MENTAL MODEL · M0485

PACELC Theorem

DecideHigh supportDecision Science

Included

Version 1.0.0 · Updated 2026-07-30

CORE DEFINITION

An extension of the CAP theorem. It states that if there is a network partition (Partition), the system must choose between availability (A) and consistency (C) (i.e., CAP); but even when there is no network partition (Else), the system must also make a trade-off between latency (Latency) and consistency (Consistency).

SCAFFOLDING EFFECT

psychology

Reduce cognitive load

Refinement of trade-offs. It reminds us that even in calm times, pursuing extreme consistency (everyone sees the same data) inevitably comes at the cost of speed. In life, if you want everyone to be in sync, decision-making will inevitably slow down.

anchor

Anchor fast decisions

CAP only forces a choice between A and C when a partition (P) occurs; PACELC points out that even without a partition (Else), the system still must trade off between latency (L) and consistency (C). That is, consistency is not 'free'; pursuing strong consistency in normal times comes at the cost of higher latency. It extends a one-time trade-off into a constant trade-off.

MINIMUM ACTION

In progress 0/4

Practice this model in one real situation:

Determine whether the system may experience network partitions (almost certainly).
When partitioned, choose A or C based on business needs.
Even without partitions, be clear: choose strong consistency (compromise on L) or low latency (weak consistency).
Based on business (e.g., payments need C, feeds can tolerate L), choose the appropriate storage.

Check to track your progress (stored locally)

Learning progress0%

account_treeGenealogyexpand_more

menu_bookReferencesexpand_more

Source support: Explicit

link
en.wikipedia.orghttps://en.wikipedia.org/wiki/PACELC_design_principleZH · Explicit
verified

RELATED MODELS