Domain
Section
Level
Progress
Medium

Users Keep Getting Logged Out

Application State & Sessions

System Design/GUIDED EDITOR
15 min
Medium

Sticky Sessions Under Failure

Application State & Sessions

System Design/GUIDED EDITOR
15 min
Hard

The Growing Session Store

Shared session storage

18 min
Hard

Stateful WebSocket Gateways

Application State & Sessions

18 min
Medium

The Query Gets Slower Every Month

Relational Databases

20 min
Medium

Too Many Connections

Relational Databases

15 min
Hard

Checkout vs Analytics

Relational Databases

25 min
Medium

Choose the Relational Model

Relational Databases

20 min
Medium

Reads Are Crushing the Primary

Database Scaling & Replication

20 min
Medium

The Replica Is Stale

System Design

15 min
Hard

Replica Lag During Import

Database Scaling & Replication

20 min
Hard

Fail the Primary

Database Scaling & Replication

20 min
Medium

The Write Ceiling

Partitioning & Sharding

20 min
Hard

Shard 7 Is Melting

Partitioning & Sharding

20 min
Hard

The Newest Shard Is Always Hot

Partitioning & Sharding

20 min
Medium

Cross-Shard Checkout

Partitioning & Sharding

20 min
Medium

Stop Putting Videos in PostgreSQL

Object Storage & File Systems

20 min
Medium

Millions of Tiny Files

Object Storage & File Systems

20 min
Medium

Shared Filesystem Bottleneck

Object Storage & File Systems

20 min
Easy

Durability vs. Temporary Data

Object Storage & File Systems

20 min
Medium

The Same Read 50,000 Times

Caching

20 min
Hard

Inventory Is Wrong

Caching

20 min
Medium

Midnight Cache Explosion

Caching

20 min
Medium

Celebrity Hot Key

Caching

20 min
Medium

Australia Is 600ms Away

CDNs & Edge Delivery

20 min
Easy

The CDN Serves Old JavaScript

CDNs & Edge Delivery

10 min
Easy

Personalized Pages at the Edge

CDNs & Edge Delivery

20 min
Easy

Origin Shield Under Viral Traffic

CDNs & Edge Delivery

10 min
Medium

Checkout Takes Eight Seconds

Queues & Background Processing

20 min
Hard

The Backlog Keeps Growing

Queues & Background Processing

20 min
Easy

The Poison Job

Queues & Background Processing

10 min
Hard

Priority Matters

Queues & Background Processing

20 min
Medium

Campaign Capacity

Servers & Compute

System Design/GUIDED EDITOR
15 min
Hard

CPU Isn't the Problem

Servers & Compute

System Design/GUIDED EDITOR
18 min
Medium

Background Jobs Are Starving the API

Servers & Compute

System Design/GUIDED EDITOR
15 min
Medium

Burst Without Waste

Servers & Compute

System Design/GUIDED EDITOR
15 min
Medium

Uneven Fleet

Load Balancers & Traffic Routing

System Design/GUIDED EDITOR
15 min
Medium

The Dead Server Still Gets Traffic

Load Balancers & Traffic Routing

System Design/GUIDED EDITOR
15 min
Medium

Long Requests Break Round Robin

Load Balancers & Traffic Routing

System Design/GUIDED EDITOR
15 min
Hard

Route by Workload

Load Balancers & Traffic Routing

System Design/GUIDED EDITOR
18 min
Medium

Users Keep Getting Logged Out

Application State & Sessions

System Design/GUIDED EDITOR
15 min
Medium

Sticky Sessions Under Failure

Application State & Sessions

System Design/GUIDED EDITOR
15 min
Hard

The Growing Session Store

Shared session storage

18 min
Hard

Stateful WebSocket Gateways

Application State & Sessions

18 min
Medium

The Query Gets Slower Every Month

Relational Databases

20 min
Medium

Too Many Connections

Relational Databases

15 min
Hard

Checkout vs Analytics

Relational Databases

25 min
Medium

Choose the Relational Model

Relational Databases

20 min
Medium

Reads Are Crushing the Primary

Database Scaling & Replication

20 min
Medium

The Replica Is Stale

System Design

15 min
Hard

Replica Lag During Import

Database Scaling & Replication

20 min
Hard

Fail the Primary

Database Scaling & Replication

20 min
Medium

The Write Ceiling

Partitioning & Sharding

20 min
Hard

Shard 7 Is Melting

Partitioning & Sharding

20 min
Hard

The Newest Shard Is Always Hot

Partitioning & Sharding

