Glossary · Phase 11 · Professional practice
Read replica
A copy of a database that serves reads, slightly behind the primary that takes the writes.
Try it now
Chains multiply, so every dependency costs you. Copies fail only if they all fail at once.
Learn it properly
System design fundamentals · Requirements, rough numbers, the standard parts and where systems break. About 40 minutes.
Other terms from the same lesson
- back-of-the-envelope estimate: Rough math, such as requests per second or storage per year, done before building to see which problems are real.
- 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.
- 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.
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