Payments Engineering in India — From UPI to Ledgers
Every app in India takes payments now. Food delivery, ed-tech, D2C brands, SaaS…
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#system-design
83 posts
Capstone: Designing Payments for a Real D2C Product
We’ve covered a lot over eleven lessons. In this last one, let’s design the…
Observability + Capstone: Designing and Debugging a Distributed System
Here’s a question I like to ask in interviews: “A customer says checkout took 1…
Capstone: Ship a Production Support Agent, End to End
We’ve covered a lot over eleven lessons. In this one, I want to walk through how…
Scaling AI Agents in Production
The first agent I put into production handled about 300 conversations a day. It…
In the Wild: Spotify's Event Delivery
Open Spotify and every interaction — pressing play, skipping a track, liking a…
In the Wild: Slack's Job Queue
Most of what a Slack message triggers does not happen in the request that sent…
In the Wild: How Shopify Survives Flash Sales
The textbook flash sale design is a clean whiteboard exercise. Shopify runs it…
In the Wild: Figma's Multiplayer Editing
Our Collaborative Editor lesson weighs Operational Transform against CRDTs for…
In the Wild: How Discord Stores Trillions of Messages
The design lessons in this course teach patterns in the abstract. This “In the…
How to Prepare
You can read every lesson in this course and still stumble in the room if you…
Design a Distributed Job Scheduler
Plenty of systems need work to happen later: send a reminder in 24 hours, run a…
Design Live Comments
“Design live comments” (Facebook Live, YouTube Live, Twitch chat) tests whether…
Design Top-K / Trending
“Show me the 10 most-watched videos in the last hour.” “What are the top…
Design an Online Auction
“Design an online auction” (eBay with live bidding) is a contention problem…
Design a Flash Sale System
At 12:00:00 sharp, a retailer drops 10,000 units of a hyped sneaker at half…
Design Yelp (Local Business Search)
“Find good coffee near me.” Behind that one tap sits a system that must filter…
Design Ticketmaster
Most of Ticketmaster is an ordinary catalogue: browse events, view a seat map…
Design a Metrics & Monitoring System
Every serious system needs eyes on it: CPU, latency, error rates, business…
Advanced: Change Data Capture (CDC)
Your data lives in a database, but that is rarely the only place it needs to be…
Advanced: Vector Databases
Keyword search matches strings: a search for “car” misses a document that only…
Advanced: Time-Series Databases
Most databases in this course are optimised for reading and updating individual…
Advanced: Data Structures for Big Data
Some questions are easy with a hash set and impossible at scale. “Has this user…
Advanced: Proximity Search
“Find the 20 nearest restaurants,” “show drivers within 2 km,” “who is nearby to…
Pattern: Managing Long-running Tasks
Not everything fits in a request/response. A user clicks “export my data” and…
Pattern: Handling Large Blobs
Sooner or later a design involves something big and binary: profile photos…
Pattern: Dealing with Contention
Some of the most memorable interview questions are really contention questions…
Pattern: Real-time Updates
Many interview questions have a line buried in the requirements like “users…
Key Technologies: Pick the Right Tool
“I’ll use Kafka” is not an answer. “This needs a durable, replayable log to…
Data Modeling
Ask a junior candidate to design a system and they start drawing tables. Ask a…
Design a Collaborative Editor (Google Docs)
“Design Google Docs” is the canonical real-time collaboration question. It looks…
Design an Ad Click Aggregator
“Design the system behind an advertising dashboard” is a favourite analytics…
Stream Processing
Most of the designs so far store data and answer queries against it. Stream…
Distributed Transactions
In a single database, a transaction is a gift: , do several writes, , and either…
Consensus and Coordination
Replication told you how to keep copies of data in sync. This lesson answers the…
Databases Deep-Dive: Indexing and Transactions
Most candidates can say “I’ll use PostgreSQL” or “I’ll use Cassandra,” but…
API Design
After you gather requirements, the next concrete artifact in a system design…
Networking and Protocols
Most candidates can draw boxes and arrows, but freeze when an interviewer asks…
Design Google Drive / Dropbox
File sync services like Google Drive and Dropbox let hundreds of millions of…
Design a Distributed Message Queue (Kafka)
A message queue decouples producers from consumers: services publish events…
Design Typeahead / Autocomplete
“Design the search box that suggests queries as you type” looks deceptively…
Design a Key-Value Store (Dynamo-style)