20 min
Medium

Cross-Shard Checkout

Partitioning & Sharding

20 min
Medium

Stop Putting Videos in PostgreSQL

Object Storage & File Systems

20 min
Medium

Millions of Tiny Files

Object Storage & File Systems

20 min
Medium

Shared Filesystem Bottleneck

Object Storage & File Systems

20 min
Easy

Durability vs. Temporary Data

Object Storage & File Systems

20 min
Medium

The Same Read 50,000 Times

Caching

20 min
Hard

Inventory Is Wrong

Caching

20 min
Medium

Midnight Cache Explosion

Caching

20 min
Medium

Celebrity Hot Key

Caching

20 min
Medium

Australia Is 600ms Away

CDNs & Edge Delivery

20 min
Easy

The CDN Serves Old JavaScript

CDNs & Edge Delivery

10 min
Easy

Personalized Pages at the Edge

CDNs & Edge Delivery

20 min
Easy

Origin Shield Under Viral Traffic

CDNs & Edge Delivery

10 min
Medium

Checkout Takes Eight Seconds

Queues & Background Processing

20 min
Hard

The Backlog Keeps Growing

Queues & Background Processing

20 min
Easy

The Poison Job

Queues & Background Processing

10 min
Hard

Priority Matters

Queues & Background Processing

20 min
Medium

Campaign Capacity

Servers & Compute

System Design/GUIDED EDITOR
15 min
Hard

CPU Isn't the Problem

Servers & Compute

System Design/GUIDED EDITOR
18 min
Medium

Background Jobs Are Starving the API

Servers & Compute

System Design/GUIDED EDITOR
15 min
Medium

Burst Without Waste

Servers & Compute

System Design/GUIDED EDITOR
15 min
Medium

Uneven Fleet

Load Balancers & Traffic Routing

System Design/GUIDED EDITOR
15 min
Medium

The Dead Server Still Gets Traffic

Load Balancers & Traffic Routing

System Design/GUIDED EDITOR
15 min
Medium

Long Requests Break Round Robin

Load Balancers & Traffic Routing

System Design/GUIDED EDITOR
15 min
Hard

Route by Workload

Load Balancers & Traffic Routing

System Design/GUIDED EDITOR
18 min
Easy

One Order, Five Consumers

Pub/Sub & Event-Driven Systems

12 min
Medium

The Slow Subscriber

Pub/Sub & Event-Driven Systems

15 min
Medium

Too Many Event Types

Pub/Sub & Event-Driven Systems

12 min
Medium

Event-Driven Workflow Challenge

Pub/Sub & Event-Driven Systems

18 min
Easy

The Payment Ran Twice

Idempotency, Ordering & Delivery Guarantees

12 min
Medium

Cancelled Before Created

Idempotency, Ordering & Delivery Guarantees

14 min
Medium

Exactly Once Requirement

Idempotency, Ordering & Delivery Guarantees

14 min
Hard

Commit Succeeded, Publish Failed

Idempotency, Ordering & Delivery Guarantees

18 min
Easy

The Chatty Mobile App

APIs & Service Communication

14 min
Medium

Internal Calls Waste Capacity

APIs & Service Communication

15 min
Medium

The Public API Needs Protection

APIs & Service Communication

16 min
Medium

Version Without Breaking Clients

APIs & Service Communication

16 min
Medium

The Monolith's Reporting Module

Service Boundaries & Microservices

18 min
Medium

Shared Database Coupling

Service Boundaries & Microservices

18 min
Hard

Too Many Tiny Services

Service Boundaries & Microservices

20 min
Hard

Synchronous Dependency Chain

Service Boundaries & Microservices

20 min
Medium

Profile Update Looks Old

Consistency & Distributed Data

15 min
Medium

Two Regions Edit the Same Record

Consistency & Distributed Data

15 min
Hard

Availability or Strong Consistency

Consistency & Distributed Data

18 min
Hard

Materialized View Is Behind

Consistency & Distributed Data

18 min
Hard

Recommendation Takes Down Product Page

System Design

18 min
Medium

Retry Storm

System Design

10 min
Medium

One Pool Consumes Everything

System Design

12 min
Easy

Recover Without a Herd

System Design

10 min
Medium

Scale on the Right Signal

System Design

15 min
Medium

Cold Start During a Flash Crowd

System Design

15 min
Medium

Workers Scale, Database Doesn't

System Design

12 min
Hard

Capacity Plan for Launch

System Design

18 min
Medium

Average Latency Looks Fine

System Design

10 min
Medium

Where Did Four Seconds Go?

