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From "Peter Schuller (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-2643) read repair/reconciliation breaks slice based iteration at QUORUM
Date Thu, 12 May 2011 16:27:47 GMT
read repair/reconciliation breaks slice based iteration at QUORUM
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                 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


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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