13.12 Backward links — where each idea came from

Idea here	Established in
Systems of record vs derived data	Ch 1 §1.8
Evolvability; minimizing irreversibility	Ch 2 §5.3
Event sourcing and CQRS	Ch 3 §3
Indexes as derived structures; the write/read trade-off	Ch 4
Rolling upgrades; schema evolution	Ch 5
The dual-write race; multi-leader conflicts; read-your-writes	Ch 6, Ch 12 §3.1
Sharding and secondary indexes (term- vs document-partitioned)	Ch 7 §5
Weak isolation levels; write skew; 2PC and XA's failures	Ch 8
System models; what we assume can and cannot fail; fencing	Ch 9
Total order broadcast = consensus; state machine replication; uniqueness needs consensus	Ch 10
Immutable inputs; reprocessing; batch fault tolerance	Ch 11
CDC, log compaction, exactly-once, time-dependence of joins	Ch 12
Ethics, regulation, and the limits of deletion	Ch 14