Mirror a source you cannot query

A real database is queried where it is. Some sources cannot be queried at all — an HTTP API, a partner's catalog that only answers ?page=&since=. Mirror<Shape> keeps a local copy of their rows, which you can then query like any other. A subclass writes one thing, how to pull:

// services/PartnerCatalog.ts
export default class PartnerCatalog extends Mirror<BookCard> {
  async *pull(since?: Date) {
    for (let page = 0; page !== null; ) {
      const body = await fetch(`${api}?page=${page}&since=${since?.toISOString() ?? ''}`);
      const { items, next } = await body.json();
      yield items.map(toCard);
      page = next;
    }
  }
}

It is a Repository<BookCard> that fills itself: the copy reads like any rows, this.list(), and a handler extending it serves the copy and refresh. Everything around the pull is the same for every mirror and lives in the base: refresh(since?) pulls from the mark it is given and writes each page in one upsertAll. The mark — the date of the last pass — is the caller's to keep, and to move only once a pass has returned: a pass that failed halfway must not skip what the source changed in the meantime.

Each page is checked like every other write, strictly: otherwise a partner renaming label would write nulls without a word. A refused page stops the pass; what earlier pages wrote stays, because an upsert is idempotent and the next run resumes from the mark.

It does not schedule itself. A boot hook, a cron, an operation behind a facade are all legitimate and none of them is this class's business.