A subledger (subsidiary ledger) is a detailed database that tracks transaction-level activity for a specific subset of accounts—such as user wallets, loans, or card issuances—before summarizing them for the General Ledger (GL). In high-volume fintechs, the subledger is the operational engine, while the GL is the financial summary.
The Granularity Problem
General Ledgers (like NetSuite or QuickBooks) choke on millions of daily transactions. They are designed for reporting, not high-throughput writes. A subledger solves this by handling the high-velocity data and only posting a consolidated "Journal Entry" to the GL at the end of the day (e.g., Debit Cash $1M / Credit User Deposits $1M).
Source of Truth vs. System of Record
For customer support and product logic, the subledger is the source of truth. If a user asks "Why is my balance $50?", the answer lies in the granular subledger history. The GL, conversely, is the System of Record for tax and compliance, caring only about the aggregate financial position.
