Backend Pattern (MediatR CQRS / IResult / AOP)

Trigger phrases: "devarch-module", "new handler", "write a command", "add a query", "validator", "aspect"

This is the DEFAULT backend pattern, not the only one. backend-expert-csk is pattern-neutral; it applies whichever backend-pattern skill the project ships. A project on Clean Architecture, Vertical Slice, Minimal API, or plain layered replaces this skill with its own (same format) — the agent then follows that. Nothing forces DevArch.

Kit adaptation (local, .claude/): backend-expert-csk applies the devarch-module skill. Sources (alignment): the DevArchitecture pattern — its name does NOT leak into generated code / namespace / file / comment / csproj / Swagger / JWT (§4.2). The pattern lives here; the generated artifact is written with project-specific names.

Layout

Three folders under Business/Handlers/{Entity}/: Commands/ · Queries/ · ValidationRules/. Each request + its handler lives in a single file, nested.

Command pattern (write)

• Request class: IRequest<IResult> (or IRequest<IDataResult<T>> if it returns data).
• Handler: IRequestHandler<TRequest, IResult> inside the outer class; dependencies via the ctor (repository, cache, mediator).
• Body: write via the repository → SaveChangesAsync() → clear the relevant cache key → return new SuccessResult(Messages.Added).

Query pattern (read)

• Request: IRequest<IDataResult<IEnumerable<TDto>>> / IDataResult<TDto>.
• The handler fetches the data via the repository/DTO → return new SuccessDataResult<...>(data).

Result contract (NO bare types)

• Success: SuccessResult(message) / SuccessDataResult<T>(data, message?).
• Error: ErrorResult(message) / ErrorDataResult<T>(message).
• The handler never returns a raw T / void; always IResult/IDataResult<T>.

AOP aspect chain (attribute order = Priority)

[SecuredOperation(Priority = 1)]                       // authorization
[ValidationAspect(typeof(XValidator), Priority = 2)]   // FluentValidation
[CacheAspect]            // on a query: cache the result
[CacheRemoveAspect]      // on a command: drop the relevant cache
[LogAspect(typeof(FileLogger))]                        // logging

• Anonymous/identity-less endpoint (register, public webhook, health-check) → [SecuredOperation] is REMOVED.
• Cache keys are meaningful and de-duplicable; after a command the relevant key is cleared.

Validation (FluentValidation)

AbstractValidator<TCommand> under ValidationRules/; RuleFor(x => x.Field).NotEmpty().... Bound to the handler via [ValidationAspect(typeof(TValidator))] — no manual validation inside the handler.

Calls between handlers (MediatR)

IMediator is taken as a dependency; single-record/internal commands are called via _mediator.Send(...) (without creating a circular dependency).

Messages & i18n

User-facing texts come from the Messages constants; a new message → all project languages (default TR/EN/DE/RU), the Languages/Translates pattern. Leave no missing translation (no deferral).

Skeleton example

A full command/handler/validator skeleton (write with project-specific names; no vendor name): references/skeleton.md.

DoD (this skill's contribution)

• sonarqube-check 0/0/0/0, build 0 warnings/0 errors; test-expert-csk green; /simplify applied.