Logs can explain what a service thought happened.

They do not prove what happened.

Klevar Docs needed an audit trail for rendered documents, invoice events, credit note applications, signatures, voids, and attachments. The usual answer is an events table. Insert a row whenever something happens. Add timestamps. Keep it forever. That is useful, but it is still just a table unless the table can detect tampering.

The hash chain is the difference.

Each entity gets its own ordered chain. A row stores the event payload, a SHA-256 hash of the canonical payload, the previous row hash, the chain index, and a link hash that binds those values together. If someone changes a payload, deletes a prior row, swaps entity rows, or reorders entries, verification fails.

The append path is transactional with the document change. That detail matters more than the hashing. If the document row commits and the chain row rolls back, the proof is incomplete. If the chain row commits and the document row rolls back, the proof references a thing that does not exist. insertChainEntry() is designed to run inside the caller's transaction.

The core logic is direct:

const allocRes = await tx.execute(
  sql`SELECT fn_allocate_chain_index(${entityId}::uuid)::text AS idx`,
);
const chainIndex = BigInt(allocRes.rows[0]!.idx);

let previousHash: string | null = null;
if (chainIndex > 1n) {
  const priorRes = await tx.execute(
    sql`SELECT payload_hash FROM document_hash_chain
         WHERE entity_id = ${entityId}::uuid
           AND chain_index = ${(chainIndex - 1n).toString()}::bigint`,
  );
  previousHash = priorRes.rows[0]!.payload_hash;
}

const chainLinkHash = computeChainLinkHash({
  content_hash: contentHash,
  previous_hash: previousHash,
  chain_index: chainIndex,
  entity_id: entityId,
});

There are two design choices hidden in that snippet.

The index comes from fn_allocate_chain_index(entity_id), not a PostgreSQL sequence. The same rollback problem that makes sequences wrong for legal document numbers also applies to chain indices. A verifier expects the chain to be contiguous. If index 19 is missing because a transaction rolled back after a sequence increment, the verifier cannot know whether that is harmless or tampering.

The link hash includes entity_id. That prevents a row from one entity being copied into another entity's chain without detection. Klevar has one group boundary, but the legal proof is per entity. FZE, LLC, and Ltd cannot share a chain just because the service is single-tenant.

The verifier is a walker, not a database query. It receives rows sorted by chain_index, checks ordering, checks the genesis row, checks each previous_hash, recomputes each link hash, and returns the first mismatch. It also reports intentionally broken indices. That last category is important because some retention or force-purge action may be documented rather than hidden. A broken chain can be honest if the break is recorded and visible.

What surprised me was the dependency on canonical JSON. Hashing JavaScript objects directly is a trap because key order and serialization details can drift. The service pins canonicalize@2.0.0 and runs an RFC 8785 boot assertion before Fastify binds a port. If canonicalization changes, the server refuses to start. That is not paranoia. It is the cost of using hashes as legal proof.

The hash chain also changed how I think about events. events_emitted is the integration outbox for Hub and Webhook Engine fanout. It is operational. document_hash_chain is proof. Those two surfaces overlap, but they are not the same thing. A notification can be retried, delayed, or dropped without changing the legal document. A chain append cannot be treated that way.

The biggest tradeoff is operational weight. A chain gives you another invariant to maintain, another verifier to run, another repair story to document, and another failure mode to alert on. The alternative is worse: a document archive that can produce files but cannot prove nobody altered the history.

For this system, the archive without the chain would be storage. The chain turns it into evidence.