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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-2643) read repair/reconciliation breaks slice based iteration at QUORUM
Date Mon, 01 Aug 2011 22:54:28 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2643?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13073980#comment-13073980
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Jonathan Ellis commented on CASSANDRA-2643:
-------------------------------------------

Looks good on the whole.  One point to clear up:

{code}
if ((maxLiveColumns >= sliceCommand.count) && (liveColumnsInRow < sliceCommand.count))
{code}

maxLiveColumns is the max from a single response, so how can it be greater than sliceCommand.count?
 Would this be a valid reformulation?

{code}
assert maxLiveColumns <= sliceCommand.count;
if ((maxLiveColumns == sliceCommand.count) && (liveColumnsInRow < sliceCommand.count))
{code}

Minor things I'd like to clean up:

- is maxLiveColumns valid on any AbstractRR subclass other than RRR? If not I'd rather move
it in there and throw an UnsupportedOperation in ARR.
- Would prefer initializing commandsToRetry to Collections.emptyList, to avoid allocating
that list in the common case that no retries are needed.  (Then clear of course needs to become
allocate.)


> 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
>            Assignee: Brandon Williams
>            Priority: Critical
>             Fix For: 1.0
>
>         Attachments: CASSANDRA-2643-poc.patch, CASSANDRA-2643-v2.patch, reliable_short_read_0.8.sh,
short_read.sh, short_read_0.8.sh, 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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