Audit ledger (immutable audit log)

An audit ledger is an append-only, tamper-evident record of who did what to which data and when, kept so that actions can be reconstructed and proven later.

Part of the Learn hub and the customer data glossary.

What it means in practice

Ordinary application logs are mutable, rotated and scattered across tools. An audit ledger is built to serve as evidence. Look for append-only writes; tamper evidence through write-once storage, hash chaining or signed entries; synchronised time; coverage of reads, writes, administrator actions, consent changes, exports and model decisions; and retention that matches your strictest rule. Tamper-evident is not tamper-proof: the goal is that any change is detectable.

Regulation points the same way. HIPAA requires mechanisms that "record and examine activity in information systems that contain or use electronic protected health information" (45 CFR 164.312(b)) and policies to protect it from improper alteration (164.312(c)(1)) 1. GDPR makes the controller responsible for demonstrating compliance (Article 5(2)) 2. For high-risk AI systems the EU AI Act requires technical capability for "automatic recording of events (logs) over the lifetime of the system" (Article 12(1)) 3.

A ledger holds personal data too, so apply data minimisation: record who, what, when, which record and which legal basis, using references or hashes rather than copies of payloads, so that erasure and audit do not collide.

Example (health insurer): internal audit asks who exported a segment of members last quarter and which consent state applied. The ledger shows the user, the query version, the destination, the field list and the consent snapshot, without storing the members' details again.

Related terms

• Consent state: consent changes are ledger events.
• Explainability: decision records feed it.
• Model lineage: links decisions to model versions.
• Event replay: reconstructs state from history.
• High-risk AI system: must keep logs.

Where it matters

• Explainable AI for customer decisions: what auditors ask for.
• On-premise, private cloud and self-hosted customer data platforms: the complete guide: audit design options.

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