The RCADO Framework for Frontend System Design Interviews

When you walk into a frontend system design interview, you’re expected to do more than list APIs or throw boxes on a diagram — you’re expected to demonstrate how you think in a modern UI framework.

Interviewers increasingly care about how you structure components, manage state, and design data flow, not just backend-inspired abstractions.

The RCADO Framework is a modern evolution of the traditional RADIO framework—tailored specifically for frontend engineers who build real UI systems.

It stands for:

Why RADIO → RCADO: The Shift in Focus

The necessity for RCADO was inspired by key feedback from experienced frontend engineers who found the traditional approach often missed the mark in specialized UI design rounds. The problem with applying RADIO directly to frontend system design isn't the framework itself, but its focus.

“I realized I was failing multiple frontend system design interviews — even though I used RADIO perfectly. The core issue wasn't the framework; it was the emphasis. RADIO naturally guides you toward backend-style thinking — APIs, interfaces, controllers — but in frontend rounds, interviewers are truly focused on just two things:
1. What components exist and what props they take and
2. How state is managed and which component owns it.

Once I replaced Interface with Components and dropped Controller entirely, my conversation flow immediately improved. I’d start with Requirements, then instantly dive into the component tree, state ownership, and data flow. The results were night and day — the same interviews turned into strong passes.”

— L5 Frontend Engineer (Facebook), via Reddit

This valuable observation reshaped how many successful frontend engineers now approach design rounds. Instead of treating UI systems like backend diagrams, RCADO makes the discussion framework-native — focusing on component architecture, data flow, and user-facing pillars like performance, accessibility, and security.

It’s not just a tweak — it’s the natural evolution of RADIO for modern frontend interviews.

R – Requirements

As before, this is your foundation. Without understanding what to build, you’ll end up optimizing the wrong thing. Always start here.

Functional Requirements

Clarify what exactly you’re building:

• What’s the scope? (e.g., Feed, Form, Dashboard)
• What features are expected? (e.g., CRUD, Search, Filtering)
• What user actions should be supported?
• Are there different user roles (e.g., admin, viewer, guest)?
• Any real-time updates or live interactions?

Non-Functional Requirements

These show your foresight:

• i18n / L10n support?
• Offline functionality?
• Accessibility (a11y) compliance?
• Performance expectations (load times, interactions)?
• Security considerations (XSS, CSRF)?

👉 Tip: List constraints upfront — “We’re targeting web only,” or “Mobile-first approach.” It keeps you aligned and time-efficient.

C – Components

This is the heart of RCADO — replacing “Interface” with Components, because frontend design = UI design. Instead of abstract controllers, jump straight into:

• What components exist
• What props they take
• Who owns the state
• How data flows between them

Example: “Design a Post Feed System” You might break it down like:

Then, explain:

• Which components manage state (e.g., Feed manages pagination, PostCard is presentational).
• Which components are controlled vs uncontrolled (e.g., input fields).
• How data flows (top-down, via props; bottom-up via callbacks).

👉 Interviewers love when you naturally talk about framework-specific patterns:

• Controlled vs uncontrolled inputs
• Lifting state up
• Context / store ownership
• Memoization & rerenders

Components Tree

If you can explain your component structure with a diagram, it will help the interviewer clearly understand how you’ve broken the project down at the component level.

OR you can abstract and show as below as well

This shows real-world understanding — not just theory.

👉 Tip: Don't focus too much on complex, backend-style diagrams. While diagrams aren't mandatory, if you have time, it's highly valuable to use a simple visual to explain how your components communicate and render efficiently, focusing on:

• Component hierarchy (parent-child relationships).
• Data flow (top-down via props; bottom-up via callbacks).
• State management boundaries (which component or store owns the data).

A – Architecture

At this stage, zoom out from individual components and describe how the application behaves as a system. The goal is to show clear thinking around rendering, data fetching, state management, performance boundaries, and transport choices. Each point below represents a topic you can introduce and expand during the interview.

