architecture-advisor

name

architecture-advisor

description

Architecture selection advisor for .NET applications. Asks structured questions about domain complexity, team size, system lifetime, compliance, and integration needs, then recommends the best-fit architecture: Vertical Slice, Clean Architecture, DDD + Clean Architecture, or Modular Monolith. Load this skill when the user asks "which architecture", "choose architecture", "set up project", "new project", "architecture decision", "restructure", or "how should I organize". Always load BEFORE any architecture-specific skill.

Architecture Advisor

Core Principles

1. Ask before recommending — Never prescribe an architecture without understanding the project. Run the questionnaire first to gather context about domain, team, lifetime, and constraints.
2. Right-size the architecture — The best architecture is the simplest one that handles the project's actual complexity. CRUD apps do not need DDD. Startups do not need Clean Architecture. Match complexity to real requirements, not aspirations.
3. Architecture is not permanent — Every architecture has an evolution path. Start simple and add structure when complexity demands it. Document the decision so the team knows when to evolve.
4. Four supported architectures — dotnet-Codex-kit provides first-class patterns for Vertical Slice Architecture (VSA), Clean Architecture (CA), DDD + Clean Architecture, and Modular Monolith. Each has specific strengths and trade-offs.

The Architecture Questionnaire

Before recommending an architecture, ask questions across these 6 categories. Not every question applies to every project — skip irrelevant ones.

Category 1: Domain Complexity

#	Question	Low Signal	High Signal
1	How many distinct business entities does the system manage?	< 10 entities	20+ entities with relationships
2	Do business rules involve multiple entities interacting?	Rules are per-entity CRUD	Complex invariants across entity groups
3	Are there business workflows with multiple steps?	Simple request → response	Sagas, approval chains, state machines
4	Do domain experts use specialized vocabulary?	Generic terms (create, update)	Ubiquitous language (underwrite, adjudicate)

Category 2: Team & Organization

#	Question	Low Signal	High Signal
5	How large is the development team?	1-3 developers	8+ developers, multiple teams
6	Do different teams own different parts of the system?	Single team owns everything	Teams aligned to business domains
7	What is the team's experience level with .NET?	Junior or mixed	Senior, experienced with patterns

Category 3: System Lifetime & Scale

#	Question	Low Signal	High Signal
8	Expected system lifetime?	< 2 years, MVP/prototype	5+ years, long-lived product
9	How many concurrent users or requests per second?	< 100 RPS	1000+ RPS, variable load
10	Will the system need to scale independently by feature area?	Uniform load	Hot spots need independent scaling

Category 4: Regulatory & Compliance

#	Question	Low Signal	High Signal
11	Are there audit trail or compliance requirements?	Basic logging sufficient	Full audit trail, SOX/HIPAA/PCI
12	Do different parts of the system have different security boundaries?	Single auth boundary	Multi-tenant, data isolation

Category 5: Existing Codebase

#	Question	Low Signal	High Signal
13	Is this greenfield or brownfield?	Greenfield, starting fresh	Brownfield, migrating from legacy
14	Are there existing architectural patterns the team follows?	No established patterns	Strong conventions in place

Category 6: Integration Complexity

#	Question	Low Signal	High Signal
15	How many external systems does this integrate with?	0-2 simple APIs	5+ systems with complex contracts
16	Are there event-driven or async communication needs?	Synchronous request/response	Event sourcing, pub/sub, eventual consistency

Decision Matrix

Map questionnaire answers to architecture recommendations:

Profile	Recommended Architecture	Why
Low domain complexity, small team, short lifetime	Vertical Slice Architecture	Minimal ceremony, fast feature delivery, easy to understand
Low-medium domain complexity, any team size, API-focused	Vertical Slice Architecture	Feature cohesion, one file per operation, natural fit for minimal APIs
Medium domain complexity, medium team, long lifetime	Clean Architecture	Enforced boundaries via project references, testable domain, clear dependency direction
High domain complexity, specialized vocabulary, complex invariants	DDD + Clean Architecture	Aggregates protect invariants, value objects model domain concepts, domain events decouple side effects
Multiple bounded contexts, team-per-domain, independent deployment potential	Modular Monolith	Module isolation, independent data stores, evolution path to microservices
Brownfield with existing layered architecture	Clean Architecture	Familiar to teams coming from N-tier, preserves layer separation with better dependency direction

When Signals Conflict

If signals point to different architectures:

1. Default to simpler — When in doubt, start with VSA and evolve
2. Domain complexity wins — High domain complexity overrides team size and lifetime signals
3. Team familiarity matters — A team experienced with CA will be more productive with CA than learning VSA, even if VSA is technically simpler
4. Compliance drives structure — Regulatory requirements often force stricter boundaries (CA or DDD)

Patterns

Vertical Slice Architecture (VSA)

Organize by feature, not by layer. Each operation is a self-contained slice.

