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API Fundamentals: Interview Prep Guide

A complete reference for API design concepts — REST, HTTP, GraphQL, RPC, IPC, WebSockets, SSE, and Status Codes. Built as a fast, visual refresher for interviews.

What's Covered

Topic Key Concepts
HTTP Methods, Headers, Versions, CORS, Caching
REST Constraints, Resources, Verbs, Idempotency
GraphQL Schema, Queries, Mutations, Subscriptions, N+1
RPC gRPC, JSON-RPC, tRPC, Protobuf
IPC Pipes, Sockets, Shared Memory, Message Queues
WebSockets Full-duplex, Handshake, Use Cases
SSE Server-Push, EventSource, vs WebSockets
Status Codes 1xx–5xx Reference, Common Codes

API Landscape Mindmap

mindmap
  root((APIs))
    Request-Response
      REST
        Resources
        HTTP Methods
        Stateless
      GraphQL
        Single Endpoint
        Schema-Driven
        Flexible Queries
      RPC
        gRPC
        tRPC
        JSON-RPC
    Real-Time
      WebSockets
        Full-Duplex
        Persistent Connection
      SSE
        Server Push Only
        HTTP-based
      Long Polling
        Fallback
    Inter-Process
      IPC
        Pipes
        Shared Memory
        Message Queues
        Unix Sockets
    Foundation
      HTTP
        Methods
        Headers
        Versions
        Status Codes

Quick Comparison: When to Use What?

┌─────────────────┬────────────────┬──────────────────────────────────────────────────┐
│ Protocol        │ Transport      │ Best For                                          │
├─────────────────┼────────────────┼──────────────────────────────────────────────────┤
│ REST            │ HTTP           │ CRUD APIs, public APIs, resource-based design     │
│ GraphQL         │ HTTP           │ Complex queries, mobile clients, aggregation      │
│ gRPC            │ HTTP/2         │ Microservices, low-latency, strongly-typed        │
│ WebSockets      │ TCP            │ Chat, live feeds, multiplayer games               │
│ SSE             │ HTTP           │ Notifications, dashboards, one-way server push    │
│ IPC             │ OS-level       │ Same-machine process communication                │
└─────────────────┴────────────────┴──────────────────────────────────────────────────┘

The Request-Response Cycle (Big Picture)

Client                         Network                        Server
  │                               │                               │
  │──── DNS Lookup ──────────────▶│                               │
  │◀─── IP Address ───────────────│                               │
  │                               │                               │
  │──── TCP Handshake ───────────▶│──── TCP Handshake ───────────▶│
  │◀─── SYN-ACK ──────────────────│◀──── SYN-ACK ─────────────────│
  │                               │                               │
  │──── TLS Handshake (HTTPS) ───▶│──── TLS Handshake ───────────▶│
  │◀─── Certificate / Keys ───────│◀──── Keys ────────────────────│
  │                               │                               │
  │──── HTTP Request ────────────▶│──── HTTP Request ────────────▶│
  │◀─── HTTP Response ────────────│◀──── HTTP Response ────────────│
  │                               │                               │

Interview tip: Be ready to explain trade-offs between REST vs GraphQL vs gRPC. Interviewers love that question. The answer always depends on the use case — there's no universal winner.