System Design

15 min
Medium

Queue Is Quiet, Customers Aren't

System Design

14 min
Hard

Production Diagnostic Challenge

System Design

22 min
Medium

Release Breaks Everyone

System Design

16 min
Medium

Zero-Downtime Version Swap

System Design

16 min
Easy

Database Change Without Downtime

System Design

10 min
Medium

Feature Launch Without Redeploy

System Design

13 min
Medium

The Architecture Costs 2x

System Design

12 min
Medium

Storage Bill Never Stops Growing

System Design

14 min
Medium

Cross-Region Bill Shock

System Design

12 min
Hard

Final Foundation Optimization Challenge

System Design

25 min
Easy

One Order, Five Consumers

System Design

12 min
Medium

The Slow Subscriber

System Design

15 min
Medium

Too Many Event Types

System Design

15 min
Hard

Event-Driven Workflow Challenge

System Design

20 min
Easy

The Payment Ran Twice

System Design

12 min
Medium

Cancelled Before Created

System Design

14 min
Medium

Exactly Once Requirement

System Design

14 min
Hard

Commit Succeeded, Publish Failed

System Design

18 min
Easy

The Chatty Mobile App

System Design

14 min
Medium

Internal Calls Waste Capacity

System Design

15 min
Medium

The Public API Needs Protection

System Design

16 min
Medium

Version Without Breaking Clients

System Design

16 min
Medium

The Monolith's Reporting Module

System Design

18 min
Medium

Shared Database Coupling

System Design

18 min
Hard

Too Many Tiny Services

System Design

20 min
Hard

Synchronous Dependency Chain

System Design

20 min
Medium

Profile Update Looks Old

System Design

15 min
Medium

Two Regions Edit the Same Record

System Design

15 min
Hard

Availability or Strong Consistency

System Design

18 min
Hard

Materialized View Is Behind

System Design

18 min
Hard

Recommendation Takes Down Product Page

System Design

18 min
Medium

Retry Storm

System Design

10 min
Medium

One Pool Consumes Everything

System Design

12 min
Easy

Recover Without a Herd

System Design

10 min
Medium

Scale on the Right Signal

System Design

15 min
Medium

Cold Start During a Flash Crowd

System Design

15 min
Medium

Workers Scale, Database Doesn't

System Design

12 min
Hard

Capacity Plan for Launch

System Design

18 min
Medium

Average Latency Looks Fine

System Design

10 min
Medium

Where Did Four Seconds Go?

