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From "Jonathan Ellis (JIRA)" <>
Subject [jira] Commented: (CASSANDRA-33) Bugs in tombstone handling in remove code
Date Fri, 17 Apr 2009 15:26:14 GMT


Jonathan Ellis commented on CASSANDRA-33:

MultiGet doesn't seem to be coming any time soon.  Guess we'll just have to deal with conflict
resolution when it does.

Re the patches provided, they follow the outline above except that it turns out we can just
use a single removeDeleted to handle both tombstones (which still supress old data "below"
them in the tree, e.g., a deleted supercolumn does not need to keep its subcolumn data around)
and GC.  So CFS and compaction can still calls removeDeleted, and then CassandraServer just
has to remove the tombstones themselves in thriftifyColumns and thriftifySuperColumns.

Patch to make GC_GRACE_IN_SECONDS configurable forthcoming.

> Bugs in tombstone handling in remove code
> -----------------------------------------
>                 Key: CASSANDRA-33
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Jonathan Ellis
>            Assignee: Jonathan Ellis
>             Fix For: 0.3
>         Attachments: 0001-preserve-tombstones-until-a-GC-grace-period-has-elap.patch,
> [copied from dev list]
> Avinash pointed out two bugs in my remove code.  One is easy to fix,
> the other is tougher.
> The easy one is that my code removes tombstones (deletion markers) at
> the ColumnFamilyStore level, so when CassandraServer does read repair
> it will not know about the tombstones and they will not be replicated
> correctly.  This can be fixed by simply moving the removeDeleted call
> up to just before CassandraServer's final return-to-client.
> The hard one is that tombstones are problematic on GC (that is, major
> compaction of SSTables, to use the Bigtable paper terminology).
> One failure scenario: Node A, B, and C replicate some data.  C goes
> down.  The data is deleted.  A and B delete it and later GC it.  C
> comes back up.  C now has the only copy of the data so on read repair
> the stale data will be sent to A and B.
> A solution: pick a number N such that we are confident that no node
> will be down (and catch up on hinted handoffs) for longer than N days.
>  (Default value: 10?)  Then, no node may GC tombstones before N days
> have elapsed.  Also, after N days, tombstones will no longer be read
> repaired.  (This prevents a node which has not yet GC'd from sending a
> new tombstone copy to a node that has already GC'd.)
> Implementation detail: we'll need to add a 32-bit "time of tombstone"
> to ColumnFamily and SuperColumn.  (For Column we can stick it in the
> byte[] value, since we already have an unambiguous way to know if the
> Column is in a deleted state.)  We only need 32 bits since the time
> frame here is sufficiently granular that we don't need ms.  Also, we
> will use the system clock for these values, not the client timestamp,
> since we don't know what the source of the client timestamps is.
> Admittedly this is suboptimal compared to being able to GC immediately
> but it has the virtue of being (a) easily implemented, (b) with no
> extra components such as a coordination protocol, and (c) better than
> not GCing tombstones at all (the other easy way to ensure
> correctness).

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