Maintaining consistency in a system comprised of separate databases can be challenging, particularly in the absence of transactions. The article discusses the importance of defining a system of record versus a system of reference and emphasizes the Write Last, Read First principle to ensure safety properties like consistency and traceability in financial transactions.

• Write to the system of record last, but read from it first — this ordering prevents referencing data that hasn't actually been durably committed.
• If a downstream write (e.g., to a search index or cache) fails after the system of record succeeds, that's recoverable via retries; failing in the other order risks phantom reads of uncommitted state.
• Treating one database as the authoritative system of record (vs. other systems of reference) gives you a clear conflict-resolution and recovery strategy when multiple stores disagree.