A Change is the unit of editing in Hypermedia: one signed step in a document's history. Each Change says which earlier Changes it builds on and lists the operations that turn that state into the new one. Replaying a document's Changes in dependency order, on any machine, produces the same document. Every Change is signed, so the author of every step is known.

This page defines the change blob type, a Hypermedia network blob that extends the signed blob envelope. Its formal schema is attached as the schemaDefinition in this document's metadata, so the app can show it and sign values of this type.

genesis names the document's first Change, which is the document's identity. deps are the heads the author saw, sorted by CID. depth is one more than the deepest dependency. body holds the operations. The genesis Change of an ordinary document is its first content Change, and it carries none of genesis, deps or depth. The daemon rejects a Change that has some but not all of the three. At replay it requires that every dependency was applied first with a smaller timestamp and depth. The home document of a space is the one exception to "the first content Change is the genesis": its genesis is an empty Change with ts: 0, deterministic for the account key.

A Change alone does not change what readers see. A Ref that points at the head Changes asserts the document's current state at an address. A Change that no Ref reaches is a proposal, and that is how branches and open editing work. Documents has the rules for ordering concurrent Changes, producing minimal Changes from an edit, and merging heads.

See also

Documents: the change graph, op ids, the CRDT rules, versions and merging.

Signed Blobs: the envelope and CIDs.

change/body and change/op: the operations.

Seed API: ListChanges lists a document's Changes.

ref: the blob that points an address at a Change's heads.