nSkillHub

System Design Quest Sheet

Sat, 18 Apr 2026 00:00:00 +0000

Classic “Design X” Questions
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#	Topic	Category	Status	Published URL	Notes
1	URL Shortener (bit.ly)	Classic	published	/system-design/classic/url-shortener	Base62, hash collisions, redirect latency
2	Twitter / Social Media Feed	Classic	todo		Fan-out on write vs read, timeline
3	Instagram	Classic	todo		Photo storage, explore, hashtag index
4	WhatsApp / Chat Messaging System	Classic	todo		WebSocket, message ordering, E2EE
5	Uber / Ride-Sharing System	Classic	todo		Geohash, supply-demand matching
6	Netflix / Video Streaming Platform	Classic	todo		CDN, adaptive bitrate, personalisation
7	YouTube	Classic	todo		Upload pipeline, transcoding, comments
8	Dropbox / Google Drive (File Sync)	Classic	todo		Block dedup, delta sync, chunking
9	Google Docs (Real-Time Collaborative Editing)	Classic	todo		OT vs CRDT, conflict-free merges
10	Search Engine (Google-scale)	Classic	todo		Crawl → index → rank → serve
11	Google Maps / Routing Engine	Classic	todo		Dijkstra, A*, road graph sharding
12	Web Crawler	Classic	todo		Politeness, dedup, frontier scheduling
13	Recommendation System (end-to-end)	Classic	todo		Collaborative filtering, two-tower, serving
14	Notification Service (email, push, SMS at scale)	Classic	todo		Fanout, deduplication, delivery tracking
15	Rate Limiter	Classic	todo		Token bucket, sliding window, Redis
16	Distributed Cache	Classic	todo		Eviction policies, clustering, consistency
17	Key-Value Store (Redis / DynamoDB internals)	Classic	todo		LSM tree, WAL, consistent hashing
18	Distributed Message Queue (Kafka)	Classic	todo		Partitions, offsets, consumer groups
19	Logging & Metrics System (Datadog / ELK)	Classic	todo		Structured logs, TSDB, alerting
20	Distributed File System (HDFS / GFS)	Classic	todo		NameNode, replication, rack awareness

Fintech
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#	Topic	Category	Status	Notes
21	Payment Processing System	Fintech	todo	Idempotency, saga, PCI DSS
22	Digital Wallet	Fintech	todo	Balance model, top-up, withdrawal
23	Money Transfer System (Venmo / Wise / Moniepoint)	Fintech	todo	Cross-border, FX, settlement rails
24	Fraud Detection System	Fintech	todo	Rule engine + ML, real-time scoring
25	Card Authorization System	Fintech	todo	Issuer, network, sub-100ms auth
26	Ledger / Double-Entry Bookkeeping System	Fintech	todo	Immutable entries, balance integrity
27	Reconciliation System (two financial systems)	Fintech	todo	Eventual consistency, diff engine
28	KYC / AML Onboarding Flow	Fintech	todo	Watchlist, PEP screening, risk scoring
29	High-Throughput Transaction Processing System	Fintech	todo	LMAX Disruptor, mechanical sympathy
30	Currency Conversion System	Fintech	todo	FX rate feed, rounding, audit trail
31	Banking Core — Account Balances at Scale	Fintech	todo	ACID at scale, regulatory reporting

E-Commerce & Marketplace
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#	Topic	Category	Status	Notes
32	Amazon — Catalog, Search & Checkout	E-Commerce	todo	Product graph, ranking, checkout saga
33	Flash Sale / High-Contention Inventory	E-Commerce	todo	Thundering herd, queue, fairness
34	Shopping Cart (multi-session, multi-device)	E-Commerce	todo	Merge strategies, guest → auth
35	Inventory Management System (multi-warehouse)	E-Commerce	todo	Oversell prevention, reservation TTL
36	Price Comparison Engine	E-Commerce	todo	Crawl-and-normalize, ranking, freshness
37	Coupon & Promotion Engine	E-Commerce	todo	Rule DSL, stacking, abuse prevention
38	Airbnb — Search, Booking & Availability	E-Commerce	todo	Geo search, calendar blocking, pricing
39	Order Management System	E-Commerce	todo	State machine, fulfilment pipeline
40	Loyalty / Points & Rewards System	E-Commerce	todo	Ledger, expiry, redemption

