🧠 Simple Definition (Word-for-word)

Core: generate a short code (6-8 chars) mapped to a long URL.

⚡ Super Simple Line

Schema: {id, shortCode, longUrl, userId, createdAt, clickCount}.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Core: generate a short code (6-8 chars) mapped to a long URL.

🧠 Simple Definition (Word-for-word)

WebSockets for persistent connections — but can't hold 1M open connections on one server.

⚡ Super Simple Line

Solution: use a pub/sub layer (Redis Pub/Sub or Kafka).

⚡ Key Details & Explanation

⚡ One-line Interview Answer

WebSockets for persistent connections — but can't hold 1M open connections on one server.

🧠 Simple Definition (Word-for-word)

Avoid sending large files through your server.

⚡ Super Simple Line

Pattern: client requests a presigned URL from your API, your API calls S3 to generate a presigned URL, client uploads directly to S3 bypassing your server.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Avoid sending large files through your server.

🧠 Simple Definition (Word-for-word)

A Rate Limiter is a system component that limits the number of requests a client can make within a given timeframe to protect API resources from abuse or overloading. The Token Bucket algorithm maintains a bucket filled with tokens at a constant rate; requests consume tokens, allowing bursts of traffic. The Sliding Window Counter tracks request timestamps within a sliding window interval, offering strict rate limits with low memory overhead.

⚡ Super Simple Line

Token Bucket = allows traffic bursts (if tokens are in bucket, requests run instantly).
Sliding Window = strictly counts requests over a rolling time window (no bursts allowed).

📊 Comparison of Rate Limiting Algorithms

🧪 Redis Implementation Concept

Redis is commonly used to build distributed rate limiters because operations are atomic:

// Sliding Window Counter using Redis sorted set (ZSET)
async function isRateLimited(userId) {
  const now = Date.now();
  const windowMs = 60000; // 1 minute
  const maxRequests = 100;
  
  const key = `rate_limit:${userId}`;
  
  await redis.multi()
    .zremrangebyscore(key, 0, now - windowMs) // Remove old timestamps
    .zcard(key) // Count remaining request logs
    .zadd(key, now, now) // Log current request
    .expire(key, 60)
    .exec();
}

⚡ One-line Interview Answer

Rate limiters protect APIs using algorithms like Token Bucket, which permits momentary bursts of traffic, or Sliding Window Counter, which strictly throttles traffic with low memory footprints.

🧠 Simple Definition (Word-for-word)

In a distributed system you can guarantee at most 2 of 3: Consistency (every read gets the latest write), Availability (every request gets a response), Partition Tolerance (system works despite network failures).

⚡ Super Simple Line

Since network partitions WILL happen, the real choice is C vs A during a partition.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

In a distributed system you can guarantee at most 2 of 3: Consistency (every read gets the latest write), Availability (every request gets a response), Partition Tolerance (system works despite network failures).

🧠 Simple Definition (Word-for-word)

Eventual consistency: in a distributed system, all replicas will eventually converge to the same value — but at any point in time, different nodes may serve different values.

⚡ Super Simple Line

Acceptable for: social media feeds (stale posts for a few seconds is fine), shopping cart (eventual sync is ok), DNS propagation, search indexes.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Eventual consistency: in a distributed system, all replicas will eventually converge to the same value — but at any point in time, different nodes may serve different values.

🧠 Simple Definition (Word-for-word)

Vertical: add more CPU/RAM to one machine — simple, no code changes, but limited and expensive, single point of failure.

⚡ Super Simple Line

Horizontal: add more machines — theoretically unlimited scale, resilient.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Vertical: add more CPU/RAM to one machine — simple, no code changes, but limited and expensive, single point of failure.

🧠 Simple Definition (Word-for-word)

Message queues decouple producers and consumers, enable async processing, and add resilience.

⚡ Super Simple Line

RabbitMQ: traditional message broker, complex routing (exchanges, bindings), good for task queues, RPC patterns, small-medium volume.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Message queues decouple producers and consumers, enable async processing, and add resilience.

🧠 Simple Definition (Word-for-word)

Microservices: split an app into independently deployable services, each owning its data and running in its own process.

⚡ Super Simple Line

Benefits: independent scaling, independent deployment, tech heterogeneity, team autonomy.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Microservices: split an app into independently deployable services, each owning its data and running in its own process.

🧠 Simple Definition (Word-for-word)

CDN (Content Delivery Network): geographically distributed servers that cache and serve static assets (images, JS, CSS, videos) from the edge server closest to the user.

⚡ Super Simple Line

How it works: request hits CDN edge, on cache miss the edge fetches from your origin server and caches it (TTL-based).

⚡ Key Details & Explanation

⚡ One-line Interview Answer

CDN (Content Delivery Network): geographically distributed servers that cache and serve static assets (images, JS, CSS, videos) from the edge server closest to the user.

🧠 Simple Definition (Word-for-word)

Long polling: client makes request, server holds it open until data is available, then responds, client immediately makes new request — lots of overhead, HTTP overhead per message.

⚡ Super Simple Line

SSE (Server-Sent Events): persistent one-directional connection (server to client only) over HTTP — simple, auto-reconnect, EventSource API.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Long polling: client makes request, server holds it open until data is available, then responds, client immediately makes new request — lots of overhead, HTTP overhead per message.

🧠 Simple Definition (Word-for-word)

Two main approaches: Operational Transformation (OT) — transforms concurrent operations to be compatible (used by Google Docs); CRDT (Conflict-free Replicated Data Types) — data structures designed to merge automatically without conflict (used by Figma, Notion).

⚡ Super Simple Line

For a simpler system: use Yjs library (CRDT-based), connect via WebSocket (Socket.io or native), broadcast operations to all connected clients.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Two main approaches: Operational Transformation (OT) — transforms concurrent operations to be compatible (used by Google Docs); CRDT (Conflict-free Replicated Data Types) — data structures designed to merge automatically without conflict (used by Figma, Notion).

🧠 Simple Definition (Word-for-word)

For a chat system, I would start with conversations, participants, and messages as the core data model.

⚡ Super Simple Line

When a user sends a message, the backend should validate the user, save the message to the database first, and then deliver it in real time to online users through WebSockets.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

For a chat system, I would start with conversations, participants, and messages as the core data model.

🧠 Simple Definition (Word-for-word)

Frontend should debounce input and cancel stale requests.

⚡ Super Simple Line

Backend options depend on scale: SQL prefix search for simple cases, Elasticsearch/OpenSearch for large-scale ranking, typo tolerance, and advanced relevance.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Frontend should debounce input and cancel stale requests.

🧠 Simple Definition (Word-for-word)

An API gateway is the single entry point for clients in front of multiple services.

⚡ Super Simple Line

It can handle authentication, rate limiting, request routing, header normalization, logging, and sometimes response aggregation.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

An API gateway is the single entry point for clients in front of multiple services.

🧠 Simple Definition (Word-for-word)

Event-driven architecture is a design pattern where the flow of the application is driven by events.

⚡ Super Simple Line

Instead of directly calling functions, components communicate by emitting and listening to events.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

Event-driven architecture is a design pattern where the flow of the application is driven by events.

🧠 Simple Definition (Word-for-word)

To handle a sudden spike in traffic: I would first ensure that my infrastructure can scale (using cloud services like AWS or GCP).

⚡ Super Simple Line

Implement auto-scaling to add more servers as needed.

⚡ Key Details & Explanation

⚡ One-line Interview Answer

To handle a sudden spike in traffic: I would first ensure that my infrastructure can scale (using cloud services like AWS or GCP).