nickva commented on code in PR #6210:
URL: https://github.com/apache/couchdb/pull/6210#discussion_r4520740507
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src/mem3/src/mem3_rep.erl:
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@@ -543,7 +552,10 @@ pull_purges(Db, Count, #shard{} = SrcShard, #tgt{} = Tgt0,
HashFun) ->
end,
Infos1 = lists:filter(BelongsFun, Infos),
{ok, _} = couch_db:purge_docs(Db, Infos1, [?REPLICATED_CHANGES]),
- Body = purge_cp_body(SrcShard, TgtShard, ThroughSeq),
+ % A checkpoint form the target but it's the same one the target
+ % uses when it pushes to us. So we write it from the target's point
+ % of view: it's the source and we are the target.
+ Body = purge_cp_body(TgtShard, SrcShard, ThroughSeq),
Review Comment:
A good question. For those case we want to keep it as `(Src,Tgt,Seq)`
because those are pushes from us (Src) to (Tgt) so we checkpoint locally on the
Src with a Src->Tgt checkpoint. For example if we're nodeA and the target is
nodeB. We'd be updating the checkpoint on nodeA: `checkpoint(Src=nodeA,
Tgt=nodeB, Seq)`.
In this case we're "pulling" from a target. Which is the same direction as a
target pushing to us. So if we go with us being nodeA and the other nodeB. Then
a pull from nodeB is like nodeB pushing to us. Then on nodeB we'll write a
`checkpoint(nodeB, nodeA)` the source then has to be nodeB (the other node) and
the target is nodeA(us).
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