Booking & Reservation
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#	Topic	Category	Status	Notes
41	Movie Ticket Booking System (BookMyShow)	Booking	todo	Seat lock, payment window, concurrency
42	Hotel Booking System	Booking	todo	Availability calendar, overbooking policy
43	Airline Reservation System	Booking	todo	PNR, seat classes, fare rules
44	Restaurant Reservation System (OpenTable)	Booking	todo	Table inventory, waitlist, no-show
45	Calendar & Scheduling System (Calendly)	Booking	todo	Availability slots, timezone, conflict

Real-Time & Streaming
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#	Topic	Category	Status	Notes
46	Live Video Streaming Platform (Twitch)	Real-Time	todo	Ingest, transcode, CDN edge, chat
47	Live Sports Scores System	Real-Time	todo	Push vs poll, SSE, fan-out
48	Online Multiplayer Game Backend	Real-Time	todo	State sync, authoritative server, lag comp
49	Stock Trading Platform	Real-Time	todo	Order matching, LMAX, market data feed
50	Real-Time Analytics Dashboard	Real-Time	todo	Kafka + Flink, OLAP, query latency
51	Collaborative Whiteboard	Real-Time	todo	CRDT, WebSocket, cursor presence

Infrastructure & Developer Tools
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#	Topic	Category	Status	Notes
52	CI/CD System	Infra & DevTools	todo	Pipeline DAG, artifact store, rollback
53	Feature Flag Service (LaunchDarkly)	Infra & DevTools	todo	Progressive rollout, targeting rules
54	Configuration Management System	Infra & DevTools	todo	Hot reload, versioning, audit
55	Secrets Management System (Vault)	Infra & DevTools	todo	Dynamic secrets, lease renewal, KMS
56	API Gateway	Infra & DevTools	todo	Auth, routing, throttling, observability
57	Service Registry & Discovery	Infra & DevTools	todo	Consul, Eureka, health checks
58	Distributed Job Scheduler (cron at scale)	Infra & DevTools	todo	Exactly-once, leader election, sharding
59	Workflow Engine (Airflow / Temporal)	Infra & DevTools	todo	DAG execution, retries, durable state

Content & Media
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#	Topic	Category	Status	Notes
60	Content Delivery Network (CDN)	Content & Media	todo	PoP placement, cache hierarchy, purge
61	Image Hosting & Serving System	Content & Media	todo	On-the-fly resize, WebP, CDN offload
62	Podcast Hosting Platform	Content & Media	todo	Audio storage, RSS, analytics
63	News Feed Aggregator	Content & Media	todo	RSS crawl, dedup, personalisation
64	Content Moderation System	Content & Media	todo	ML classifier + human review pipeline
65	Comment System at Scale	Content & Media	todo	Threading, voting, spam, hot content

AI / ML Systems
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#	Topic	Category	Status	Notes
66	Recommendation System — Full Pipeline	AI/ML	todo	Candidate gen → ranking → serving
67	LLM-Powered Chatbot at Scale	AI/ML	todo	Streaming tokens, session, cost control
68	RAG System over Enterprise Documents	AI/ML	todo	Chunking, embeddings, retrieval, grounding
69	A/B Testing & Experimentation Platform	AI/ML	todo	Assignment, metrics, stat significance
70	Feature Store for ML	AI/ML	todo	Online vs offline, point-in-time correctness
71	ML Model Serving Infrastructure	AI/ML	todo	Shadow mode, canary, latency SLO
72	Vector Database & Semantic Search	AI/ML	todo	HNSW, ANN, embedding freshness

Data & Storage Systems
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#	Topic	Category	Status	Notes
73	Search Engine Internals (Elasticsearch)	Data & Storage	todo	Inverted index, relevance scoring
74	Time-Series Database	Data & Storage	todo	InfluxDB, downsampling, retention
75	Graph Database & Social Network Queries	Data & Storage	todo	Neo4j, shortest path, friend-of-friend
76	Data Warehouse & Lakehouse Architecture	Data & Storage	todo	Iceberg, Parquet, partitioning
77	Change Data Capture (CDC)	Data & Storage	todo	Debezium, Binlog tailing, event propagation
78	Consistent Hashing Deep Dive	Data & Storage	todo	Virtual nodes, hot spots, rebalancing
79	Bloom Filter & Probabilistic Data Structures	Data & Storage	todo	HyperLogLog, Count-Min Sketch
80	LRU / LFU Cache Implementation	Data & Storage	todo	LinkedHashMap, Caffeine, eviction

