Grokking Modern API Design Interview

The API Design Interview

Introduction: The API Design Interview

What Is an API?

How API Design Shows Up in Interviews

What Interviewers Actually Grade

A Framework for API Design Answers

Requirements Gathering for an API

The API as a Product

Capstone: Design a URL Shortener API

Chapter Assessment

Designing a REST API

What Are REST APIs?

Introduction: Resource Modeling and REST Core

Resources, Not Actions

HTTP Methods and Their Semantics

URL Design

Request and Response Shapes

Field Types That Break Contracts

Status Codes and Error Design

Pagination from the Consumer's View

Filtering, Sorting, and Search

When REST Doesn't Fit

Bulk and Batch Operations

Capstone: Design the Twitter API

Chapter Assessment

When REST Is Not Enough

Introduction: Beyond REST

gRPC Contract Design

GraphQL Schema Design

REST vs gRPC vs GraphQL

Real Time Delivery: WebSockets, SSE, and Long Polling

Webhooks: Designing the API That Calls Back

Async APIs for Long Running Work

Large Uploads and Downloads

Capstone: Design a Live Notification API

Chapter Assessment

The Hard Parts: Failures, Security, and Change

Introduction: The Hard Parts

Idempotency Keys

Rate Limiting in the Contract

Authentication and Authorization for APIs

Designing for Many Tenants

Versioning and Backward Compatibility

Concurrency and Conditional Requests

Read Your Writes

Caching in the Contract

The Reliability Contract

Capstone: Design a Payment API

Chapter Assessment

APIs for AI Products

Introduction: APIs in the AI Era

Streaming LLM APIs

Designing APIs for AI Agents

Usage Metering and Billing APIs

Rate Limiting Expensive Compute

Capstone: Design an LLM Inference API

Chapter Assessment

More Interview Questions

Introduction: More Interview Questions

Interview Question: Design a Chat API

Interview Question: Design a Booking API

Interview Question: Design a Feature Flag API

Interview Question: Design a Checkout API

Interview Question: Design a Calendar API

Interview Question: Design a Collaborative Document API

Chapter Assessment

The Product Architecture Round

Introduction: The Product Architecture Round

From Feature to Data Model

Designing for One Screen

Offline, Sync, and Conflict

Interview Question: Design the API Behind a News Feed Screen

Interview Question: Design the API Behind a Messaging App

Chapter Assessment

Full Answers and Quick Review

Introduction: Interview Questions and Rapid Review

The 10 Minute API Answer

Interview Question: Design a Ride Sharing API

Interview Question: Design a File Storage API

Interview Question: Critique This API

Rapid Fire Questions

Red Flags and Decision Cheat Sheets

Chapter Assessment

Conclusion: What You Now Know, and Where to Go Next

Concurrency and Conditional Requests

Two support agents read the same order.

Agent A changes the shipping instruction. Agent B changes the contact phone number. Each client sends a representation based on the copy it read.

If B writes last, B's stale copy may restore the old shipping instruction. Both requests return success. One person's work disappears.

This is the lost update problem. The API needs a way for a writer to say:

"Apply this change only if the resource is still the state I read."

HTTP provides that condition through an ETag and If-Match.