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Adapter Pattern: Make Two Interfaces That Weren't Built to Talk, Talk

Your code expects one interface; the useful class only speaks another — and you can't edit either. Adapter is the thin impedance-matching layer that lets them work together without a rewrite.

Asked at Amazon, Stripe, Uber +2

Aug 8, 2026·16 min read

Facade Pattern: One Door Into a Room Full of Machinery

When 'place an order' means orchestrating inventory, payment, shipping, and notifications — every caller re-learns the whole subsystem. A Facade gives them one door, and leaves the machinery open for anyone who still needs it.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·14 min read

Singleton: One Instance, and the Global It Smuggles In

The GoF pattern everyone knows and half of us regret. Watch a naive getInstance() spawn two instances under load — then see why modern apps get one instance from a DI container, not a global.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·20 min read

Strategy Pattern: Swap Behavior Without Touching the Caller

Why a checkout that grows an if-branch every quarter is a design smell — and how the Strategy pattern turns runtime behavior into something you plug in, not rewrite.

Asked at Amazon, Google, Microsoft +2

Jul 27, 2026·14 min read

Template Method: Freeze the Recipe, Swap the Ingredients

When CSV, JSON, and XML importers all do the same four steps in the same order but differ in two of them, three near-identical pipelines is the smell. Template Method puts the order in one place and lets subclasses fill only the steps that vary.

Asked at Amazon, Google, Microsoft +2

Aug 8, 2026·15 min read

Easylow level design oop design patterns

Designing Tic-Tac-Toe: A Low-Level Design Walkthrough

A complete object-oriented design for Tic-Tac-Toe — a playable board, the full class model (players, board, pluggable rules), an O(1) win check, a codeable interface you can implement, and the patterns that scale it to N×N, K-in-a-row, and bots.

Asked at Google, Amazon, Microsoft +3

Jul 24, 2026·20 min read

Medium33

Abstract Factory: Ship a Whole Family, Never a Mismatched Pair

One factory hands you a Button, a Checkbox, and a Menu that already agree with each other — so a macOS button can never end up next to a Windows checkbox.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·16 min read

Mediumhigh level design distributed systems microservices

The API Gateway: One Front Door for a Hundred Services

When a mobile screen fires a dozen calls to a dozen services, each doing its own auth and its own CORS, the answer isn't a smarter client — it's a single entry point that owns the cross-cutting concerns once.

Asked at Netflix, Amazon, Uber +2

Aug 8, 2026·15 min read

Behavioral Patterns: How Objects Talk and Decide

The eleven behavioral patterns aren't eleven tricks — they're eleven answers to one question: when responsibility and communication are tangled inside a class, which one do you pull out, and where does it go? A map, not a re-teach.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·8 min read

Builder: Assemble Complex Objects Without the Telescoping Constructor

Ten positional arguments and a fistful of nulls is not construction — it's a guessing game. The Builder pattern turns object assembly into readable, validate-once, immutable steps.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·17 min read

Mediumhigh level design distributed systems caching

Caching Strategies: The Bet That Reads Repeat

A cache is a wager that the same data gets read again before it changes. Get the read/write pattern, the eviction policy, and — the hard part — invalidation right, and one box absorbs the load that would have flattened your database.

Asked at Meta, Amazon, Netflix +3

Aug 8, 2026·13 min read

Mediumhigh level design distributed systems consistency

CAP Theorem: The Choice You Only Make When the Network Breaks

Everyone quotes 'pick 2 of 3' and everyone is wrong. CAP is a decision forced only during a partition — and PACELC is the sharper lens for the 99.9% of the time when the network is fine.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·12 min read

Mediumlow level design oop design patterns

Designing a Parking Lot: A Low-Level Design Walkthrough

A complete object-oriented design for a multi-level parking lot — the class model, pluggable spot-allocation and pricing strategies, an O(1) availability lookup, a live simulator you can drive cars into, and a codeable interface you can implement against.

Asked at Google, Amazon, Microsoft +3

Jul 27, 2026·30 min read

Chain of Responsibility: Let the Request Find Its Own Handler

Why the sender of a request should never hard-code who processes it — and how linking handlers into a chain turns a brittle escalation switch into the reorderable pipeline behind every middleware stack.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·15 min read

Mediumdistributed systems algorithms databases

How Consistent Hashing Powers Distributed Systems

A deep dive into the algorithm that lets systems like Cassandra and Amazon's Dynamo route keys without reshuffling the entire keyspace when nodes come and go.

Asked at Amazon, Google, Meta +2

Jul 23, 2026·27 min read

Creational Patterns: Who Decides What Gets Built

The five creational patterns aren't five clever tricks — they're five answers to one question: how does an object come into existence without the code that uses it hard-wiring the code that makes it? A map, and one interactive way to pick the right one.

Asked at Amazon, Google, Microsoft +3

Jul 28, 2026·5 min read

Mediumhigh level design distributed systems databases

Database Indexing: Why B-Trees Read Fast and LSM-Trees Write Fast

A missing index turns a one-user lookup into a scan of fifty million rows. Here's how the two great index families — B+ trees and LSM-trees — buy that lookup back, and what each one charges you for it.

Asked at Amazon, Google, Meta +3

Aug 8, 2026·13 min read

Mediumhigh level design distributed systems databases

Database Replication: Copies That Disagree With Each Other

Why the same query returns different answers on two servers a second apart — and the failover, lag, and conflict traps that turn 'just add a read replica' into a 3am incident.

Asked at Amazon, Google, Meta +3

Aug 8, 2026·15 min read

Decorator Pattern: Stack Behavior at Runtime Without a Subclass Bomb

You need buffering, then compression, then encryption — in any combination. Subclassing that gives you BufferedEncryptedCompressedInputStream. The Decorator pattern lets you wrap those responsibilities on one object at a time instead.

