1. Monolith, modular monolith and microservices

Monolith — all features in one process and one database.

• ✅ Simple development, debugging, transactions, deployment.
• ❌ Grows into a "big ball of mud"; one bug or slow module affects all; the whole app scales together.

Modular monolith — one deployable, but modules are isolated:

Shop.Api (one process)
 ├── Modules/Orders     (orders schema, IOrdersModule public API)
 ├── Modules/Catalog    (catalog schema)
 └── Modules/Billing    (billing schema)
Rule: modules talk only through public interfaces or in-process events — never each other's tables.

Microservices — each module becomes its own service with its own database and pipeline.

Interview tip: "I would start with a modular monolith and extract a service when a module needs independent scaling, release cadence or ownership." This is the answer most senior interviewers want.