Reliability & Operations
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#	Topic	Category	Status	Notes
81	Distributed Tracing System	Reliability	todo	OpenTelemetry, sampling, tail-based
82	Circuit Breaker & Bulkhead Patterns	Reliability	todo	Resilience4j, half-open, fallback
83	Disaster Recovery — RTO / RPO Planning	Reliability	todo	Backup strategies, failover runbook
84	Chaos Engineering Framework	Reliability	todo	Steady state, blast radius, game days
85	Zero-Downtime Deployments & Schema Migrations	Reliability	todo	Blue-green, expand-contract, canary
86	Distributed Lock Service	Reliability	todo	Redlock, fencing tokens, ZooKeeper
87	Leader Election & Consensus (Raft / Paxos)	Reliability	todo	Split-brain, quorum, term numbers
88	Multi-Region Active-Active Design	Reliability	todo	Conflict resolution, CRDT, global LB

Security & Compliance
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#	Topic	Category	Status	Notes
89	Identity & Access Management (IAM)	Security	todo	RBAC vs ABAC, policy engine
90	OAuth2 & OpenID Connect Deep Dive	Security	todo	Token lifecycle, PKCE, refresh rotation
91	Zero-Trust Network Architecture	Security	todo	mTLS, BeyondCorp, SPIFFE/SPIRE
92	Audit Logging & Compliance Trail	Security	todo	Immutable log, SOC2, GDPR
93	GDPR Right-to-Erasure Implementation	Security	todo	Crypto-shredding, propagation
94	Data Masking & Tokenisation Service	Security	todo	PCI DSS, PII vault
95	Healthcare — Patient Record System (EHR)	Compliance	todo	HIPAA, consent management

Architecture Patterns
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#	Topic	Category	Status	Notes
96	Event Sourcing + CQRS	Architecture	todo	Append-only log, projection rebuild
97	Saga Pattern (Distributed Transactions)	Architecture	todo	Choreography vs orchestration
98	Strangler Fig & Anti-Corruption Layer	Architecture	todo	Monolith migration, domain boundary
99	Multi-Tenant SaaS Platform Architecture	Architecture	todo	Isolation models, noisy neighbour
100	Outbox Pattern + Transactional Messaging	Architecture	todo	At-least-once, idempotent consumers

Java Deep Dives
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#	Topic	Category	Status	Notes
101	Virtual Threads vs Reactive (Loom vs WebFlux)	Java Deep Dive	todo	Java 21, I/O bound, thread-per-request
102	JVM GC Tuning for Production	Java Deep Dive	todo	G1 vs ZGC vs Shenandoah, Generational ZGC
103	Spring Boot 3 + GraalVM Native Image	Java Deep Dive	todo	AOT, reflection hints, startup time
104	Structured Concurrency (Java 21)	Java Deep Dive	todo	StructuredTaskScope, cancellation
105	CompletableFuture Pitfalls in Production	Java Deep Dive	todo	Error propagation, thread pool starvation
106	Domain-Driven Design with Records & Sealed Classes	Java Deep Dive	todo	Value objects, aggregates, exhaustive switch
107	Database Connection Pool Tuning (HikariCP)	Java Deep Dive	todo	Pool sizing formula, leak detection
108	Reactive Streams & Backpressure (Project Reactor)	Java Deep Dive	todo	Flux, Mono, scheduler selection

Geospatial & Location
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#	Topic	Category	Status	Notes
109	Ride-Hailing Pricing Engine (Surge)	Geospatial	todo	Real-time demand model, elasticity
110	Location Tracking & Geo-Fencing Service	Geospatial	todo	Moving objects, polygon queries, alerts
111	Food Delivery Dispatch System	Geospatial	todo	Assignment optimisation, ETA, batching

High Performance
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#	Topic	Category	Status	Notes
112	High-Frequency Trading Infrastructure	High Performance	todo	Kernel bypass, co-location, FPGA
113	Video Conferencing (WebRTC Infrastructure)	High Performance	todo	SFU vs MCU, TURN/STUN, jitter buffer
114	IOT Device Management Platform	High Performance	todo	MQTT, device shadow, OTA updates
115	Service Mesh + Observability (Istio / Envoy)	High Performance	todo	mTLS, traffic policy, telemetry

