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Random Notes

System Design​

Unreliable clocks​

  • Clocks in PCs are unreliable - up to 7 seconds per full day.
  • PCs use Network Time Protocol - protocol for synchronising time of local machine with the time server.
  • There are skip seconds - minute can have 59 seconds, so scheduling something on n second of a minute can lead to bugs.
  • Instead, use just time deltas.
  • For even better reliability use vector event something...

Caching​

Data Storage Systems​

SQL vs NoSQL vs Other​

  • Feels like another iteration over old dispute between document-based IBM db and networked DBs
  • Document oriented DB - good when data does not have too many links
  • Relational DB - good when you need better control over data links
  • Graph DBs - good when links are starting to become as or even more important than just data.
  • There is no such thing as "no-schema" DBs. Less structured DBs such as Document DB just have "schema-on-a-read".

Postgres​

MySQL​