changes feed has negative pending value

[robsonvn]

Hi folks,

I'm running a test in which two entries are updated in the following order: test1, test2.

when I query http://127.0.0.1:5984/doctrine_test_database/_changes

Sometimes I get this order:

{
  "results": [
    {
      "seq": "1-g1AAAAF1eJzLYWBg4MhgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTIkyf___z8rgzmRMRcowG5pbmloYWKKTQMeY5IUgGSSPcikRAZ86hxA6uIJq0sAqasnqC6PBUgyNAApoNL5xKhdAFG7nxi1ByBq7xOj9gFELci9WQBea3jf",
      "id": "test2",
      "changes": [
        {
          "rev": "1-c86e975fffb4a635eed6d1dfc92afded"
        }
      ]
    },
    {
      "seq": "2-g1AAAAHleJzLYWBg4MhgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTIkyf___z8rgzmRMRcowG5pbmloYWKKTQMeY5IUgGSSPdQkBrBJaQYWJkmWFikMnKV5KalpmXmpKfhMcACZEI9igqmxcYqhiRGxJiSATKhHMcHEIC0x2TyNSBPyWIAkQwOQAhoyHxEmpskmFimmliSGCcS0BRDT9mclMhBUewCi9j4xah9A1P4Hqs0CAIaTl3s",
      "id": "test1",
      "changes": [
        {
          "rev": "1-4c6114c65e295552ab1019e2b046b10e"
        }
      ]
    }
  ],
  "last_seq": "2-g1AAAAIzeJyV0EEOgjAQBdAqJurSE-gRCrTSruQmSjttKqmwUNZ6E72J3kRvgkVMgMQQ2cwkM5mXybcIoZnxAC1kXkgDIra5TKzJjyfrVuMEiWVZlqnxktHBDaY84j4j9NdBDyNWrorNV0IfSWNGBGeA5kUGSu8zBX1CXAnbjkDDEHwS_CvsKuHcEQjWiYz0n0I2cRVdXHPItcmESsKA8oGZ1Nqt1u7NT1j5igp_0E-PWnk2SkgjoGyY8qqVVj7rKMCKkbaSvgGlZaup",
  "pending": 0
}

sometimes this:

{
  "results": [
    {
      "seq": "1-g1AAAAHDeJzLYWBg4MhgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTIkyf___z8rkQGPoiQFIJlkD1KXwZzIkAvksacZWJgkWVqkMHCW5qWkpmXmpabgM8EBZEI8igmmxsYphiZGxJqQADKhHsUEE4O0xGTzNCJNyGMBkgwNQApoyHyQKYwQdySbWKSYWmLTR9C0BRDT9uMPP4jaAxC194lR-wCiFhQvWQC-343K",
      "id": "test1",
      "changes": [
        {
          "rev": "1-4c6114c65e295552ab1019e2b046b10e"
        }
      ]
    },
    {
      "seq": "2-g1AAAAHleJzLYWBg4MhgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTIkyf___z8rgzmRMRcowG5pbmloYWKKTQMeY5IUgGSSPdQkBrBJaQYWJkmWFikMnKV5KalpmXmpKfhMcACZEI9igqmxcYqhiRGxJiSATKhHMcHEIC0x2TyNSBPyWIAkQwOQAhoyHxEmpskmFimmliSGCcS0BRDT9mclMhBUewCi9j4xah9A1P4Hqs0CAIaTl3s",
      "id": "test2",
      "changes": [
        {
          "rev": "1-c86e975fffb4a635eed6d1dfc92afded"
        }
      ]
    }
  ],
  "last_seq": "2-g1AAAAIzeJyV0EEOgjAQBdAqJurSE-gRCrTSruQmSjttKqmwUNZ6E72J3kRvgkVMgMQQ2cwkM5mXybcIoZnxAC1kXkgDIra5TKzJjyfrVuMEiWVZlqnxktHBDaY84j4j9NdBDyNWrorNV0IfSWNGBGeA5kUGSu8zBX1CXAnbjkDDEHwS_CvsKuHcEQjWiYz0n0I2cRVdXHPItcmESsKA8oGZ1Nqt1u7NT1j5igp_0E-PWnk2SkgjoGyY8qqVVj7rKMCKkbaSvgGlZaup",
  "pending": 0
}

Assuming that the documentation says that I must use descending=true to sort by the most recent change, and there's no ascending parameter, so it should be always sorted in ascending way.

The descending parameter is not working properly as well

{
  "results": [
    {
      "seq": "2-g1AAAAGpeJzLYWBg4MhgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTIkyf___z8rkRGPoiQFIJlkD1KXwZzIkAvksacZWJgkWVqkMHCW5qWkpmXmpabgM8EBZEI82CYGfOoSQOrqUWwyMUhLTDZPI9KmPBYgydAApICGzAeZwgg2xTTZxCLF1BKbPoKmLYCYth-_2yFqD0DU3idG7QOIWlCYZAEAn36HNw",
      "id": "test1",
      "changes": [
        {
          "rev": "1-4c6114c65e295552ab1019e2b046b10e"
        }
      ]
    },
    {
      "seq": "2-g1AAAAHleJzLYWBg4MhgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTIkyf___z8rgzmRMRcowG5pbmloYWKKTQMeY5IUgGSSPdQkBrBJaQYWJkmWFikMnKV5KalpmXmpKfhMcACZEA8yIZEBn7oEkLp6FJtMDNISk83TiLQpjwVIMjQAKaAh8xE-N002sUgxtSTR5xDTFkBM24_f7RC1ByBq7yPcb2xqnmJqYUiS-x9ATAGFVhYACduXhg",
      "id": "test2",
      "changes": [
        {
          "rev": "1-c86e975fffb4a635eed6d1dfc92afded"
        }
      ]
    }
  ],
  "last_seq": "2-g1AAAAIzeJyV0EEOgjAQBdAqJurSE-gRCrTSruQmSjttKqmwUNZ6E72J3kRvgkVMgMQQ2cwkM5mXybcIoZnxAC1kXkgDIra5TKzJjyfrVuMEiWVZlqnxktHBDaY84j4j9NdBDyNWrorNV0IfSWNGBGeA5kUGSu8zBX1CXAnbjkDDEHwS_CvsKuHcEQjWiYz0n0I2cRVdXHPItcmESsKA8oGZ1Nqt1u7NT1j5igp_0E-PWnk2SkgjoGyY8qqVVj7rKMCKkbaSvgGlZaup",
  "pending": -2
}

Besides the order is wrong (should be test2,test1) the pending value is -2 but there are only two changes.

[dmunch]

I'm not entirely sure about pending, but I think it's important to understand the concept of _changes ordering - in CouchDB 2.0 the _changes feed is only partially ordered. You can find more information on this discussion.

[wohali]

What @dmunch says is correct - we do not guarantee strict ordering, but we do guarantee that for any specific given document, you will get a consistent order from us such that the changes can be replayed in an idempotent fashion.

However, the negative pending value is unexplained, and I'm changing the bug report title to reflect that as the ongoing concern in this issue.

[wohali]

Per IRC chat with @davisp:

shard combinations can cause that. because we're not sending an update_seq with the pending changes request, so if it gets a different set of shards it can go wonky like that.

In other words, both behaviours are expected with CouchDB 2.0.