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https://issues.apache.org/jira/browse/HBASE-15968?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15486867#comment-15486867
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Phil Yang commented on HBASE-15968:
-----------------------------------
After reading code introduced by HBASE-7663 and HBASE-10885, if I am not wrong,
visibility labels will not change the logic of versions. If the maxVersion is
3, we can only read first three versions and check if it can be seen according
to tags. For delete, we can set tags for it and it only mask Put with same
tags(no tag is also a kind of tag).
For mvcc-sensitive semantics, we have VERSION_MASKED which means this Put is
masked by enough number of versions and can never be read. So for
mvcc-sensitive with visibility labels. If we have 4 Puts whose tags are a, a,
b, a. The first one will never be read no mater what labels the reader has. But
if we delete the third Put after we put it. The order is Put(a), Put(a),
Put(b), Delete(b), Put(a). What should be the result of reader if its label is
a? The latest two Put(a), or all three of them?
[~anoop.hbase][~ram_krish][~Apache9] What do you think? Thanks.
> MVCC-sensitive semantics of versions
> ------------------------------------
>
> Key: HBASE-15968
> URL: https://issues.apache.org/jira/browse/HBASE-15968
> Project: HBase
> Issue Type: New Feature
> Reporter: Phil Yang
> Assignee: Phil Yang
> Attachments: HBASE-15968-v1.patch
>
>
> In HBase book, we have a section in Versions called "Current Limitations" see
> http://hbase.apache.org/book.html#_current_limitations
> {quote}
> 28.3. Current Limitations
> 28.3.1. Deletes mask Puts
> Deletes mask puts, even puts that happened after the delete was entered. See
> HBASE-2256. Remember that a delete writes a tombstone, which only disappears
> after then next major compaction has run. Suppose you do a delete of
> everything ⇐ T. After this you do a new put with a timestamp ⇐ T. This put,
> even if it happened after the delete, will be masked by the delete tombstone.
> Performing the put will not fail, but when you do a get you will notice the
> put did have no effect. It will start working again after the major
> compaction has run. These issues should not be a problem if you use
> always-increasing versions for new puts to a row. But they can occur even if
> you do not care about time: just do delete and put immediately after each
> other, and there is some chance they happen within the same millisecond.
> 28.3.2. Major compactions change query results
> …create three cell versions at t1, t2 and t3, with a maximum-versions
> setting of 2. So when getting all versions, only the values at t2 and t3 will
> be returned. But if you delete the version at t2 or t3, the one at t1 will
> appear again. Obviously, once a major compaction has run, such behavior will
> not be the case anymore… (See Garbage Collection in Bending time in HBase.)
> {quote}
> These limitations result from the current implementation on multi-versions:
> we only consider timestamp, no matter when it comes; we will not remove old
> version immediately if there are enough number of new versions.
> So we can get a stronger semantics of versions by two guarantees:
> 1, Delete will not mask Put that comes after it.
> 2, If a version is masked by enough number of higher versions (VERSIONS in
> cf's conf), it will never be seen any more.
> Some examples for understanding:
> (delete t<=3 means use Delete.addColumns to delete all versions whose ts is
> not greater than 3, and delete t3 means use Delete.addColumn to delete the
> version whose ts=3)
> case 1: put t2 -> put t3 -> delete t<=3 -> put t1, and we will get t1 because
> the put is after delete.
> case 2: maxversion=2, put t1 -> put t2 -> put t3 -> delete t3, and we will
> always get t2 no matter if there is a major compaction, because t1 is masked
> when we put t3 so t1 will never be seen.
> case 3: maxversion=2, put t1 -> put t2 -> put t3 -> delete t2 -> delete t3,
> and we will get nothing.
> case 4: maxversion=3, put t1 -> put t2 -> put t3 -> delete t2 -> delete t3,
> and we will get t1 because it is not masked.
> case 5: maxversion=2, put t1 -> put t2 -> put t3 -> delete t3 -> put t1, and
> we can get t3+t1 because when we put t1 at second time it is the 2nd latest
> version and it can be read.
> case 6:maxversion=2, put t3->put t2->put t1, and we will get t3+t2 just like
> what we can get now, ts is still the key of versions.
> Different VERSIONS may result in different results even the size of result is
> smaller than VERSIONS(see case 3 and 4). So Get/Scan.setMaxVersions will be
> handled at end after we read correct data according to CF's VERSIONS setting.
> The semantics is different from the current HBase, and we may need more logic
> to support the new semantic, so it is configurable and default is disabled.
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