1. CSR vs SSR vs SSG

Clarify how the UI will be delivered to the client.
CSR (Client-Side Rendering) offers lighter servers and highly interactive experiences but requires more client resources and may delay initial render.
SSR (Server-Side Rendering) improves first paint and SEO, ideal for content-heavy or public-facing applications.
SSG (Static Site Generation) pre-renders stable pages at build time, offering excellent performance and scalability where data does not change frequently.
Explain why your chosen approach aligns with the product’s latency, SEO, and interactivity needs.

2. SPA vs MPA

Define whether the solution is a single-page app or a multi-page application.
SPA provides seamless transitions and rich interaction but requires careful handling of routing, hydration, and large bundle sizes.
MPA offers isolated pages, simpler performance characteristics, and clearer security boundaries, but may feel less fluid.
Connect this choice to user expectations, caching strategies, and deployment complexity.

3. REST vs GraphQL vs gRPC

Explain how data enters the application and how it is cached or synchronized.
REST, GraphQL, and gRPC each affect granularity, overfetching, and client-side schema design.
Pagination strategies (cursor-based or offset-based) determine how lists scale.
Caching layers—memory cache, HTTP cache, React Query caches—prevent redundant requests and improve responsiveness.
If mutations are required, clarify whether you use optimistic updates, background refetching, or rollback strategies.

4. Realtime Updates (WebSocket, SSE, Long Polling, Short Polling)

Specify how the application receives live data when required.
WebSockets support bi-directional communication for highly interactive features.
Server-Sent Events offer a lightweight one-way stream ideal for dashboards or notifications.
Long polling keeps a request open until new data arrives, suitable when persistent sockets aren't permitted.
Short polling periodically triggers requests and works for low-frequency or non-critical updates.
Highlight how the UI integrates updates into existing state while maintaining consistency and minimizing rerenders.

5. Transport Protocol Choice (HTTP/1.1 vs HTTP/2 vs HTTP/3)

Demonstrate awareness of the underlying network protocol and its impact on performance.
HTTP/1.1 handles only one request per connection, leading to head-of-line blocking on resource-heavy pages.
HTTP/2 adds multiplexing, stream prioritization, and header compression, significantly improving concurrency and API performance.
HTTP/3, based on QUIC, removes transport-level head-of-line blocking and offers faster, more resilient connections—especially valuable for realtime features and mobile networks.
Connecting protocol capabilities to rendering and data-fetching decisions shows senior-level architectural awareness.

D – Data Model (API Interface + UI State Architecture)

In RCADO, the Data section explains two equally important aspects of frontend system design:

1. API Interface – the contract between frontend and backend
2. UI State Architecture – how the frontend stores, normalizes, transforms, and synchronizes that data

A strong system design answer covers both sides: how data arrives, and how it lives inside the UI.

1. API Interface (Network Contract)

The API Interface is a critical discussion in any frontend design interview. As a senior frontend engineer, you’re expected to help define an ideal, extendable, and predictable FE–BE contract. This ensures that UI rendering, caching, pagination, and mutations behave correctly and scale cleanly.

1.1 Endpoint Design

A resource-based approach keeps endpoints consistent and future-proof.
Example for a Post Feed system:

This structure remains extendable even as new features are introduced.

1.2 URL Strategy and Naming Conventions

A senior engineer validates backend routes by ensuring the following:

• Resource-first nouns: /posts, /comments, /users
• Nesting only when relationships are scoped: /posts/:id/comments
• Use of query parameters for sorting, filtering, pagination
• Predictable, REST-style semantics rather than action-style URLs

This consistency simplifies caching strategies, routing, and client-side data flows.

1.3 Response Structure

Backend responses must align with UI needs to avoid unnecessary additional requests or transformations. A typical post structure may look like:

Key frontend-focused fields such as likedByUser significantly simplify UI logic.

1.4 Pagination Contract

Clarifying the pagination model is essential for feed-based systems.

Preferred: Cursor-based pagination
This approach integrates naturally with infinite scroll, incremental fetching, and client-side cache merging.

