Append-only log
Data structure where entries are only-added, never modified or deleted. Tamper-evident: any modification is detectable via a hash chain. Foundation for audit logs, blockchain, certificate transparency, Sigstore Rekor.
Implementation patterns: Merkle tree (efficient inclusion proofs), hash chain (each entry references the previous), blockchain (distributed append-only). Use cases: audit logs where compliance requires immutability (PCI DSS, HIPAA), supply-chain transparency (in-toto, Sigstore), Certificate Transparency (Google logs all-TLS certs). Modern tools: Trillian (Google open-source append-only-log engine, used for CT + Sigstore Rekor), Project Nessie for data versioning. Anti-pattern: a "tamper-resistant" log that can-be-rewritten by an admin = not append-only.