Bonus: Platform & FinOps
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#	Topic	Category	Status	Notes
116	Internal Developer Platform (IDP)	Architecture	todo	Golden paths, self-service, paved road
117	Cost Optimisation Framework (FinOps)	Architecture	todo	Right-sizing, spot strategy, waste
118	gRPC vs REST vs GraphQL — Protocol Trade-offs	Architecture	todo	When to pick which, streaming, contracts
119	Event-Driven Architecture Deep Dive	Architecture	todo	Domain events, eventual consistency
120	Ad Click Aggregation & Attribution System	Scalability	todo	Lambda arch, exactly-once, privacy

Progress
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Total topics: 120
Published: 1 (1%)
In progress: 0
Todo: 119

Last updated: 2026-04-18

URL Shortener (bit.ly)

Sat, 18 Apr 2026 00:00:00 +0000

1. Hook
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Every time you click a bit.ly or t.co link, a distributed system silently resolves a 7-character code to a full URL and redirects you — in under 10 milliseconds — before your browser even renders the loading spinner. Behind that invisible handshake sits a deceptively rich design problem: how do you build a service that creates billions of short codes, never loses a mapping, and serves hundreds of thousands of reads per second with single-digit millisecond latency, all while preventing abuse, surviving data-centre failures, and staying profitable?

Microservices Patterns: Saga, CQRS, Event Sourcing, BFF, and More

Tue, 07 Apr 2026 20:00:00 +0530

Microservices patterns are the vocabulary of distributed systems design. Knowing when to apply each one — and when not to — separates an architect who reads pattern books from one who’s shipped production systems.

Engineering Leadership Trade-offs: Build vs Buy, Tech Debt, and Rewrite vs Refactor

Tue, 07 Apr 2026 19:00:00 +0530

EM interviews often end with “the harder framing” — questions about judgment, decision-making under pressure, and how you navigate disagreement. These don’t have right answers; they have reasoned answers that demonstrate how you think. Here’s a framework for the most common ones.

Data Pipeline and Analytics: OLTP vs OLAP, Batch vs Streaming, CDC

Tue, 07 Apr 2026 18:00:00 +0530

As systems grow, the gap between operational data (what your application uses to run) and analytical data (what your business uses to make decisions) becomes significant. Understanding how to design data pipelines that bridge this gap is an EM-level concern.

Testing Strategy: Test Pyramid, Contract Testing, and Coverage Pragmatics

Tue, 07 Apr 2026 17:00:00 +0530

Testing strategy is an EM-level concern because it directly affects delivery velocity, production reliability, and onboarding speed. Too little testing = production incidents. Too much ceremony = slow CI and frustrated engineers. The goal is the right tests in the right places.

Build, Deploy, and Release: Trunk-Based Dev, Deployment Strategies, Zero-Downtime DB Migrations

Tue, 07 Apr 2026 16:00:00 +0530

How you deploy code is as important as how you write it. The gap between writing a feature and it running in production reliably is where most engineering organizations lose velocity. This post covers the decisions that shape that gap.

Cloud and Infrastructure: AWS vs GCP vs Azure, Kubernetes vs Serverless

Tue, 07 Apr 2026 15:00:00 +0530

Cloud infrastructure decisions are often more political than technical. The right answer depends on where your team’s expertise is, what your customers require, and what you’re willing to operate. Here’s how to frame these decisions at the EM level.

Security and Authentication: JWT, OAuth2, and Secrets Management

Tue, 07 Apr 2026 14:00:00 +0530

Security architecture decisions have higher stakes than most — the cost of getting them wrong is a data breach, not a performance degradation. This post covers the trade-offs that come up in EM-level interviews: authentication approaches, identity protocols, and secrets management.

Observability: Logs, Metrics, Traces, and Alerting

Tue, 07 Apr 2026 13:00:00 +0530

Observability is the ability to understand what’s happening inside your system from the outside — from its outputs. The three pillars (logs, metrics, traces) are complementary tools, each answering different questions. Getting the combination right is what separates systems that you can reason about from systems that require tribal knowledge to debug.

Reliability and Resilience: Circuit Breakers, Retries, SLOs, and Failure Modes

Tue, 07 Apr 2026 12:00:00 +0530

Reliability isn’t about preventing failures — it’s about building systems that fail gracefully, recover quickly, and maintain user trust even when things go wrong. This post covers the patterns that keep systems running under degraded conditions.