src/MyApp.Api/
  Features/
    Orders/CreateOrder.cs    # Request + Handler + Response + Endpoint
    Orders/GetOrder.cs
  Common/
    Behaviors/ValidationBehavior.cs
    Persistence/AppDbContext.cs

Best for: CRUD-heavy apps, APIs, MVPs, small-medium teams, short-medium lifetime. Load skill: vertical-slice

Clean Architecture (CA)

Concentric layers with dependency inversion. Domain at the center, infrastructure at the edge.

src/
  MyApp.Domain/              # Entities, interfaces, domain logic
  MyApp.Application/         # Use cases, DTOs, validation
  MyApp.Infrastructure/      # EF Core, external services
  MyApp.Api/                 # Endpoints, middleware

Best for: Medium complexity, long-lived systems, teams familiar with layered patterns. Load skill: clean-architecture

DDD + Clean Architecture

Clean Architecture with tactical DDD patterns: aggregates, value objects, domain events.

src/
  MyApp.Domain/              # Aggregates, value objects, domain events, domain services
  MyApp.Application/         # Use cases orchestrating aggregates
  MyApp.Infrastructure/      # Persistence, external service adapters
  MyApp.Api/                 # Thin endpoints

Best for: Complex domains, specialized vocabulary, strict invariants, experienced teams. Load skill: ddd + clean-architecture

Modular Monolith

Independent modules in a single deployable unit, each with its own architecture internally.

src/
  MyApp.Host/                # Wires modules together
  Modules/
    Orders/                  # Own features, own DbContext, own architecture
    Catalog/                 # Can use VSA, CA, or DDD internally
  MyApp.Shared/              # Integration event contracts only

Best for: Multiple bounded contexts, team-per-domain, future microservices extraction. Load template: modular-monolith

Evolution Paths

Architecture is not a one-time decision. Systems evolve. Here are the common migration paths:

From	To	Trigger	How
VSA	CA	Domain logic growing beyond handlers	Extract Domain + Application layers, keep features as use cases
VSA	Modular Monolith	Multiple bounded contexts emerging	Group features into modules, add module boundaries
CA	DDD + CA	Invariants becoming complex, primitive obsession	Introduce aggregates, value objects, domain events
Monolith	Modular Monolith	Teams stepping on each other, shared database coupling	Split into modules with own DbContexts and schemas
Modular Monolith	Microservices	Independent scaling needs, independent deployment	Extract modules into separate deployable services

Anti-patterns

Picking Clean Architecture for a CRUD App

// BAD — 4 projects, 6+ files per feature for simple CRUD
src/MyApp.Domain/Entities/Product.cs
src/MyApp.Application/Products/CreateProduct/CreateProductCommand.cs
src/MyApp.Application/Products/CreateProduct/CreateProductHandler.cs
src/MyApp.Application/Products/CreateProduct/CreateProductValidator.cs
src/MyApp.Infrastructure/Persistence/ProductRepository.cs
src/MyApp.Api/Endpoints/ProductEndpoints.cs

// GOOD — VSA: 1 file for a simple CRUD feature
src/MyApp.Api/Features/Products/CreateProduct.cs

DDD Everywhere

// BAD — value objects and aggregates for a settings table
public class UserSettings : AggregateRoot  // overkill
{
    public ThemeName Theme { get; private set; }  // value object for "dark"/"light"?
    public void ChangeTheme(ThemeName theme) { /* domain event? really? */ }
}

// GOOD — simple entity for simple data
public class UserSettings
{
    public Guid UserId { get; init; }
    public string Theme { get; set; } = "light";
}

Premature Microservices

// BAD — splitting into 5 microservices on day one with 2 developers
OrderService (own repo, own DB, own CI/CD)
CatalogService (own repo, own DB, own CI/CD)
IdentityService (own repo, own DB, own CI/CD)
NotificationService (own repo, own DB, own CI/CD)
GatewayService (own repo, own CI/CD)

// GOOD — start as a modular monolith, extract when you have evidence
src/
  Modules/Orders/
  Modules/Catalog/
  Modules/Identity/
  Modules/Notifications/

Skipping the Questionnaire

// BAD — "I always use Clean Architecture"
User: "Set up a new project for a todo app"
Agent: *immediately scaffolds 4-project CA solution*

// GOOD — ask first, then recommend
User: "Set up a new project for a todo app"
Agent: "Let me ask a few questions about your project to recommend the best architecture..."
Agent: *runs questionnaire, recommends VSA for low-complexity app*

Decision Guide

Scenario	Recommendation
New project, unknown requirements	Run the questionnaire
Simple CRUD API, 1-3 developers	VSA
Medium complexity, long-lived, experienced team	Clean Architecture
Complex domain, specialized vocabulary	DDD + Clean Architecture
Multiple bounded contexts, multiple teams	Modular Monolith
Existing N-tier codebase, needs modernization	Clean Architecture (familiar migration)
MVP / startup, speed is priority	VSA
Regulatory / compliance-heavy	Clean Architecture or DDD (enforced boundaries)
Will need independent scaling later	Modular Monolith (extraction-ready)