System Design

15 min
Medium

Queue Is Quiet, Customers Aren't

System Design

14 min
Hard

Production Diagnostic Challenge

System Design

22 min
Medium

Release Breaks Everyone

System Design

16 min
Medium

Zero-Downtime Version Swap

System Design

16 min
Easy

Database Change Without Downtime

System Design

10 min
Medium

Feature Launch Without Redeploy

System Design

13 min
Medium

The Architecture Costs 2x

System Design

12 min
Medium

Storage Bill Never Stops Growing

System Design

14 min
Medium

Cross-Region Bill Shock

System Design

12 min
Hard

Final Foundation Optimization Challenge

System Design

25 min
Medium

URL Shortener

System Design

20 min
Medium

Notification Platform

System Design

22 min
Hard

Distributed Rate Limiter

System Design

22 min
Hard

Dropbox

System Design

25 min
Hard

WhatsApp

System Design

28 min
Hard

Twitter/X Feed

System Design

24 min
Hard

Instagram

System Design

25 min
Hard

YouTube

System Design

30 min
Hard

Netflix

System Design

24 min
Hard

Uber

System Design

26 min
Hard

Ticketmaster

System Design

24 min
Hard

Stripe

System Design

24 min
Hard

Slack

System Design

26 min
Hard

Google Drive

System Design

26 min
Medium

Search Autocomplete

System Design

18 min
Hard

Full-Text Search

System Design

22 min
Hard

Collaborative Editor

System Design

22 min
Hard

Webhook Platform

System Design

24 min
Hard

Distributed Scheduler

System Design

26 min
Hard

Metrics Platform

System Design

24 min
Hard

Logging Platform

System Design

25 min
Hard

Feature Flag Platform

System Design

24 min
Hard

Distributed Cache

System Design

26 min
Hard

Object Storage Service

System Design

26 min
Expert

Kafka-Like Streaming Platform

System Design

35 min
Easy

From Notebook to Production

ML System Architecture Fundamentals

12 min
Easy

Batch vs. Online, Not Both at Once

ML System Architecture Fundamentals

14 min
Medium

A Safe Path to Raw Data

ML System Architecture Fundamentals

15 min
Medium

A Small, Complete ML Lifecycle

ML System Architecture Fundamentals

20 min
Easy

Sampling Bias in Event Collection

Data Collection & Labeling

12 min
Medium

Joining Late-Arriving Labels

Data Collection & Labeling

14 min
Medium

Human Labeling Throughput

Data Collection & Labeling

15 min
Medium

Filtering Sensitive Data Before Training

Data Collection & Labeling

15 min
Medium

Outgrowing the Nightly Window

Data Pipelines & Preprocessing

15 min
Medium

A Realtime Feature Arrives Too Late

Data Pipelines & Preprocessing

14 min
Medium

Duplicate Events Corrupt Training Examples

Data Pipelines & Preprocessing

13 min
Medium

Backfills Overwhelm the Pipeline

Data Pipelines & Preprocessing

16 min
Medium

Training-Serving Skew

Feature Engineering & Feature Stores

15 min
Medium

Online Feature Lookups Under Peak Load

Feature Engineering & Feature Stores

16 min
Medium

Freshness SLAs Differ by Feature

Feature Engineering & Feature Stores

14 min
Hard

Six Teams, One Feature

Feature Engineering & Feature Stores

18 min
Medium

Shrinking a 46-Hour Training Run

Training Infrastructure

16 min
Medium

GPU Utilization Bottlenecked by Data Loading

Training Infrastructure

16 min
Medium

Multiple Teams Competing for GPUs

Training Infrastructure

15 min
Easy

Checkpoint and Resume After Failure

Training Infrastructure

12 min
Easy

Same Code, Different Results

Experiment Tracking & Reproducibility

13 min
Medium

Untraceable Hyperparameters

Experiment Tracking & Reproducibility

14 min
Medium

Hundreds of Untraceable Experiments

Experiment Tracking & Reproducibility

15 min
Hard

Reproducing a Six-Month-Old Model

Experiment Tracking & Reproducibility

20 min
Easy

Aggregate Accuracy Hides a Segment Regression

Model Evaluation & Validation

12 min
Medium

Undeployable Despite Good Offline Metrics

Model Evaluation & Validation

15 min
Easy

Release Without a Champion Comparison

Model Evaluation & Validation

12 min
Hard

Data Leakage Inflates the Eval Score

Model Evaluation & Validation

18 min
Easy

Deploying Straight From the Training Machine

Model Registry & Versioning

12 min
Easy

Rolling Back in Minutes, Not Hours

Model Registry & Versioning

13 min
Medium

Different Regions, Different Versions

Model Registry & Versioning

16 min
Medium

Candidate to Staging to Production

Model Registry & Versioning

20 min
Medium

Score Everyone Before 6 AM

Batch Inference

18 min
Medium

One Partition, All the Stragglers

Batch Inference

18 min
Medium

Rescore Only What Changed

Batch Inference

18 min
Easy

Publishing a Half-Finished Run

Batch Inference

14 min
Easy

Behind a Production API, Finally

Online Inference & Model Serving

15 min
Medium

Cold Starts Make New Servers Slow to Help

Online Inference & Model Serving

18 min
Medium

Feature Retrieval Dominates Latency

Online Inference & Model Serving

20 min
Hard

One Slow Dependency Blocks a Healthy Request

Online Inference & Model Serving

22 min
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