Building Modular APIs with ASP.NET Core

Build ASP.NET Core APIs that stay maintainable as your application grows.

Business applications rarely become difficult because they lack features. They become difficult because boundaries disappear. Database access spreads across the codebase, business rules are duplicated, modules become tightly coupled, and even a small change can affect parts of the system nobody expected.

Building Modular APIs with ASP.NET Core shows you how to solve that problem with a practical, production-oriented modular architecture.

Using .NET 10 LTS, ASP.NET Core 10, C# 14, Entity Framework Core 10, and PostgreSQL, you will build AtlasOps, a realistic business application composed of Customers, Work Orders, Billing, and Identity modules. Instead of treating modularity as an architectural diagram, the book shows how to enforce ownership and boundaries directly in application code, persistence, contracts, tests, and production operations.

You will learn how to:

• Design a modular monolith around real business capabilities

• Keep module dependencies explicit and prevent architectural drift

• Structure ASP.NET Core solutions for clear ownership and maintainability

• Give modules independent EF Core DbContexts, schemas, and migrations

• Build practical Customers, Work Orders, Billing, and Identity modules

• Communicate between modules using contracts, integration events, and the transactional outbox

• Implement authentication, roles, claims, permissions, and resource-level authorization

• Design stable API contracts with validation, Problem Details, versioning, and OpenAPI

• Handle transactions, optimistic concurrency, retries, and idempotent commands safely

• Test domain behavior, PostgreSQL persistence, endpoints, and architectural boundaries

• Add structured logging, OpenTelemetry, health checks, metrics, and resilience

• Optimize EF Core queries, PostgreSQL indexes, caching, and API performance

• Containerize and deploy the application with production-focused CI/CD practices

• Recognize when a module genuinely needs to become an independent service

Rather than pushing every growing application toward microservices, this book demonstrates how a well-designed modular monolith can preserve simple deployment while providing strong boundaries, reliable data ownership, testability, and room for future evolution.

You will also learn how to identify the real signals for service extraction-such as independent scaling, deployment cadence, data ownership, and team autonomy-without introducing distributed-system complexity before it is justified.

Building Modular APIs with ASP.NET Core is designed for C# and ASP.NET Core developers, backend engineers, software architects, and development teams who want to build business APIs that remain understandable, testable, and production-ready as they grow.

If you want the maintainability benefits associated with well-designed distributed systems without prematurely paying the operational cost of microservices, this book provides a practical path forward.