nickva commented on issue #4640:
URL: https://github.com/apache/couchdb/issues/4640#issuecomment-1593507302
Thanks for creating the issue @jcoglan !
> to the captured seq value returns results out of order
How did we determine they are out of order? Is that based on the `N-...`
first part of the sequence number?
> The descending value of the numeric part of the seq value looked curious
to me so I ran these values through a parser tool and obtained:
```
original:
'30-g1AAAAKVeJzLYWBg4MhgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTIkyf___z8rgzmRJRcowJ6alGySlmqJTQMeY5IUgGSSPcykDKYUBtbigpzMErCZpslJZmbmBqSa6QAyMx5qJgPYJAPLRAsjcwtSTUoAmVRPVdflsQBJhgYgBTR2PshcNjRzzUxTTSzMU8gydwHE3P0InxsZJ5obmpuSZdoBiGn3qe3KBxBz_5NobhYAREXOrQ',
parsed: [
[ 'couchdb@localhost-00000000-1fffffff', 4 ],
[ 'couchdb@localhost-20000000-3fffffff', 4 ],
[ 'couchdb@localhost-40000000-5fffffff', 0 ],
[ 'couchdb@localhost-60000000-7fffffff', 4 ],
[ 'couchdb@localhost-80000000-9fffffff', 6 ],
[ 'couchdb@localhost-a0000000-bfffffff', 0 ],
[ 'couchdb@localhost-c0000000-dfffffff', 6 ],
[ 'couchdb@localhost-e0000000-ffffffff', 6 ]
]
```
```
original:
'23-g1AAAAKBeJzLYWBg4MhgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTIkyf___z8rgzmRJRcowJ6alGySlmqJTQMeY5IUgGSSPdQkBohJQGMMTA1INckBZFI8ikkGlokWRuYWpJqUADKpHmoSI9ikREtTU_OkJBJNymMBkgwNQApo2HyQaWwZTCkMrMUFOZklYHPNTFNNLMxTyDJ3AcTc_Qj_Ghknmhuam5Jl2gGIafep7coHEHP_k2huFgBjZMkL',
parsed: [
[ 'couchdb@localhost-00000000-1fffffff', 4 ],
[ 'couchdb@localhost-20000000-3fffffff', 0 ],
[ 'couchdb@localhost-40000000-5fffffff', 0 ],
[ 'couchdb@localhost-60000000-7fffffff', 1 ],
[ 'couchdb@localhost-80000000-9fffffff', 6 ],
[ 'couchdb@localhost-a0000000-bfffffff', 0 ],
[ 'couchdb@localhost-c0000000-dfffffff', 6 ],
[ 'couchdb@localhost-e0000000-ffffffff', 6 ]
]
```
I think the parser tools is ignoring an important `split` marker and just
shows the sequence number. Let's see what it's hiding:
```
fabric_view_changes:decode_seq(<<"30-g1AAAAKVeJzLYWBg4MhgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTIkyf___z8rgzmRJRcowJ6alGySlmqJTQMeY5IUgGSSPcykDKYUBtbigpzMErCZpslJZmbmBqSa6QAyMx5qJgPYJAPLRAsjcwtSTUoAmVRPVdflsQBJhgYgBTR2PshcNjRzzUxTTSzMU8gydwHE3P0InxsZJ5obmpuSZdoBiGn3qe3KBxBz_5NobhYAREXOrQ">>).
[{couchdb@localhost,[0,536870911],
{4,<<"ebc4fe9">>,couchdb@localhost}},
{couchdb@localhost,[536870912,1073741823],
{4,{split,<<"5cb6670">>},couchdb@localhost}},
{couchdb@localhost,[1073741824,1610612735],
{0,<<"09a8278">>,couchdb@localhost}},
{couchdb@localhost,[1610612736,2147483647],
{4,{split,<<"5cb6670">>},couchdb@localhost}},
{couchdb@localhost,[2147483648,2684354559],
{6,{split,<<"65e487d">>},couchdb@localhost}},
{couchdb@localhost,[2684354560,3221225471],
{0,<<"23a7175">>,couchdb@localhost}},
{couchdb@localhost,[3221225472,3758096383],
{6,{split,<<"65e487d">>},couchdb@localhost}},
{couchdb@localhost,[3758096384,4294967295],
{6,{split,<<"65e487d">>},couchdb@localhost}}]
```
We notice how `{4,{split,<<"5cb6670">>},couchdb@localhost}}` looks a bit
different than `{0,<<"09a8278">>,couchdb@localhost}}`. Specifically the
`{split,<<"5cb6670">>}` part. That part shows a split "marker", that it looks
that particular range was likely the result of a split from the original
sequence with (Q=2) with original since seq node uuid for the now missing
(split) range. The `4` part shows the original sequence from the Q=2 sequence.
In general looking at the `N-....` is not too helpful. To make the sequences
look "nicer" we could avoid adding the sequence to the total sum when computing
`N` if the sequence is a split marker. That would make the `N-` value increment
"nicer". But that value is effectively meaningless and it is thrown away when
we parse the sequence.
It would still be a nice minor ergonomic fix but so far nothing here
violates correctness, we're not throwing changes away or returning them out of
order.
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