A Banking API Is Not Just CRUD: What Building a Money-Movement Ledger Taught Me

Short summary

Financial systems cannot use mutable balance storage due to race conditions in concurrent transactions. The solution is ledger-based event sourcing: every transaction is an immutable fact in an ordered log, and balances are computed from the log at read time. This requires infrastructure that guarantees event ordering to prevent data loss and enable replay-based auditing.

• •Mutable balance columns cause race conditions when concurrent requests read the same state and both proceed, corrupting the account
• •Ledger architecture stores immutable transaction events in sequence and derives balance as a computed projection at read time
• •Event ordering must be guaranteed at the infrastructure level, not application code, to handle financial correctness under load

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