What data model does CouchDB use and how is a document structured?

CouchDB uses a document data model: all data is stored as discrete JSON objects grouped into databases. There is no enforced schema — documents in the same database can have entirely different fields.

Every CouchDB document has two mandatory system fields:

• _id — the unique primary key. If omitted, CouchDB generates a UUID. Typed prefix conventions like "order:2024-001" co-locate related documents in the B-tree.
• _rev — the current revision token in the format {generation}-{md5hash}. You must supply the current _rev on every update or delete.

{
  "_id": "order:2024-00188",
  "_rev": "2-7d3a9f012b4e8c56ab1d2ef3",
  "type": "order",
  "customer_id": "user:42",
  "items": [
    { "sku": "WIDGET-01", "qty": 3, "price": 9.99 },
    { "sku": "GADGET-07", "qty": 1, "price": 49.00 }
  ],
  "total": 78.97,
  "status": "pending",
  "created_at": "2024-03-15T08:22:00Z",
  "_attachments": {
    "invoice.pdf": {
      "content_type": "application/pdf",
      "length": 48312,
      "stub": true
    }
  }
}

Values can be any valid JSON type: strings, numbers, booleans, arrays, or nested objects. Binary data is stored as attachments under the reserved _attachments key. Special system documents — design documents (_design/) and local documents (_local/) — live in the same database namespace but are handled differently by the replication engine.