An append-only audit log is a log where no row is ever updated or deleted after insertion. New events are appended; nothing is mutated. Regulatory audit-trail requirements (EU AI Act Art. 12 automatic event logging, ISO/IEC 42001 A.8.2 testing and evaluation, NIST AI 600-1 MANAGE-3.2 incident response) effectively require this property even when they do not say so verbatim.
The implementation pattern varies. The application-level constraint is enforced through SQL grants where the writer role has INSERT-only privilege and no UPDATE or DELETE grant. A storage-layer tamper-evident mirror (a database-level audit channel that logs every write independently of the application) gives the auditor a second-source trail that an actor with database-administrator access cannot rewrite.
Retention is typically tenant-configurable. Regulated industries often keep the audit log for ten years to match the prudential or healthcare evidence retention requirement.