Asked at Amazon, Google, Microsoft +2

Aug 8, 2026·14 min read

Dependency Inversion: Point the Arrow at an Abstraction

A service that news-up its own email sender can't be tested or swapped. Dependency Inversion flips the arrow so high-level policy and low-level detail both depend on an abstraction.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·8 min read

Mediumsystem design interview algorithms concurrency databases

Design a Distributed ID Generator

Auto-increment doesn't survive sharding, and random UUIDs wreck your database's index. How do you mint billions of unique, roughly time-sortable 64-bit IDs across a fleet with no coordinator? This is the Snowflake design — bit for bit.

Asked at Twitter, Instagram, Discord +2

Aug 9, 2026·16 min read

Factory Method: Let the Subclass Decide What to Build

A switch(type) buried inside your shared workflow is a design smell. Factory Method moves the 'new' to a subclass so the logic that uses an object never learns which one it got.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·17 min read

Flyweight: One 'e' for a Million Letters

When you must hold millions of near-identical objects, one-object-per-instance quietly blows the heap. Flyweight splits shared shape from per-use position — and hands out one copy for all of them.

Asked at Google, Microsoft, Adobe +3

Aug 8, 2026·14 min read

Mediumhigh level design distributed systems reliability

Idempotency: How to Retry a Payment Without Charging Twice

The network fails after your server does the work but before the answer gets home, so the client retries — and a naive charge endpoint bills the card twice. Idempotency is the discipline that makes every retry safe.

Asked at Stripe, Amazon, PayPal +3

Aug 8, 2026·15 min read

Interface Segregation: Don't Make Callers Depend on What They Don't Use

One fat Notifier interface forces every channel to stub methods it can't support and every caller to depend on methods it never calls. Interface Segregation splits it into roles.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·7 min read

Mediumhigh level design distributed systems networking

Load Balancing: One Front Door for a Fleet That Keeps Changing

A single server melts under Black-Friday traffic. Load balancing is how you put ten servers behind one address — and the L4-vs-L7 choice, the routing algorithm, and the health checks are where systems live or die.

Asked at Amazon, Google, Netflix +3

Aug 8, 2026·16 min read

Liskov Substitution: If Clients Must Check the Type, You Broke It

An SmsChannel that can't do what a Channel promised isn't a safe stand-in for its base — so callers start type-checking. Liskov, explained simply: a subtype must work anywhere its parent does.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·8 min read

Mediator Pattern: Turn a Mesh of Dependencies Into a Star

When every object holds a reference to every other, change becomes impossible — an O(N²) tangle where one edit ripples everywhere. The Mediator pulls the wiring into one place so components can stay gloriously ignorant of each other.

Asked at Google, Microsoft, Meta +3

Aug 8, 2026·15 min read

Mediumhigh level design distributed systems messaging

Message Queues & Event Streaming: Stop Making Users Wait for Work They Don't Care About

A checkout that emails, bills, and updates inventory before it answers is one flaky partner away from a timeout. How queues and logs let producers and consumers stop holding each other hostage.

Asked at Amazon, LinkedIn, Netflix +3

Aug 8, 2026·17 min read

Observer Pattern: Broadcast State Without Knowing Who's Listening

When one object changes and an unknown, shifting crowd must react, wiring the subject to each dependent is a maintenance trap. Observer inverts it — dependents subscribe, the subject just broadcasts.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·15 min read

Open/Closed: Add a Channel Without Touching the Core

Every new notification channel meant another edit to the same switch — and another regression. Open/Closed makes new behavior a drop-in, not a rewrite.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·7 min read

Prototype: Clone a Ready-Made Object Instead of Rebuilding It

Constructing an object can be expensive and the client shouldn't need its class — so copy a preconfigured instance. The whole game is shallow vs deep copy, and getting it wrong aliases your original.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·17 min read

Proxy Pattern: The Stand-In Your Code Can't Tell Apart

The @Transactional annotation on your service method does nothing on its own — a proxy quietly wrapping the bean is what makes it work. Understand that stand-in and half of Spring's magic stops being magic.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·14 min read

Mediumhigh level design distributed systems networking

Real-Time Updates: Polling, Long Polling, SSE, and WebSockets

HTTP can't push — the server only answers when asked. Here's the ladder of techniques that fake or fix that, each one solving the last one's flaw, and the fan-out problem that breaks all of them at scale.

Asked at Slack, Discord, WhatsApp +3

Aug 8, 2026·16 min read

SOLID, Actually: Five Rules for Code That Survives Change

SOLID isn't five unrelated rules — it's one idea about managing dependencies and change. Watch one messy NotificationService become clean, one principle at a time.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·4 min read

Mediumhigh level design distributed systems databases

SQL vs NoSQL: Model the Access Pattern, Then Pick the Store

The choice isn't a religion — it's a consequence. Design how your data is read and written first, and the datastore that fits falls out of it. Pick the store first and you'll hand-join in application code for the next three years.

Asked at Amazon, Google, Netflix +3

Aug 8, 2026·15 min read

Single Responsibility: One Reason to Change, One Owner

SRP isn't 'do one thing' — it's 'answer to one actor'. Watch a Marketing copy tweak break Compliance's audit log, then split one NotificationService by who owns each change.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·7 min read

State Pattern: Make Illegal Transitions Impossible

An order that can be paid, shipped, delivered, or cancelled grows a switch(status) block in every method — until one of them forgets an arm and a shipped order gets refunded. The State pattern moves both behavior and the legal next-steps into the state itself.

Asked at Amazon, Google, Microsoft +2

Aug 8, 2026·16 min read