Expert-level PostgreSQL: index design and query plans, transaction isolation levels and locking, deadlock diagnosis, large-table partitioning, and writing and tuning complex SQL without an ORM; solid Redis knowledge including data structures and common cache failure modes (penetration, breakdown, avalanche, consistency)
Core financial systems experience: have personally led the data model, transaction design, and reconciliation design of a ledger, wallet, payment, or billing system
Deep expertise in idempotency design, concurrency control, and consistency strategies, with trade-offs justified from production experience rather than theory
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