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https://issues.apache.org/jira/browse/CASSANDRA-14496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16502639#comment-16502639
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Robert Tarrall commented on CASSANDRA-14496:
--------------------------------------------
I definitely agree with respect to "don't enable by default."
However, I believe if I have set {{unchecked_tombstone_compaction}} to true,
I'm asking for these compactions. I may however be missing something – is
there another purpose for that option? I.e. might someone else be setting that
to true who would be surprised to find it enables tombstone compactions? I
can't find any documentation which explains that
{{'unchecked_tombstone_compaction': 'true'}} has no effect unless you also
explicitly set other options, and I see discussions in blog posts that suggest
people think that setting is how you enable tombstone compactions in TWCS, and
I had to rummage around in source code for a while to work out why I wasn't
getting those compactions.
Coming at this from the other direction may help. If I have the following
compaction defined:
{code:java}
'class': 'org.apache.cassandra.db.compaction.TimeWindowCompactionStrategy',
'compaction_window_size': '1', 'compaction_window_unit': 'HOURS',
'unchecked_tombstone_compaction': 'true'{code}
... I don't get tombstone compactions. When I change that to:
{code:java}
'class': 'org.apache.cassandra.db.compaction.TimeWindowCompactionStrategy',
'compaction_window_size': '1', 'compaction_window_unit': 'HOURS',
'tombstone_compaction_interval': '86400', 'unchecked_tombstone_compaction':
'true'
{code}
... I have just switched from "no tombstone compactions" to "tombstone
compactions". This seems like a surprising side effect – one would not expect
that explicitly setting an option to its default value would change behavior
like this.
If there's another purpose for unchecked_tombstone_compaction, I'd recommend
the defaults for TWCS make it clear that tombstone compactions intentionally
act differently from STCS; instead of interval & threshold defaults of 86400 &
0.2, they should be infinitely high, and documented as such, so that you must
specify non-default values in order to get tombstone compactions.
> TWCS erroneously disabling tombstone compactions
> ------------------------------------------------
>
> Key: CASSANDRA-14496
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14496
> Project: Cassandra
> Issue Type: Bug
> Components: Compaction
> Reporter: Robert Tarrall
> Priority: Minor
>
> This code:
> {code:java}
> this.options = new TimeWindowCompactionStrategyOptions(options);
> if
> (!options.containsKey(AbstractCompactionStrategy.TOMBSTONE_COMPACTION_INTERVAL_OPTION)
> &&
> !options.containsKey(AbstractCompactionStrategy.TOMBSTONE_THRESHOLD_OPTION))
> {
> disableTombstoneCompactions = true;
> logger.debug("Disabling tombstone compactions for TWCS");
> }
> else
> logger.debug("Enabling tombstone compactions for TWCS");
> }
> {code}
> ... in TimeWindowCompactionStrategy.java disables tombstone compactions in
> TWCS if you have not *explicitly* set either tombstone_compaction_interval or
> tombstone_threshold. Adding 'tombstone_compaction_interval': '86400' to the
> compaction stanza in a table definition has the (to me unexpected) side
> effect of enabling tombstone compactions.
> This is surprising and does not appear to be mentioned in the docs.
> I would suggest that tombstone compactions should be run unless these options
> are both set to 0.
> If the concern is that (as with DTCS in CASSANDRA-9234) we don't want to
> waste time on tombstone compactions when we expect the tables to eventually
> be expired away, perhaps we should also check unchecked_tombstone_compaction
> and still enable tombstone compactions if that's set to true.
> May also make sense to set defaults for interval & threshold to 0 & disable
> if they're nonzero so that setting non-default values, rather than setting
> ANY value, is what determines whether tombstone compactions are enabled?
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