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CS Fundamentals

The concept notes an interviewer probes - hashing, concurrency, OS, networking, DB internals, security, OOP, memory, estimation. This pillar answers the "explain X in 2 minutes" and "why X vs Y" questions that come up outside of coding and system-design rounds (and inside them, as follow-ups).

How to use this

  • Each file is interview-length: intuition, key trade-offs, and "when would you use X vs Y" - not a textbook. If you want the 2-minute spoken answer, read the "Explain in 2 min" box at the top of each concept.
  • Topics are tagged [core] (must-know for entry-to-mid level) or [advanced] (rarely asked, nice to have for senior roles).
  • Concepts with a canonical deep-dive elsewhere (caching, sharding, CAP) are linked, not re-explained - go there for depth.
  • Pair each concept with its Q&A file (see map below) for rapid-fire drilling.

Concept files

# File Covers Depth
A1 Operating Systems process vs thread, context switch, scheduling, virtual memory/paging, deadlock, syscalls core
A2 Concurrency & Parallelism concurrency vs parallelism, threads vs async, locks, race conditions, deadlock, thread pools, Amdahl, GIL core
A3 Networking OSI/TCP-IP, TCP vs UDP, HTTP/1.1/2/3, TLS handshake, DNS, "URL → page", WebSockets, CDN core
A4 Database Internals ACID, isolation levels, indexing, normalization, SQL vs NoSQL, replication, partitioning, CAP/PACELC, MVCC core
A5 Hashing hash functions, collision handling, load factor/resizing, consistent hashing, bloom filters, crypto vs table hash, HMAC core
A6 Security Fundamentals authN vs authZ, sessions/JWT/OAuth2, password hashing, TLS, XSS/CSRF/SQLi/SSRF, rate limiting core
A7 OOP & Language Concepts 4 OOP pillars, SOLID, composition vs inheritance, interface vs abstract, immutability, generics core
A8 Memory & Runtime stack vs heap, garbage collection, memory leaks, reference counting, Python GIL/generators/decorators advanced
A9 Latency Numbers & Estimation numbers every engineer should know, back-of-envelope drills, QPS/storage/bandwidth math core
A10 Distributed Systems CAP/PACELC, replication, consensus, sagas advanced
A11 Bloom Filters & Probabilistic Structures bloom/counting bloom, count-min sketch, HyperLogLog - approximate answers, tiny memory advanced
A12 Serialization & Schemas JSON vs Protobuf vs Avro, schema evolution/compat, schema registry core
A13 Idempotency & Delivery Semantics idempotency keys, at-least/at-most/exactly-once, dedup, retries core
A14 Time & Ordering wall-clock skew/NTP, happens-before, Lamport clocks, vector clocks advanced

Concept → Q&A map

The Q&A Bank holds short, defensible interview answers. Each concept note maps to a Q&A file for drilling:

Concept Q&A file
Operating systems Operating Systems Q&A
Concurrency & parallelism Concurrency Q&A
Networking Networking Q&A
Databases Databases Q&A
Hashing Hashing Q&A
Caching Caching Q&A
Security Security Q&A
OOP & design OOP & Design Q&A
System design concepts System Design Concepts Q&A
Python language Python Language Q&A