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From "Sylvain Lebresne (JIRA)" <>
Subject [jira] Commented: (CASSANDRA-2101) support deletes in counters
Date Thu, 03 Feb 2011 13:08:29 GMT


Sylvain Lebresne commented on CASSANDRA-2101:

Before commenting on the patch itself, I want to use this ticket to recall that counter deletes
are intrinsically broken. It has been said already but I'll use this comment to explain in
more depth how so and keep a trace of this for the record.

First, I'll use the following notation:
  c(x, 3)@[4, 2] - for a counter column of name x, value 3, timestamp 4 and timestampOfLastDelete
2 (I'll use -1 as the min timestampOfLastDelete).
  d(x)@[5] - for a tombstone of name x and timestamp 5

And now suppose that the following inserts are done (in that order):
   c(x, 1)@[1, -1]
   c(x, 1)@[3, -1]

If these inserts are resolved in that order, everything is fine:
   c(x, 1)@[1, -1]
 + d(x)@[2]
=> d(x)@[2]
 + c(x, 1)@[3, -1]
=> c(x, 1)@[3, 2]

However, some reordering don't work. Namely, if you merge the two counts together, before
you merge one of the count with the delete:
   c(x, 1)@[1, -1]
 + c(x, 1)@[3, -1]
=> c(x, 2)@[3, -1]
 + d(x)@[2]
=> c(x, 2)@[3, 2]

The problem is, the resolve operation is not commutative when you consider counter columns
and tombstones. But Cassandra rely heavily on resolve being commutative (as a side note, I
never understood the reason of the CommutativeType terminology in the code. It suggest that
regular columns are not commutative, while they are as far as resolve is concerned. Resolve
is not idempotent on counters however).

Not only is there no guarantee on which order the insert will be received by each node, but
even if they are in the right order, there is no guarantee that (minor) compaction won't screw
up this.

Hence I think that there is not much guarantee we can give on deletes. The only one I can
think of is that when on issue a delete, you must wait to issue any following update that
the delete have reach all the nodes and all of them have been fully compacted.

That being said, we can keep counter deletes. It's at least useful for cases where you know
that you won't reuse a counter ever and want to get rid of the disk space. But I would add
a very strong warning to its documentation.

Lastly, the deletion of full counter rows or super columns suffers the same problem for the
same reason.

> support deletes in counters
> ---------------------------
>                 Key: CASSANDRA-2101
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 0.8
>            Reporter: Kelvin Kakugawa
>            Assignee: Kelvin Kakugawa
>             Fix For: 0.8
>         Attachments: 0001-CASSANDRA-2101-fix-timestampOfLastDelete-reconciliat.patch
> Obey timestampOfLastDelete during reconciliation.

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