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ADR-002: PostgreSQL + Drizzle ORM

Status: Accepted Date: 2026-02-26

Context

A financial system requires a database that can enforce data integrity, support multi-step atomic transactions, and provide row-level locking for concurrent access.

Decision

Use PostgreSQL as the database and Drizzle as the ORM.

Reasoning

PostgreSQL:

  • SELECT FOR UPDATE enables pessimistic row-level locking — the correct solution for concurrent purchase requests
  • Full ACID transaction support for multi-step wallet operations
  • UNIQUE constraints enforced at the database level for idempotency keys
  • NUMERIC and integer types for exact monetary arithmetic

Drizzle over Prisma: Prisma abstracts SQL, hiding the locking queries that are central to this system's correctness. Drizzle is SQL-first — SELECT FOR UPDATE is explicit in the service code, making the concurrency strategy visible and intentional rather than buried in ORM internals.

Consequences

  • Locking queries are explicit — easier to review and reason about
  • Schema is defined in TypeScript with full type safety
  • Migrations managed via drizzle-kit