Monoliths

📘What it really is

• A monolith is an application built and deployed as one unit: one codebase, one deployable (one container, Module 69), usually one database. Your CRM is a monolith.

• Strengths:

• simple to build, test, deploy and debug: one place to look;
• fast in-process function calls between parts, no network in between;
• transactions across features are easy (one database, Module 56);

• one set of infrastructure to run and pay for.

• Weaknesses as it grows:

• without discipline, parts become tangled (a "big ball of mud"), and every change risks breaking something unrelated;
• the whole app is deployed for every small change;
• one heavy part can't be scaled separately (though you can run several copies of the whole thing, Module 70);

• large teams step on each other in one codebase.

• The modular monolith keeps one deployable but divides the code into modules with clear boundaries: bookings, availability, payments, notifications, each with its own folder, public interface and, ideally, its own tables. Modules call each other only through those interfaces, never by reaching into each other's internals or tables.

• Most products should start as a (modular) monolith. Many successful companies run large, well-structured monoliths for years.