three Months of Drift
Conferences and Speakers disagree on layout, naming, id policy, exception visibility, mapping style and route constraints - plus a 200 that returns nothing and a broker that threw NotImplementedException for four months. The honest ledger.
The Road Out of the Monolith
Which of ModularMonolith's three modules could become a service tomorrow, what blocks the other two, and the hardening bill the sample never pays - a retrospective on the modular monolith as a station, not a destination.