Hi,
I recently noticed the sort order in MongoMissingLastRevSeeker on
_modified. The query sorts descending. I think this may be a problem
given the isolation guarantees provides by MongoDB. See also discussion
in OAK-4535.
In particular, I'm thinking of the following behaviour mentioned in the
MongoDB documentation: "Reads may miss matching documents that are
updated during the course of the read operation."
This happens when a query scans an index and a matching document is
concurrently updated and moves from the front of the cursor to the part
of the index that was already scanned. In this case MongoDB will not
return the document even if it matched the query before and after the
update.
Given the query in MongoMissingLastRevSeeker, it is quite likely MongoDB
will scan the _modified index from (currently) newest to specified
startTime. While doing this, a document may get updated, its index entry
jumps over the cursor and becomes the newest entry in the index.
A can't really think of a reason why the sort order must be descending
in MongoMissingLastRevSeeker. The base class doesn't specify a specific
order.
Does anyone remember why this sort order is in place?
Regards
Marcel
