Part of Stem. This page defines the Snapshot blob, the second state representation beside the Change graph.

A Snapshot is one signed value, typed by a schema, that is the whole state of a resource at a point in its history.

Fields

The schema extends blob.

field	type	required	meaning
`type`	literal `Snapshot`	yes	Blob type tag.
`schema`	hm URL	yes	The schema the value conforms to, normally the `schema` of the resource's Kind.
`prev`	list of CID	no	The Snapshot or Change blobs this state builds on, sorted by CID. Omitted on the first snapshot.
`value`	any	yes	The complete state.

Rules

A Snapshot is reached only through a Node whose target is snapshot. The peer validates value against schema, strictly for the kinds Stem ships and advisory for user-defined kinds, and refuses a Snapshot whose schema is not the kind's schema or an extension of it. prev gives causal order: a Snapshot that lists another is newer than it. A Snapshot may list Changes in prev, which is how a document can be compacted into a snapshot, and a Change may list a Snapshot in deps, which is how editing continues from it. Retention follows the kind: latest lets a peer drop superseded snapshots once no Node, link or version pin reaches them. References inside the value (node references, ipfs:// URLs, hm:// URLs) become links according to the kind's link rules. A Snapshot carries no visibility: its audience is its node's.

Today (HM24)

Comment, Contact and Profile are each a separate blob type with a replace-the-whole-value edit model, a TSID or key identity and their own index code. Stem expresses all three as Snapshot-backed kinds and gains schemas, kinds, files and policies for free.

Example

A comment's snapshot:

{
  "type": "Snapshot",
  "signer": {"/": {"bytes": "7QEE...Commenter"}},
  "sig": {"/": {"bytes": "..."}},
  "ts": 1759910520000,
  "schema": "hm://z6MkiAKDcRSzQ4zPZfnJcS5HYx5MwgN6MU9foHihJGrhqNBj/stem/kinds/comment/value",
  "value": {
    "target": {"space": {"/": {"bytes": "7QEE...Starlight"}}, "node": "bafyreiujzdegxdncf32epf3dhodzdocis2jhtlgmxgedn73u55xtplpft7", "version": "bafyreib3nq5mkq2w7e4w3slkz2n7m7xq6x4hw4x2x3vl6tz2cmkfnq5a3e"},
    "body": [{"id": "b1", "type": "Paragraph", "text": "Should the roadmap mention groups?"}]
  }
}

Fields

Rules

Today (HM24)

Example

See also