Glossary · Phase 11 · Professional practice

Back-of-the-envelope estimate

Also written: back-of-the-envelope, back-of-the-envelope math

Rough math, such as requests per second or storage per year, done before building to see which problems are real.

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Chains multiply, so every dependency costs you. Copies fail only if they all fail at once.

System design fundamentals · Requirements, rough numbers, the standard parts and where systems break. About 40 minutes.

CAP theorem: When the network between copies breaks, a system must choose: stay consistent and refuse some requests, or stay available and risk stale answers.
consistent hashing: Placing servers and keys on a ring so that adding or removing a server moves only a small share of keys.
eventual consistency: Copies of data may briefly disagree after a write, but come to agree soon after.
non-functional requirement: How well a system must work rather than what it does: latency, scale, availability, consistency, cost and security.
read replica: A copy of a database that serves reads, slightly behind the primary that takes the writes.
sharding: Splitting data across several databases by a key, such as user id, so each holds only part of it.
single point of failure: A part the whole system needs that has no backup, so its failure takes everything down.

Teach a small one to write, right in your browser. It takes about 20 seconds and needs no account.