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From "Peter Schuller (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-2643) read repair/reconciliation breaks slice based iteration at QUORUM
Date Fri, 13 May 2011 08:15:47 GMT


Peter Schuller commented on CASSANDRA-2643:

I realized I failed to make the possibly most important point: That you can indeed get short
reads such that iteration will stop early. Consider 3 nodes, RF=3, QUORUM.

* Node A has [10,40] of columns
* Node B has [10,40] of columns
* Node C has [10,20] of column deletions for the columns that A/B has,
but does NOT have any for [21,40] because it was down when those were written

Now a client slices [10,1000] with count=11. The co-ordinating node will reconcile that; C's
tombstones will override A/B (I'm assuming tombstones are later than A+B's columns), but since
C is lacking the "remainder" of columns you don't just get some columns at lowered consistency
level - you actually get a "short" result, and the application or high-level client will believe
that the iteration is complete.

This was the primary reason why I said in the OP that I believed "iterating over columns is
impossible to do reliably with QUORUM". I somehow lost this when re-phrasing the JIRA post
a couple of times.

Note: The short read case is not something I have tested and triggered, so is based on extrapolation
from my understanding of the code.

> read repair/reconciliation breaks slice based iteration at QUORUM
> -----------------------------------------------------------------
>                 Key: CASSANDRA-2643
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>    Affects Versions: 0.7.5
>            Reporter: Peter Schuller
>            Priority: Critical
>         Attachments:
> 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
> 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
> 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) ./ hostname_of_node_C insert # let it run for a few seconds, then ctrl-c
> (4) stop node A
> (5) ./ 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) ./ hostname_of_node_A slice # make A co-ordinator though it doesn't necessarily
> 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

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