Stage 1 · Lesson 9 of 10
CAP and PACELC
Sometimes the network breaks and splits the copies () of your data into groups that cannot talk. This is a . The says the system must then choose.
It can pick consistency: refuse or delay requests so nobody reads . Or it can pick availability: answer from whichever side you reach, accepting stale or conflicting data. CAP is only about partitions, which are rare but certain to happen.
adds the everyday case. Its name spells out the rule: if Partition, choose Availability or Consistency; Else, choose Latency or Consistency. Even with no partition, waiting for replicas to agree takes time.
A bank balance usually leans towards consistency. A like counter or a product catalogue can lean towards availability and low latency.
Common trapcalling a whole system "CP" or "AP": real systems make this choice per operation and often let you tune it.
A question to check yourself, an AI practice session and 2 cheat sheet entries come with the full roadmap. Unlock
Resources for this stage
Start with· follow one of these
- CoursePaidGrokking System Design Fundamentals
Design Gurus · Beginner · Subscription
Load balancing, API gateway, proxies, caching, CAP and Bloom filters: week 1 of the plan in order
- GitHub repoFreeThe System Design Primer
Donne Martin · Intermediate
Free: its scalability, latency, availability, CDN, load balancer and cache sections
Keep handy
Also worth your time
Why CAP only describes partitions, and what PACELC adds for normal operation