Dual Writes Diverge — and the Ordered-Log Fix Diagram

Segment 1 · The Dual-Write Race

Two clients concurrently write the same record: A sets x = 1, B sets x = 2
Each client writes to both stores directly — the application updates every representation itself
Unlucky timing: the database applies A then B (x = 2), while the search index applies B then A (x = 1)
Both stores accepted both writes — no error occurred anywhere, yet they now disagree

Why the Divergence Is Permanent

Neither the database nor the search index is in charge of deciding the order of writes
The two stores make contradictory decisions and become permanently inconsistent (Figure 12-4)
Nothing afterwards can say which order was right: one write simply overwrote the other
Keeping copies in several storage systems demands clarity about inputs and outputs — where data is written first, and which representations are derived from which sources

Segment 2 · One Ordering Authority

Funnel all user input through a single system that decides on a total order for every write
CDC captures the changes and applies them to the search index in the same total order
Derived updates become deterministic and idempotent, which makes recovery from faults easy
Whether you use CDC or an event sourcing log matters less than the principle of deciding on a total order