What is the "revpos" value used for in CouchDB attachments?

Question

Here is an example:

   "_attachments": {
       "002-imgsq.jpg": {
           "stub": true,
           "content_type": "image/jpeg",
           "length": 80074,
           "revpos": 3
       },
       "016-imgsq.jpg": {
           "stub": true,
           "content_type": "image/jpeg",
           "length": 100095,
           "revpos": 4
       },
       "MASTER_img-square.jpg": {
           "stub": true,
           "content_type": "image/jpeg",
           "length": 70611,
           "revpos": 2
       }
   }

Answer 1

The attachment revpos is the doc's rev prefix (a sequential number) from when the attachment was added or updated. I believe it's only really used during replication to avoid copying attachments that have not changed since the last replication.