Scaling Strategies: A Decision Framework

Tue, 07 Apr 2026 11:00:00 +0530

Scaling is not a synonym for “add more servers.” Each scaling lever has different costs, trade-offs, and appropriate circumstances. Reaching for the wrong one wastes money, adds complexity, or misses the actual bottleneck.

Consistency, Availability, and the CAP/PACELC Trade-off

Tue, 07 Apr 2026 10:00:00 +0530

Consistency and availability trade-offs show up in nearly every system design discussion. The theory (CAP, PACELC) is well-known; the practical application — knowing which choice to make for a specific use case — is what separates a design-literate engineer from one who just quotes theorems.

Microservices vs Monolith: Making the Right Architecture Call

Tue, 07 Apr 2026 09:00:00 +0530

The microservices vs monolith debate is one of the most over-indexed topics in software architecture — teams decompose too early, pay operational costs they’re not ready for, and spend months untangling the mess. The decision framework is simpler than the discourse suggests.

API Design: REST vs GraphQL vs gRPC

Tue, 07 Apr 2026 08:00:00 +0530

API design decisions have long tails — once you publish an API and clients integrate with it, changing it is expensive. The choice of protocol, versioning strategy, and backwards compatibility approach should be deliberate, not defaults.

Messaging and Event-Driven Architecture: Kafka vs RabbitMQ vs SQS

Tue, 07 Apr 2026 07:00:00 +0530

The choice between a message queue and an event streaming platform shapes your architecture more than almost any other infrastructure decision. Getting it wrong means rebuilding — not reconfiguring. Here’s how to think through it.

Caching Strategies: Placement, Patterns, and Pitfalls

Tue, 07 Apr 2026 06:00:00 +0530

Caching is the single highest-leverage performance tool available — and also one of the most common sources of production bugs. The decision isn’t just “should we cache?” — it’s where, how, and what the consistency implications are.

NoSQL Families: Choosing the Right Tool

Tue, 07 Apr 2026 05:00:00 +0530

NoSQL isn’t a single thing — it’s five different database families with fundamentally different data models, consistency guarantees, and use cases. Using the wrong family (or the wrong database within a family) is a common and costly mistake. Here’s how to think through each one.

SQL Flavors: Postgres vs MySQL vs SQL Server

Tue, 07 Apr 2026 04:00:00 +0530

SQL is SQL until it isn’t. When you’re making a database selection for a new service, the choice between PostgreSQL, MySQL, and SQL Server comes down to features, ecosystem, operational model, and political reality. Here’s how to reason through it.

SQL vs NoSQL: Making the Right Call

Tue, 07 Apr 2026 03:00:00 +0530

“Should we use SQL or NoSQL?” is one of the most common — and most misunderstood — architecture questions. Teams default to NoSQL because it sounds modern or scalable, or to SQL because it’s familiar. Neither is the right reason. The decision should come from your data’s shape, consistency requirements, and access patterns.

Spring Boot Evolution: 1.x to 3.4 — What Every EM Needs to Know

Tue, 07 Apr 2026 02:00:00 +0530

Spring Boot is the backbone of most Java microservice ecosystems. As an EM, you’re not expected to know every annotation — but you should be able to drive the architectural decisions: MVC vs WebFlux vs virtual threads, Boot 2 vs 3 migration, observability strategy, and testing approach. Here’s the full evolution with the trade-offs that matter.

JVM Garbage Collection: From Java 8 to 21

Tue, 07 Apr 2026 01:00:00 +0530

Garbage collection is one of those topics where “I let the JVM handle it” is a perfectly valid answer until it isn’t — and for EMs, that inflection point usually shows up as unexplained latency spikes in production, OOM kills in containers, or a team paralyzed by which GC flag to tweak. Here’s the full picture from Java 8 through 21.

Java 8 to 21: Language Features Every EM Should Know

Tue, 07 Apr 2026 00:00:00 +0530

Java has changed dramatically since Java 8. As an engineering manager, you don’t need to recite the JLS — but you do need to understand why these features exist, the trade-offs they carry, and how they affect the decisions your team makes every day. Here’s a curated tour.

Using Generics for Datastructures

Sun, 27 Oct 2024 17:16:11 +0530

In Java programming, generics provide a way to create reusable classes, methods, and interfaces with type parameters. They allow us to design components that can work with any data type, providing type safety and flexibility. In this blog post, we will explore the use of generics in creating a data structure from scratch, emphasizing object-oriented programming principles and step-by-step explanations.

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