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Phil Yang updated HBASE-15968: ------------------------------ Summary: New behavior of versions considering mvcc and ts rather than ts only (was: MVCC-sensitive semantics of versions) > New behavior of versions considering mvcc and ts rather than ts only > -------------------------------------------------------------------- > > 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, HBASE-15968-v2.patch, > HBASE-15968-v3.patch, HBASE-15968-v4.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. -- This message was sent by Atlassian JIRA (v6.4.14#64029)