How NestJS Manages External Bank Transaction Data at Ingestion Boundaries

In real fintech applications, a significant share of transaction data originates from external banks via Open Banking APIs, not from the system itself, making it inherently less trustworthy than internally generated records. Unlike self-generated transactions, externally sourced data can arrive late, duplicated, or with inconsistent metadata, requiring careful validation before being treated as fact. NestJS provides architectural structure to clearly separate external provider data from internal records, treating incoming bank data as unverified input until reconciled against an existing ledger. To prevent duplicate entries — common due to network retries or overlapping polling windows — developers are advised to key deduplication logic on the bank's own transaction ID rather than a system-generated one. Additionally, since external data syncs can fail mid-operation, tracking a per-provider sync checkpoint ensures subsequent attempts neither miss transactions nor re-record already ingested ones.