PACELC Theorem
Updated 2026-08-02
INTRODUCTION
English translation pending.
CORE DEFINITION
The PACELC theorem was formulated by Daniel Abadi in a 2010 blog post and a 2012 paper in IEEE Computer, extending the CAP theorem. CAP covers only the partition case; PACELC adds the else branch. If a Partition occurs, a system chooses between Availability and Consistency; Else, when the network is healthy, it chooses between Latency and Consistency. Abadi's point is that the second trade-off operates almost all of the time, so a design described only by CAP says nothing about its ordinary behavior. Systems are classified accordingly, as PA/EL or PC/EC.
SCAFFOLDING EFFECT
Reduce cognitive load
- Classify systems: label a database by both branches, not just its CAP posture. - Budget latency: ask what response time you will pay for stronger consistency. - Revisit defaults: remember the everyday trade-off dominates, since partitions are rare in practice and latency is always present.
Anchor fast decisions
Partitions are rare, so the everyday constraint is response time. Strong consistency forces a node to wait for acknowledgement from replicas before answering, and that coordination cost appears directly as latency. Relaxing consistency lets a node answer from local state immediately, at the price of possibly stale reads. Because the two regimes pose different questions, availability versus consistency under failure and speed versus accuracy in normal operation, they must be reasoned about separately.
MINIMUM ACTION
In progress 0/1Practice this model in one real situation:
account_treeGenealogyexpand_more
menu_bookReferencesexpand_more
Source support: Explicit
- en.wikipedia.orghttps://en.wikipedia.org/wiki/PACELC_design_principleverified
PRIVATE NOTES · Only visible to you
SAVED Q&A
ENTRY Q&A · Private saving available
Ask with a clear boundary
thinkingmodels answers from published entry context only.
Your question is sent to thinkingmodels. The answer uses public entry context only.
RELATED MODELS