The Consistency Ladder: Eventual to Strong

Subtitle: Five levels of read guarantee, weakest to strongest.

• Panel 1 - THE LADDER (full-width schematic: five stacked rungs, a vertical arrow up the side labelled STRONGER GUARANTEE, HIGHER LATENCY):
• EVENTUAL - replicas agree, eventually
• READ-YOUR-WRITES - you see your own write
• MONOTONIC READS - time never moves backwards
• CAUSAL - cause is read before effect
• STRONG (LINEARIZABLE) - one global order

Panel 2 - WHAT BREAKS WITHOUT IT:

Eventual: saved profile still shows old name No read-your-writes: your comment vanishes No monotonic reads: a reply appears, then gone No causal: the answer loads before the question Strong: nothing stale, but slower and fragile

Panel 3 - THE COST AXIS:

Up the ladder = more coordination More coordination = more latency Strong reads need a leader or a quorum PACELC: else-latency vs else-consistency Availability drops as freshness rises

Panel 4 - USE IT WHEN:

Eventual: likes, view counts, feeds Read-your-writes: profile edits, drafts Monotonic reads: timelines, chat history Causal: threads and comment trees Strong: balances, inventory, locks

Warning note - COMMON MISTAKE:

Choosing one level for the whole system. Consistency is a per-read choice, not a per-database setting. Session guarantees usually beat global strong.