Optional ReadonlyfoldFolds a batch at once, equal to stepping each event; lets a reducer copy its state once per batch rather than once per event, which keeps a long fold linear. The state passed in is never modified.
ReadonlyidNames the reducer and its state shape; a stored checkpoint under a different id is never read back.
ReadonlyinitialOptional ReadonlyrebindPoints decoded refs at a database, since a stored ref keeps its URI but not what resolves it.
ReadonlyschemaReadonlystep
A reducer whose state can be stored, so a reader resumes from a saved checkpoint instead of refolding the feed.