[couchdb] 01/01: Improve emitted change feed sequence after a split

[va...@apache.org]

This is an automated email from the ASF dual-hosted git repository.

vatamane pushed a commit to branch prettify-split-sequences
in repository https://gitbox.apache.org/repos/asf/couchdb.git

commit ead9f67aa60de1f89d5958420956d9309269d525
Author: Nick Vatamaniuc <va...@gmail.com>
AuthorDate: Thu Jun 15 20:32:46 2023 -0400

    Improve emitted change feed sequence after a split
    
     When we get sequence before the split, we'll fill in the missing (now split)
    ranges with a special `{split, OldNodeUUid}` marker. However, when sequences
    are emitted in the changes API, that will make the N- prefix (SeqSum) bounce
    around from higher to lower numbers, while users expect those to be mostly
    incrementing. So take a conservative approach and assume it will be full
    rewind for that ramge, and use 0 for that range. This is a purely cosmetic
    thing, when we decode the sequence that prefix gets thrown away anyway.
    
    Fixes a part of issue: #4640
---
 src/fabric/src/fabric_view_changes.erl | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/src/fabric/src/fabric_view_changes.erl b/src/fabric/src/fabric_view_changes.erl
index fd439c5c3..84046d174 100644
--- a/src/fabric/src/fabric_view_changes.erl
+++ b/src/fabric/src/fabric_view_changes.erl
@@ -425,6 +425,15 @@ pack_seqs(Workers) ->
     Opaque = couch_util:encodeBase64Url(?term_to_bin(SeqList, [compressed])),
     ?l2b([integer_to_list(SeqSum), $-, Opaque]).
 
+% When we get sequence before the split, we'll fill in the missing (now split)
+% ranges with a special {split, OldNodeUUid} marker. However, when sequences
+% are emitted in the changes API, that will make the N- prefix (SeqSum) bounce
+% around from higher to lower numbers, while users expect those to be mostly
+% incrementing. So take a conservative approach and assume it will be full
+% rewind for that ramge, and use 0 for that range. This is a purely cosmetic
+% thing, when we decode the sequence that prefix gets thrown away anyway.
+%
+seq({Seq, {split, <<_/binary>>}, _Node}) when is_integer(Seq) -> 0;
 seq({Seq, _Uuid, _Node}) -> Seq;
 seq({Seq, _Uuid}) -> Seq;
 seq(Seq) -> Seq.
@@ -1065,4 +1074,18 @@ find_replacement_sequence_test() ->
     ?assertEqual(0, find_replacement_sequence(Dead4, [0, 10])),
     ?assertEqual(0, find_replacement_sequence(Dead4, [0, 5])).
 
+pack_split_seq_test() ->
+    Shards = [{"n1", 0, 10}, {"n2", 11, ?RING_END}],
+    Workers = mk_workers(Shards, {42, {split, <<"abc1234">>}, 'node1@127.0.0.1'}),
+    PackedSeq = pack_seqs(Workers),
+    ?assertMatch(<<"0-", _/binary>>, PackedSeq),
+    DecodedSeq = decode_seq(PackedSeq),
+    ?assertEqual(
+        [
+            {n1, [0, 10], {42, {split, <<"abc1234">>}, 'node1@127.0.0.1'}},
+            {n2, [11, 4294967295], {42, {split, <<"abc1234">>}, 'node1@127.0.0.1'}}
+        ],
+        DecodedSeq
+    ).
+
 -endif.