Example response:

1.5 Mutation Contract

Mutations should ideally return the updated entity rather than just a partial patch. This avoids extra refetching and ensures caches remain consistent.

Example:
POST /api/posts/:id/like returns the full updated post object.

1.6 Error Modeling

Consistent error structures enable predictable UI notifications and retry logic:

1.7 Extendability

The contract should support future features without breaking existing ones.
The chosen URL and data model structure should make it easy to add:

• Post reactions
• Sharing
• Nested comments
• Feed filters or sorting
• Search and advanced query capabilities

A clean API contract reduces friction for both backend and frontend teams.

2. How Data Is Stored in the UI (State Architecture)

Once the API delivers data, the next step is determining how the UI stores, modifies, and synchronizes it. This is where state architecture becomes crucial.

2.1 Local State vs Global State vs Server State

2.1.1 Local State (Component-Level)

Use local state for temporary or component-specific information:

• Text input in a comment box
• Modal visibility
• UI-only flags such as hover or collapse state

Examples include useState in React or ref in Vue.

2.1.2 Global State (Redux, Zustand, Jotai, Context)

Use global state only for:

• Data shared widely across the app (authenticated user, theme, filters)
• Configuration or metadata
• UI states required across multiple pages or modules

Avoid putting server-fetched entities (like posts or products) into global state unless they require cross-page synchronization. Modern patterns often replace Redux for server data.

2.1.3 Server State (React Query, SWR, RTK Query)

Server state tools are now the recommended standard for managing API-driven data. They automatically handle:

• Fetching
• Caching
• Pagination
• Background refresh
• Retries
• Stale-while-revalidate
• Optimistic updates

This separates remote data concerns from UI-specific concerns.

2.2 Data Normalization

Normalization is important when:

• You have large or repeated lists
• Entities appear in multiple places (e.g., the same post in multiple views)
• Updates must reflect consistently across the UI
• You want constant-time lookups

Normalized example:

React Query combined with a lightweight global store (like Zustand) is often the most efficient modern pattern.

2.3 Derived State

Only store what is truly the source of truth. Compute the rest.

Examples:

• Determining whether the current user is the post author
• Sorted lists
• Filtered views

Storing both source and derived values causes contradictions and inconsistencies.

2.4 Mutation Handling and Optimistic UI

A robust mutation workflow includes:

1. Apply optimistic updates immediately
2. Trigger API mutation
3. Update or refetch based on server response
4. Roll back if the mutation fails

Modern tools like React Query simplify this dramatically and reduce boilerplate.

O – Other (Performance, Accessibility, Security)

Rather than a long optimization dump, RCADO suggests a live subheading approach — bring up topics when relevant during the interview.

🏎 Performance

• Virtualization for long lists
• Bundle/code splitting (lazy loading)
• Memoization (React.memo, useMemo, useCallback or framework equivalents like v-memo)
• Debounce/throttle inputs
• Prefetch next-page data

♿ Accessibility

• Semantic HTML (e.g., <button>, not <div>)
• ARIA roles for dynamic/custom elements
• Keyboard navigation and focus states
• Color contrast checks

🔐 Security

• Escape all dynamic HTML (XSS prevention)
• Avoid dangerouslySetInnerHTML
• Secure tokens in memory (not localStorage)
• CSRF-aware form submissions

👉 Mention these organically — e.g., “While rendering comments dynamically, I’d sanitize input to avoid XSS.”

Why RCADO Works Better

RADIO works well for backend-leaning designs, but RCADO is frontend-first — it mirrors how modern interviews and real-world apps are built.

Putting It All Together

When asked to design a system, start with this line:

“I’ll use the RCADO framework to structure my thought process — starting with Requirements, then diving into Components, Architecture, Data, and finally Other considerations like performance and security.”

That single line immediately signals structured, senior-level thinking. RCADO = Framework-native system design thinking. It’s clean, real, and built for how frontend engineers actually work today.