[{"_1":2,"_72":-5,"_73":-5},"loaderData",{"_3":4,"_44":45},"root",{"_5":-5,"_6":-5,"_7":8,"_22":23,"_24":25,"_26":27,"_30":31,"_36":37,"_38":39,"_40":41},"user","shoppingCartId","shoppingCart",{"_9":10,"_11":12,"_13":14,"_15":16,"_17":18,"_19":20,"_21":14},"id","no-cart","count",0,"free",true,"updatedAt","","paymentGateway","STRIPE","cartItems",[],"loading","locale","en","readerTheme","light","constants",{"_28":29},"paypalClientId","AX53Cwz8jR6rVVzzZiZNWWXl0dGagi-GZrjG-S3Ua14E5nrKMn7DxDQ730O4R8LusLPp49ALn6aRVaEy","publishers",{"_32":33,"_34":35},"companies",[],"universities",[],"learnerOrganizations",[],"needsLocaleCookieSync",false,"announcements",{"_42":-5,"_43":-5},"announcement_store","announcement_authors","routes/_store+/_readers+/email_author.$slug/route",{"_46":47,"_71":-5},"book",{"_9":48,"_49":50,"_51":52,"_53":54,"_55":56,"_57":58},"Z2lkOi8vbGVhbnB1Yi9TdG9yZTo6Qm9va1Byb2R1Y3QvNzc3NzM","title","Layerd FP in Scala","state","published","contactAuthorMethod","email","aboutText","

Clean Architecture, hexagonal (ports and adapters), and the onion model are one family of ideas: dependencies point inward, boundaries stay explicit, and the core stays testable while technology churn stays at the edges. This book treats them that way—decision-grade material for experienced Scala developers, tech leads, and architects who already ship services and want the dependency rule enforced in real modules, not only on diagrams.

The worked example is SaaS billing: plans, subscriptions, cancellation, plan changes, payment notify, and idempotent webhooks. The domain is a functional core (opaque identifiers, pure SubscriptionRules, DomainError) with no ZIO in production domain code. The application ring defines outbound ports, ApplicationError, and use-case services (for example subscribe and webhook handling) in ZIO. Adapters supply in-memory stores, PostgreSQL with Flyway migrations and JDBC, JDK HttpServer, Circe JSON, and an outbound HTTP payment gateway—then the composition root (AppLayers, Main) wires ZLayer so one repository instance serves the whole graph.

The manuscript mirrors a multi-module sbt companion repository: Chapters 3–7 print complete, line-faithful production sources in reading order (no elided bodies in repository code—only Chapter 11 uses labelled wrong snippets on purpose). You get guided reading (source-order commentary), placement in Clean/Hex/Onion, production tension (retries, clocks, bad HTTP), and master synthesis sections that tie each chapter to review and shipping concerns. Later chapters cover reliability (what the code proves and what it does not), a testing strategy (munit for pure domain, zio-test for effects, optional Docker integration tests), compile-time module boundaries, anti-patterns and fixes, and a summary linking vertical slices to sibling topics (event sourcing, saga).

Audience: Scala 3 (given, enums, opaque types), basic sbt, and algebraic effects / ZIO at least in outline; working SQL and HTTP. Not a Stripe or PCI hardening guide—the payment paths are architecturally realistic but minimal so boundaries remain visible.

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