TigerBeetle is a financial transactions database designed to ensure correctness in processing double-entry bookkeeping with accounts and transfers. It utilizes a separation of data storage, relying on Postgres for master data (like account holder information) while managing transactional data independently. This structure allows for scalability and meets various security and compliance needs. However, managing consistency across these two databases poses challenges, especially since they don’t share a transaction boundary. The system must handle operations without the assurance of standard transactions, meaning it can temporarily become inconsistent. To maintain consistency, the article introduces the "Write Last, Read First" principle. It emphasizes designating a "System of Record" (TigerBeetle) and a "System of Reference" (Postgres). The system of record dictates the existence of accounts, while the reference system serves only as a supporting layer. Operations must be executed in a specific order: write to the reference before the record. This order is crucial to ensure safety properties like consistency and traceability. If the wrong system is designated, it could lead to scenarios where accounts exist without proper ownership records, violating traceability. The article also highlights the role of an Application Programming Interface (API) in orchestrating operations and ensuring consistency through a framework called Distributed Async Await. This framework provides checkpoints to manage disruptions, allowing operations to resume without repeating non-idempotent actions. Each subsystem has to implement idempotent functions for account creation, ensuring that repeated operations don’t cause inconsistencies. The article underscores the importance of understanding system interactions and correctly implementing operations to avoid critical failures in financial transactions.