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https://issues.apache.org/jira/browse/CASSANDRA-2643?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13032589#comment-13032589
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Peter Schuller commented on CASSANDRA-2643:
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Unless I'm off, the problem with deleted columns is fairly similar to that of
deleted rows. Like with range ghosts, an option would be to propagate
tombstones to clients.
Another option is to make read responses include the range for which they are
authoritative, and then only consider the intersection of all responses'
authoritative ranges when reconciling results. The authoritative range of the
response would have to be communicated back to the client, such that it can
continue iterating from the appropriate column name even without actually
receiving a column for that name.
Other than failing requests with a new kind of error, I don't see a good way to
fix the tombstone case (the over-shoot case can be fixed by just capping
results) without changing the protocol. For obvious reason's we don't want the
co-originating node to go into a potentially unbounded re-try spin until
sufficient results are obtained from all nodes participating.
FWIW, returning iteration state feels pretty clean to me (it's what our layer
implements on top to enable easier iteration). It is also compatible with
future improvements to e.g. cap the size of a response by bytes for safely
slicing over columns whose size you do not know. By removing the strict
requirement to deliver exactly the number of asked-for columns else it be
interpreted as "out of columns", significant flexibility is gained. So if the
only option for a clean fix is truly to change the protocol, at least other
positive benefits may be had.
> read repair/reconciliation breaks slice based iteration at QUORUM
> -----------------------------------------------------------------
>
> Key: CASSANDRA-2643
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2643
> Project: Cassandra
> Issue Type: Bug
> Affects Versions: 0.7.5
> Reporter: Peter Schuller
> Priority: Critical
> Attachments: slicetest.py
>
>
> In short, I believe iterating over columns is impossible to do reliably with
> QUORUM due to the way reconciliation works.
> The problem is that the SliceQueryFilter is executing locally when reading on
> a node, but no attempts seem to be made to consider limits when doing
> reconciliation and/or read-repair (RowRepairResolver.resolveSuperset() and
> ColumnFamily.resolve()).
> If a node slices and comes up with 100 columns, and another node slices and
> comes up with 100 columns, some of which are unique to each side,
> reconciliation results in > 100 columns in the result set. In this case the
> effect is limited to "client gets more than asked for", but the columns still
> accurately represent the range. This is easily triggered by my test-case.
> In addition to the client receiving "too many" columns, I believe some of
> them will not be satisfying the QUORUM consistency level for the same reasons
> as with deletions (see discussion below).
> Now, there *should* be a problem for tombstones as well, but it's more
> subtle. Suppose A has:
> 1
> 2
> 3
> 4
> 5
> 6
> and B has:
> 1
> del 2
> del 3
> del 4
> 5
> 6
> If you now slice 1-6 with count=3 the tombstones from B will reconcile with
> those from A - fine. So you end up getting 1,5,6 back. This made it a bit
> difficult to trigger in a test case until I realized what was going on. At
> first I was "hoping" to see a "short" iteration result, which would mean that
> the process of iterating until you get a short result will cause spurious
> "end of columns" and thus make it impossible to iterate correctly.
> So; due to 5-6 existing (and if they didn't, you legitimately reached
> end-of-columns) we do indeed get a result of size 3 which contains 1,5 and 6.
> However, only node B would have contributed columns 5 and 6; so there is
> actually no QUORUM consistency on the co-ordinating node with respect to
> these columns. If node A and C also had 5 and 6, they would not have been
> considered.
> Am I wrong?
> In any case; using script I'm about to attach, you can trigger the
> over-delivery case very easily:
> (0) disable hinted hand-off to avoid that interacting with the test
> (1) start three nodes
> (2) create ks 'test' with rf=3 and cf 'slicetest'
> (3) ./slicetest.py hostname_of_node_C insert # let it run for a few seconds,
> then ctrl-c
> (4) stop node A
> (5) ./slicetest.py hostname_of_node_C insert # let it run for a few seconds,
> then ctrl-c
> (6) start node A, wait for B and C to consider it up
> (7) ./slicetest.py hostname_of_node_A slice # make A co-ordinator though it
> doesn't necessarily matter
> You can also pass 'delete' (random deletion of 50% of contents) or
> 'deleterange' (delete all in [0.2,0.8]) to slicetest, but you don't trigger a
> short read by doing that (see discussion above).
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