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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-11475) MV code refactor
Date Fri, 01 Apr 2016 09:16:25 GMT
Sylvain Lebresne created CASSANDRA-11475:
--------------------------------------------

             Summary: MV code refactor
                 Key: CASSANDRA-11475
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11475
             Project: Cassandra
          Issue Type: Bug
            Reporter: Sylvain Lebresne
            Assignee: Sylvain Lebresne
             Fix For: 3.0.x, 3.x


While working on CASSANDRA-5546 I run into a problem with TTLs on MVs, which looking more
closely is a bug of the MV code. But one thing leading to another I reviewed a good portion
of the MV code and found the following correction problems:
* If a base row is TTLed then even if an update remove that TTL the view entry remained TTLed
and expires, leading to an inconsistency.
* Due to calling the wrong ctor for {{LivenessInfo}}, when a TTL was set on the base table,
the view entry was living twice as long as the TTL. Again leading to a temporary inconsistency.
* When reading existing data to compute view updates, all deletion informations are completely
ignored (the code uses a {{PartitionIterator}} instead of an {{UnfilteredPartitionIterator}}).
This is a serious issue since it means some deletions could be totally ignored as far as views
are concerned especially when messages are delivered to a replica out of order. I'll note
that while the 2 previous points are relatively easy to fix, I didn't find an easy and clean
way to fix this one on the current code.

Further, I think the MV code in general has inefficiencies/code complexities that should be
avoidable:
* {{TemporalRow.Set}} is buffering both everything read and a pretty much complete copy of
the updates. That's a potentially high memory requirement. We shouldn't have to copy the updates
and we shouldn't buffer all reads but rather work incrementally.
* {{TemporalRow}}/{{TemporalRow.Set}}/{{TemporalCell}} classes are somewhat re-inventing the
wheel. They are really just storing both an update we're doing and the corresponding existing
data, but we already have {{Row}}/{{Partition}}/{{Cell}} for that. In practice, those {{Temporal*}}
class generates a lot of allocations that we could avoid.
* The code from CASSANDRA-10060 to avoid multiple reads of the base table with multiple views
doesn't work when the update has partition/range tombstones because the code uses {{TemporalRow.Set.setTombstonedExisting()}}
to trigger reuse, but the {{TemporalRow.Set.withNewViewPrimaryKey()}} method is used between
view and it does not preseve the {{hasTombstonedExisting}} flag.  But that oversight, which
is trivial to fix, is kind of a good thing since if you fix it, you're left with a correction
problem.
  The read done when there is a partition deletion depends on the view itself (if there is
clustering filters in particular) and so reusing that read for other views is wrong. Which
makes that whole reuse code really dodgy imo: the read for existing data is in {{View.java}},
suggesting that it depends on the view (which again, it does at least for partition deletion),
but it shouldn't if we're going to reuse the result across multiple views.
* Even ignoring the previous point, we still potentially read the base table twice if the
update mix both row updates and partition/range deletions, potentially re-reading the same
values.
* It's probably more minor but the reading code is using {{QueryPager}}, which is probably
an artifact of the initial version of the code being pre-8099, but it's not necessary anymore
(the reads are local and locally we're completely iterator based), adding, especially when
we do page. I'll note that despite using paging, the current code still buffers everything
in {{TemporalRow.Set}} anyway .

Overall, I suspect trying to fix the problems above (particularly the fact that existing deletion
infos are ignored) is only going to add complexity with the current code and we'd still have
to fix the inefficiencies. So I propose a refactor of that code which does 2 main things:
# it removes all of {{TemporalRow}} and related classes. Instead, it directly uses the existing
{{Row}} (with all its deletion infos) and the update being applied to it and compute the updates
for the view from that. I submit that this is more clear/simple, but this also avoid copying
every cell of both the existing and update data as a {{TemporalCell}}. We can also reuse codes
like {{Rows.merge}} and {{Rows.diff}} to make the handling of deletions relatively painless.
# instead of dealing with each view one at a time, re-iterating over all updates each time,
it iterates over each individual updates once and deal with each view for that update. This
makes it more clear that the reads has to care about every view involved, but more importantly
allow to deal with the read data incrementally, never buffering it all.



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