Shopify/shopify_app is a Ruby on Rails engine gem (v23.0.3, MIT) that provides the foundational integration layer for building Shopify Apps. It handles OAuth 2.0 authentication with both legacy and new embedded-API token exchange flows, session management (online/offline with access scopes), webhook registration and verification, script tag management, billing, and App Bridge support. The gem ships with dozens of Rails generators to scaffold models, controllers, jobs, views, and routes tailored to embedded and non-embedded app patterns. Architecture is a mounted engine with mixin-based controller concerns (e.g., LoginProtection, EnsureHasSession, WebhookVerification) and configurable session storage backends (default ActiveRecord via host app models, with in-memory stores for development). Key data flows: the app directs users through Shopify OAuth, stores session tokens (with automatic refresh for expiring tokens), then uses the Shopify API (via shopify_api gem) for authenticated calls. The codebase demonstrates disciplined Rails engine design, robust security practices (HMAC verification, CSP, CSRF protection, sanitized redirects), and strong test coverage with 300+ test files. Notable craftsmanship includes the session storage mixins (ShopSessionStorage, UserSessionStorage) that separate concerns via method injection, the access-scope verification strategies, and the full-page redirect pattern for embedded apps. Maintenance is active—despite some doc-coverage inconsistencies (e.g., deprecated modules still tested) and two coexisting auth strategies that add conditional complexity, the gem tracks modern Shopify API versions and Ruby/Rails requirements (>= 3.2 / >= 7.1). The large generator surface and i18n support (20 locales) show attention to developer experience.