1,173 Free System Design Lessons to Crack SDE Interviews in 2027

Published September 2026 · avivashishta.com

You commented system, so here are all 1,173 lessons, free, with a link for each. They come from three GitHub repos, and I checked every link on 28 September 2026 and dropped the dead ones. A "lesson" here is one page, video, solved problem, paper or engineering write-up you can finish in one sitting. No paid books or courses.

The short version: 160 concept lessons from the System Design Primer, 13 solved problems with full write-ups, 137 more concepts, interview problems and papers from Awesome System Design Resources, and 863 real engineering write-ups on how companies like Netflix, Uber and Discord actually scale. Follow the 4-week plan, then use the rest as a reference.

The three repos

donnemartin/system-design-primer ~372K ★

github.com/donnemartin/system-design-primer

One long, well-organised guide with trade-offs for every topic, plus solved interview problems with diagrams and object-oriented design solutions in notebooks.

ashishps1/awesome-system-design-resources ~41.8K ★

github.com/ashishps1/awesome-system-design-resources

Short concept lessons, interview problems from easy to hard (many are YouTube walkthroughs), must-read engineering articles and the classic distributed systems papers.

binhnguyennus/awesome-scalability ~74K ★

github.com/binhnguyennus/awesome-scalability

The biggest part of this list: real write-ups from company engineering blogs on scaling, availability, stability, performance and data platforms. This is what makes your interview answers sound like you've done it before.

The 4-week plan I'd follow

Week 1, the building blocks. In Part 1: how to approach a system design question, back-of-the-envelope calculations, performance vs scalability, availability vs consistency, DNS, CDN, load balancers and reverse proxies.
Week 2, data and communication. Still in Part 1: replication, sharding, SQL vs NoSQL, caching, async queues, HTTP, TCP/UDP, REST vs RPC. Then the fundamentals sections at the top of Part 3 to fill gaps.
Week 3, problems. Work through the solved problems in Part 2, then the Easy and Medium problems in Part 3. Draw each one yourself before you look at the answer.
Week 4, real systems. Do the Hard problems in Part 3, then read 10 write-ups from Part 4 for companies you'd actually want to work at. Use them as examples in your answers.

Part 1: Core concepts (160)

Source: github.com/donnemartin/system-design-primer

Anki flashcards (3)

How to approach a system design interview question (5)

Back-of-the-envelope calculations (1)

Use back of the envelope calculations

System design topics: start here (4)

Performance vs scalability (2)

Latency vs throughput (1)

Understanding latency vs throughput

Availability vs consistency (4)

Consistency patterns (1)

Transactions across data centers

Domain name system (8)

Weighted round robin
Latency-based
Geolocation-based
governments, ISPs, and large companies
DDoS attack: , preventing users from accessing websites such as Twitter without knowing Twitter's IP address(es).
DNS architecture
Wikipedia
DNS articles

Content delivery network (3)

Load balancer (8)

Reverse proxy (web server) (3)

Application layer (5)

Master-master replication (1)

Multi-master replication

Federation (2)

Use good indices (1)

B-tree: that keeps data sorted and allows searches, sequential access, insertions, and deletions in logarithmic time.

Tune the query cache (1)

query cache: could lead to performance issues.

Key-value store (4)

Document store (4)

Wide column store (4)

Graph database (3)

NoSQL (4)

SQL or NoSQL (1)

SQL vs NoSQL differences

Cache (5)

Asynchronism (4)

Hypertext transfer protocol (HTTP) (3)

Transmission control protocol (TCP) (2)

checksum fields: for each packet
Acknowledgement: packets and automatic retransmission

User datagram protocol (UDP) (5)

Remote procedure call (RPC) (1)

RPC calls are properly cached: on caching servers such as Squid.

Representational state transfer (REST) (1)

HATEOAS: (HTML interface for HTTP) - your web service should be fully accessible in a browser.

RPC and REST calls comparison (6)

Security (5)

Powers of two table (1)

Powers of two

Latency numbers every programmer should know (4)

Company engineering blogs (41)

Part 2: Solved interview problems (13)

Source: github.com/donnemartin/system-design-primer

System design interview questions with solutions (7)

Object-oriented design interview questions with solutions (6)

Part 3: Concepts, interview problems and papers (137)

Source: github.com/ashishps1/awesome-system-design-resources

Core Concepts (9)

Networking Fundamentals (8)

API Fundamentals (8)

Database Fundamentals (9)

Caching Fundamentals (5)

Asynchronous Communication (3)

Distributed System and Microservices (8)

Architectural Patterns (5)

System Design Tradeoffs (12)

Easy (10)

Medium (22)

Hard (12)

Courses (2)

Books (1)

Designing Data-Intensive Applications

YouTube Channels (7)

Must-Read Engineering Articles (6)

Must-Read Distributed Systems Papers (10)

Part 4: How real companies scale (863)

Source: github.com/binhnguyennus/awesome-scalability

Principle (56)

Scalability (411)

Availability (70)

Stability (19)

Performance (77)

Intelligence (134)

Architecture (46)

Interview (18)

Talk (32)

How I'd use this

Don't read all 1,173 in order. Follow the 4-week plan, then use the rest as a reference when a topic comes up.
For every problem, draw it yourself first (Excalidraw is free), then compare with the solution.
Say your answer out loud with a 45-minute timer. Interviews test how you talk through trade-offs, not what you memorised.
Use Part 4 for stories. "Discord moved from Cassandra to ScyllaDB because..." is the kind of line that makes an interviewer lean in.
Build one piece for real: a URL shortener with a cache and a rate limiter, deployed with a live link. That's a project and interview prep in one.

Want to know what else interviews test now? Read what 2027 interviews will actually ask you.

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