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Appendix — Back-of-the-Envelope Estimates & References

Useful when asked to estimate by hand (e.g., "how long to generate 100 image thumbnails?" or "how much memory does this data structure use?").

Powers of Two Table

Power Exact Value Approx Value Bytes
7 128 — —
8 256 — —
10 1,024 1 thousand 1 KB
16 65,536 — 64 KB
20 1,048,576 1 million 1 MB
30 1,073,741,824 1 billion 1 GB
32 4,294,967,296 — 4 GB
40 1,099,511,627,776 1 trillion 1 TB

Latency Numbers Every Programmer Should Know

L1 cache reference                           0.5 ns
Branch mispredict                            5   ns
L2 cache reference                           7   ns           14x L1 cache
Mutex lock/unlock                           25   ns
Main memory reference                      100   ns           20x L2, 200x L1
Compress 1K bytes with Zippy            10,000   ns  = 10 us
Send 1 KB over 1 Gbps network          10,000   ns  = 10 us
Read 4 KB randomly from SSD*           150,000   ns  = 150 us
Read 1 MB sequentially from memory     250,000   ns  = 250 us
Round trip within same datacenter      500,000   ns  = 500 us
Read 1 MB sequentially from SSD*     1,000,000   ns  = 1 ms
HDD seek                            10,000,000   ns  = 10 ms
Read 1 MB sequentially from 1 Gbps   10,000,000   ns  = 10 ms
Read 1 MB sequentially from HDD      30,000,000   ns  = 30 ms
Send packet CA -> Netherlands -> CA 150,000,000   ns  = 150 ms

Notes: 1 ns = 10^-9 s, 1 us = 10^-6 s = 1,000 ns, 1 ms = 10^-3 s = 1,000 us.

Handy derived metrics

  • Read sequentially from HDD at 30 MB/s
  • Read sequentially from 1 Gbps Ethernet at 100 MB/s
  • Read sequentially from SSD at 1 GB/s
  • Read sequentially from main memory at 4 GB/s
  • 6–7 world-wide round trips per second
  • 2,000 round trips per second within a data center

Mental model (round numbers)

  • L1 cache ≈ 1 ns; main memory ≈ 100 ns (100x); SSD random read ≈ 100 us (1000x memory); HDD seek ≈ 10 ms; cross-continent packet ≈ 150 ms.

Additional System Design Interview Questions

Practice these with resources you find online:

  • Design a file sync service (Dropbox)
  • Design a search engine (Google)
  • Design a scalable web crawler
  • Design Google Docs
  • Design a key-value store (Redis)
  • Design a cache system (Memcached)
  • Design a recommendation system (Amazon)
  • Design a URL shortener (Bitly)
  • Design a chat app (WhatsApp)
  • Design a photo-sharing system (Instagram)
  • Design a news feed / timeline (Facebook)
  • Design a graph search (Facebook)
  • Design a CDN (Cloudflare)
  • Design a trending-topics system (Twitter)
  • Design a random ID generator (Snowflake)
  • Return the top-k requests during a time interval
  • Design multi-data-center data serving
  • Design an online multiplayer card game
  • Design a garbage collection system
  • Design an API rate limiter
  • Design a stock exchange (NASDAQ/Binance)

Real-World Architectures (study list)

Identify shared principles and patterns rather than memorizing details:

  • Data processing: MapReduce (Google), Spark, Storm

  • Data stores: Bigtable, HBase, Cassandra, DynamoDB, MongoDB, Spanner, Memcached, Redis

  • File systems: Google File System (GFS), Hadoop File System (HDFS)

  • Misc: Chubby (lock service), Dapper (tracing), Kafka (pub/sub), ZooKeeper (coordination)

Company Engineering Blogs & Architectures

Great sources for real-world patterns: Amazon, Dropbox, Google, Instagram, Facebook, Flickr, Netflix, Twitter, and others via High Scalability.

Question → Topic Map

Each common interview question is walked through as a collapsible on its topic page. Use this to find them.

Data, caching, and databases
Question Topic page
Design a URL shortener Full walkthrough, 11 RDBMS, 16 Security
Design a key-value store (Redis) 12 NoSQL
Design a cache (Memcached) 13 Cache
Design a search-engine query cache 13 Cache
Design a social network graph 12 NoSQL
Design a recommendation system 12 NoSQL
Design Amazon's sales ranking 11 RDBMS
Design a news feed / timeline 11 RDBMS, 13 Cache
Networking, routing, and delivery
Question Topic page
Design a CDN (Cloudflare) 06 DNS, 07 CDN
Design Instagram (media) 07 CDN
Design a globally available service 06 DNS
Design an API gateway 09 Reverse Proxy
Design an API rate limiter 02 Latency vs Throughput
Application, communication, and async
Question Topic page
Design a chat app (WhatsApp) 10 App Layer, 15 Communication
Design Google Docs 04 Consistency, 15 Communication
Design a ride-hailing service (Uber) 10 App Layer
Design a web crawler 14 Asynchronism
Design a notification system 14 Asynchronism
Design Mint.com 14 Asynchronism
Design a multiplayer game 15 Communication
Scale, availability, and consistency
Question Topic page
Design a system that scales to millions of users 01 Performance vs Scalability
Design a multi-data-center system 01 Performance vs Scalability
Design a globally distributed database 03 CAP
Design a 99.99% available system 05 Availability Patterns
Design a key-value store (consistency) 03 CAP, 04 Consistency

Further reading