Financial Audit Logs: What to Record and What Regulators Want

Building financial software means living under a microscope. Every transaction, every access attempt, every configuration change — regulators expect a complete, tamper-proof record. After years of PCI-DSS compliance work, here's what I've learned about audit logging that actually satisfies auditors.

The Immutable Foundation

At the core of any financial audit system is immutability. Once a log entry is written, it must never change. No updates, no deletes, no soft-deletes. Append-only is the only acceptable pattern. This sounds simple, but implementing it correctly across distributed systems requires careful thought about partition tolerance and consistency guarantees.

What Regulators Actually Look For

Regulators care about three things: who did what, when they did it, and whether the system detected anything unusual. They don't care about your fancy log aggregation pipeline — they want to see that you capture authentication events, authorization decisions, data modifications, and access to sensitive fields. Every write to a financial record needs a before-and-after snapshot. Every failed login needs a timestamp and source IP.

Designing for Scale

The challenge isn't capturing logs — it's capturing them without degrading transaction throughput. Financial systems often need to process thousands of transactions per second while recording every state change. The solution is async logging with bounded buffers and backpressure. If the logging pipeline falls behind, the system must degrade gracefully — slowing transactions rather than losing audit records.

In practice, I've found that a well-tuned PostgreSQL with append-only tables and strategic partitioning handles audit workloads far better than specialized solutions for most teams. Add a materialized view for query performance and you've got a system that satisfies both auditors and engineers.