Agent Skills: Modern Web App Architecture (SPA/SSR/SSG/RSC)

Modern Web App Architecture (SPA/SSR/SSG/RSC)

Overview

Comprehensive guidance for designing and building modern web applications (including SPAs and hybrid rendering apps). This skill emphasizes trade-offs, explicit boundaries, and production-ready practices (performance, accessibility, security, testing, delivery).

Core principle: Everything in architecture is a trade-off. There are no "right" answers, only least-worst combinations for your specific context.

Operating Mode (How to Use This Skill)

When activated, work in this order:

1. Clarify context (5–10 questions max) → users, routes, SEO, interactivity, data, auth, team, constraints.
2. Choose a rendering strategy per route (not “one strategy for the whole app”).
3. Define boundaries → feature/domain modules, shared libraries, ownership, and stable interfaces.
4. Plan state + data → local/shared/global/server/URL state, cache strategy, invalidation, optimistic updates.
5. Plan non-functionals → performance budgets + measurement, accessibility plan, security posture, observability.
6. Produce artifacts → short recommendations with explicit trade-offs, plus concrete next steps (folder structure, ADRs, checklists).

If the user can’t answer a question, state reasonable assumptions and continue (don’t block).

When to Use

• Starting a new SPA or web application
• Choosing rendering strategies (CSR, SSR, SSG, ISR, RSC)
• Implementing state management
• Optimizing Core Web Vitals (LCP/INP/CLS)
• Scaling for multiple frontend teams
• Making architecture trade-off decisions
• Migrating from legacy to modern frontend

Quick Discovery Questions (Ask First)

• What are the top 3 user journeys and their target latency (e.g., “search → product → checkout”)?
• Is SEO required for any routes? Which are public vs behind auth?
• What’s the data shape: mostly CRUD, real-time, offline-first, heavy forms, large tables/charts?
• What are the constraints: browser support, bundle limits, time-to-market, compliance (SOC2/HIPAA/PCI)?
• What’s the team topology: how many devs/teams, release cadence, ownership boundaries?
• What’s your preferred stack (React/Vue/Angular/vanilla), and are you open to TypeScript?

Reference Files

| Topic | When to Load | |-------|--------------| | @references/design-patterns.md | Implementing JS patterns (Module, Observer, Factory, etc.) | | @references/react-patterns.md | React components, hooks, state, composition | | @references/spa-fundamentals.md | SPA architecture, routing, module organization | | @references/micro-frontends.md | Scaling teams, independent deployments | | @references/performance.md | Bundle size, loading, Core Web Vitals | | @references/architecture-decisions.md | Trade-offs, coupling, fitness functions | | @references/rendering-strategies.md | CSR vs SSR vs SSG vs ISR vs RSC | | @references/state-management.md | Local, global, server state patterns | | @references/security-and-auth.md | Auth choices, token storage, XSS/CSRF, CSP, API boundaries | | @references/accessibility-and-i18n.md | WCAG basics, SPA focus mgmt, inclusive components, i18n pitfalls | | @references/testing-and-quality.md | Testing strategy, CI quality gates, a11y checks, contract tests | | @references/tooling-and-delivery.md | Bundling, environments, deployment, observability, feature flags |

Quick Architecture Decision Tree

Project Requirements?
├─ SEO critical + dynamic content → SSR (or SSR+streaming)
├─ SEO critical + mostly static → SSG/ISR (or hybrid)
├─ Mostly behind auth + app-like UX → CSR SPA (or hybrid with pre-rendered shell)
├─ Mixed (marketing + app) → Hybrid/Islands (route-level strategy)
│
Team Size?
├─ <5 developers → Modular monolith SPA
├─ 5-15 developers → Well-structured SPA or Service-based
├─ >15 developers, multiple teams → Consider micro-frontends
│
Domain Complexity?
├─ Simple CRUD → Layered architecture
├─ Complex workflows → Domain-partitioned (DDD)
├─ Multiple bounded contexts → Micro-frontends

Default Outputs (What You Should Produce)

Depending on the user request, aim to output:

• Route strategy map: a small table of routes → CSR/SSR/SSG/ISR/RSC + why
• Module boundary sketch: feature folders, shared libs, interface contracts
• State map: local/shared/global/server/URL, plus the chosen tooling pattern
• Data plan: caching/invalidation, error states, optimistic updates, pagination
• Quality plan: testing layers + CI gates + accessibility checks
• Performance plan: budgets + measurement + concrete loading strategy (split points)
• Risk register: top 5 risks + mitigations (e.g., hydration cost, auth posture, team coupling)

Essential Patterns Quick Reference

Component Patterns

| Pattern | Use When | |---------|----------| | Container/Presentational | Separating data from UI | | Compound Components | Building composable APIs (Select, Menu) | | Hooks | Sharing stateful logic without HOCs | | Provider | Avoiding prop drilling for global data |

State Management

| Approach | Use When | |----------|----------| | useState/useReducer | Local component state | | Context | Theme, auth, low-frequency global state | | Zustand/Jotai | Simple global state, minimal boilerplate | | Redux Toolkit | Complex state, time-travel debugging | | React Query/SWR | Server state, caching, background refresh | | XState | Complex flows with explicit state machines |

Performance Essentials

| Technique | Impact | |-----------|--------| | Code splitting | Reduce initial bundle | | Lazy loading | Defer non-critical | | React.memo | Prevent unnecessary re-renders | | useMemo/useCallback | Stable references | | Virtual lists | Handle large datasets |

Architecture Characteristics (Pick 3-7)

| Characteristic | Questions to Ask | |----------------|------------------| | Scalability | How many concurrent users? Growth rate? | | Performance | What's acceptable TTI? LCP target? | | Deployability | How often do you ship? Independent deploys? | | Testability | How easy to verify changes? | | Maintainability | What's the expected lifespan? | | Modularity | How often do requirements change? |

Anti-patterns to Avoid

| Anti-pattern | Problem | Fix | |--------------|---------|-----| | Prop drilling | Tight coupling | Context or state management | | God components | Too many responsibilities | Split by concern | | Premature optimization | Complexity without evidence | Profile first | | Shared mutable state | Race conditions, bugs | Immutable patterns | | Monolithic bundle | Slow initial load | Code splitting | | Over-fetching | Wasted bandwidth | GraphQL or BFF | | LocalStorage tokens by default | XSS turns into account takeover | Prefer httpOnly cookies + CSP (see security refs) | | Global store for server data | Cache invalidation pain | Use React Query/SWR for server state |

Performance Budgets

Budgets must be calibrated to your users/devices, but these are good starting points for a “fast by default” app:

| Metric | Target | Needs Work | |--------|--------|------------| | LCP | <2.5s | 2.5–4s | | INP | <200ms | 200–500ms | | CLS | <0.1 | 0.1–0.25 | | TTFB | <800ms | 800ms–1.8s | | Route JS (initial) | <170KB gzip | <300KB gzip |

Sources

Synthesized from:

• Learning JavaScript Design Patterns (Osmani, 2023)
• React in Depth (Barklund, 2024)
• SPA Design and Architecture (Scott)
• Single Page Web Applications (Mikowski & Powell)
• Building Micro-Frontends (Mezzalira)
• Micro Frontends in Action (Geers)
• Responsible JavaScript (Wagner)
• High Performance Browser Networking (Grigorik)
• Web Performance in Action (Wagner)
• Frontend Architecture for Design Systems (Godbolt)
• Fundamentals of Software Architecture (Richards & Ford)
• Software Architecture: The Hard Parts (Ford et al.)
• Patterns.dev