Meta

41 design problems known to be asked at Meta.

Easy4

Easylow level design design patterns oop

Iterator Pattern: Walk a Collection Without Knowing Its Shape

Every for-each loop you've ever written is the Iterator pattern in disguise. Here's how one hasNext()/next() surface lets you traverse an array, a tree, or an unbounded paged API the exact same way — and why mutating mid-loop blows up.

Asked at Amazon, Google, Microsoft +2

Aug 8, 2026·16 min read

Easylow level design design patterns oop

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

Easylow level design design patterns oop

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

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

Medium28

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumhigh level design distributed systems data structures

Bloom Filters: The Yes/No That Saves a Disk Seek

A tiny bit-array that answers 'definitely not here' or 'maybe here' in constant time — how Cassandra, HBase, and Chrome skip millions of expensive lookups for keys that were never there.

Asked at Google, Amazon, Meta +3

Aug 8, 2026·12 min read

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

Composite Pattern: One size() Call for a File or Ten Thousand

When your data is a tree — files and folders, elements and containers, parts and assemblies — the Composite pattern lets client code treat a single leaf and a whole subtree with the exact same call. The recursion hides inside the tree, not in your if-statements.

Asked at Amazon, Google, Microsoft +2

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

Mediumlow level design design patterns oop

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

Mediumlow level design oop design principles

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

Mediumlow level design design patterns oop

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

Mediumlow level design oop design principles

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

Mediumlow level design oop design principles

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design oop design principles

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design oop design principles

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

Mediumlow level design oop design principles

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

Mediumlow level design design patterns oop

Structural Patterns: Composing Objects Into Bigger Things

The seven structural patterns aren't seven clever tricks — they're seven answers to one question: how do you compose classes and objects into larger structures without the composition itself going rigid? A map, and how to tell the confusing ones apart.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·8 min read

Hard9

Hardsystem design interview distributed systems message queues realtime

Design a Chat System

WhatsApp-scale messaging, built end to end: persistent WebSocket gateways, a user→server registry, store-and-forward for offline devices, the one-tick / two-tick / blue-tick delivery state machine, message ordering, and group fan-out. Drive the delivery protocol yourself, then take it apart.

Asked at Meta, WhatsApp, Slack +3

Aug 9, 2026·14 min read

Hardhigh level design distributed systems consensus

Consensus & Leader Election: How a Cluster Agrees on One Truth

Split-brain corrupts data quietly. This is how Raft gets N crash-prone nodes to agree on a leader, a log, and a lock — and why a majority is the only quorum that can't lie to itself.

Asked at Google, Amazon, Microsoft +3

Aug 8, 2026·17 min read

Hardhigh level design distributed systems databases

Database Sharding: When One Machine Stops Being Enough

Your 4 TB users table no longer fits — or no longer keeps up with writes — on a single node. Sharding is the answer, and the shard key you pick quietly decides everything that follows.

Asked at Amazon, Google, Meta +3

Aug 8, 2026·16 min read

Hardhigh level design distributed systems transactions

Distributed Transactions: When One Business Action Spans Many Services

The payment succeeded but the inventory reservation failed — and there's no ROLLBACK that spans both. How 2PC, Sagas, and the Outbox pattern give you atomicity when a single ACID transaction is no longer on the table.

Asked at Amazon, Uber, DoorDash +3

Aug 8, 2026·17 min read

Hardsystem design interview databases algorithms concurrency

Design a Key-Value Store

The capstone that ties the whole series together: consistent hashing to place data, quorums to tune consistency, vector clocks to detect conflicts, and hinted handoff to stay writable through failure. This is Amazon's Dynamo, rebuilt from its four moving parts.

Asked at Amazon, Netflix, Uber +2

Aug 9, 2026·23 min read

Hardsystem design interview distributed systems caching scalability

Design a News Feed

The Twitter/Instagram timeline, worked end to end. The whole system pivots on one decision — do you build a follower's feed when someone posts (push), or when they open the app (pull)? Toggle between them, watch the write and read cost flip, and see why one celebrity breaks the naive answer.

Asked at Meta, Twitter, Instagram +3

Aug 9, 2026·13 min read

Hardhigh level design distributed systems consensus

Raft, Driven: Watch a Cluster Elect, Replicate, and Heal

Everyone can recite 'Raft elects a leader by majority.' The hard part is the log — how a follower that missed writes during a partition gets repaired, and the one subtle rule that stops a committed entry from ever being lost. Drive a live 5-node cluster, then take it apart.

Asked at Google, Amazon, Microsoft +4

Aug 9, 2026·20 min read

Hardhigh level design distributed systems databases storage

Storage Engines: LSM-Trees vs B-Trees

Every database makes one foundational choice — how to lay bytes on disk. B-trees update in place; LSM-trees never overwrite, they append and compact. That single decision sets your write throughput, read latency, and space usage. Drive an LSM write path — memtable, flush, compaction — and see the tradeoff for yourself.

Asked at Google, Meta, Amazon +3

Aug 9, 2026·14 min read

Hardlow level design design patterns oop

Visitor Pattern: Add Operations Without Editing the Classes

When your class hierarchy is stable but you keep bolting new operations onto every node — typecheck, pretty-print, evaluate, export — the Visitor pattern moves each operation into its own class so the nodes never change again.

Asked at Google, Microsoft, Amazon +3

Aug 8, 2026·15 min read