“Design a distributed key-value store” — the Dynamo question — is the deepest…
Design a Unique ID Generator
Generating unique IDs sounds trivial until you remove the single database that…
Design a News Feed (Twitter/Instagram)
“Design Twitter’s timeline” (or Instagram’s feed, or Facebook’s News Feed) is…
Design a Notification System
Every product sends notifications — order shipped, someone mentioned you, your…
Design a URL Shortener (TinyURL/bit.ly)
A URL shortener looks trivial — map a long URL to a short code and redirect…
Scaling Pitfalls: Hotspots, Stampedes and Failure
A design that looks correct on the whiteboard can still collapse the first time…
Replication and Consistency
Almost every scalable system keeps more than one copy of its data. The moment…
Scaling Reads
Almost every system you will be asked to design is read-heavy. A social network…
Scaling Writes
In the previous lesson, reads scaled by making copies: a cache, a replica, a CDN…
Cracking the System Design Interview
System design interviews are the most feared round in senior engineering…
Mock Interview Walkthrough
Reading about system design is one thing. Performing under interview pressure is…
Mistakes That Kill Your Interview
I’ve seen hundreds of system design interviews from both sides of the table. The…
Design a Distributed Cache
Every high-traffic system hits the same wall: the database cannot handle the…
Design a Rate Limiter
Every public API needs a rate limiter. Without one, a single misbehaving client…
Design a Web Crawler
Web crawlers are the backbone of search engines. Google’s crawler discovers and…
Design a Payment System
Payment systems are the backbone of e-commerce. A bug here does not just cause a…
Design a Search Engine
A search engine is one of the most complex distributed systems ever built…
Design an E-Commerce Platform
E-commerce platforms handle some of the most demanding distributed systems…
Design a Chat System (WhatsApp)
“Design WhatsApp” is one of the most popular system design interview questions…
Design a Video Streaming Platform (YouTube)
“Design YouTube” is the quintessential system design interview question. It…
Design a Ride-Sharing App (Uber)
Ride-sharing platforms like Uber and Lyft process millions of rides daily…
Common Components and When to Use Them
Every system you design in an interview is assembled from a small set of well…
The Framework — How to Structure Your Answer
In the previous lesson, we covered what interviewers actually evaluate. Now we…
Estimation and Back-of-Envelope Math
When an interviewer asks you to estimate the storage requirements for a…
What Interviewers Actually Look For
Most candidates walk into a system design interview thinking they need to…
System Design Interview Checklist
System design interviews are not trivia tests. The interviewer is not checking…
Designing a File Storage System (Dropbox/Google Drive)
A cloud file storage system seems straightforward until you think about what…
Designing a Notification System
Notification systems look simple on the surface. Something happens, you tell the…
System Design Masterclass
System design is the skill that separates senior engineers from everyone else…
Real-Time Systems — WebSockets, SSE & Polling
Most web applications are request-response: the client asks, the server answers…
Rate Limiting & Throttling at Scale
Your API has a problem. A single misbehaving client is sending 10,000 requests…
Designing a News Feed (Facebook/Twitter)
Every social platform has a feed. Facebook’s News Feed, Twitter’s Timeline…
Distributed Transactions & the SAGA Pattern
In a monolith, transactions are easy. You wrap your database operations in a…
Message Queues & Event-Driven Architecture
Every system starts synchronous. User clicks a button, server does the work…
Database Selection — SQL vs NoSQL vs NewSQL
Database selection is one of the highest-impact decisions in system design. Get…
Designing for High Availability
High availability is the property of a system that operates continuously without…
Designing a URL Shortener
The URL shortener is the “Hello World” of system design interviews. It seems…
How to Approach Any System Design Problem
Most engineers fail system design interviews not because they lack knowledge…
Load Balancing Patterns and Algorithms
A load balancer sits between clients and servers, distributing incoming requests…
Scaling Reads — Caching, Replicas & CDN
Most real-world systems are read-heavy. Twitter serves 600K reads per second but…
Scaling Writes — Sharding, Write-Ahead Logs & Partitioning
Scaling reads is straightforward — add caches, replicas, CDN. Scaling writes is…
Staff Engineer Study Plan for MAANG Interviews — The Complete 12-Week Roadmap
If you’re a Senior Engineer (L5) preparing for Staff (L6+) roles at MAANG…