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https://issues.apache.org/jira/browse/HIVE-13369?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15379865#comment-15379865
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Eugene Koifman edited comment on HIVE-13369 at 7/15/16 6:34 PM:
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What do you mean "making it through"? getChildState() only sets
"bestBase.status" if isValidBase() is true...
was (Author: ekoifman):
What do you mean "making it through"? It only sets "bestBase.status" if
isValidBase() is true...
> AcidUtils.getAcidState() is not paying attention toValidTxnList when choosing
> the "best" base file
> --------------------------------------------------------------------------------------------------
>
> Key: HIVE-13369
> URL: https://issues.apache.org/jira/browse/HIVE-13369
> Project: Hive
> Issue Type: Bug
> Components: Transactions
> Affects Versions: 1.0.0
> Reporter: Eugene Koifman
> Assignee: Eugene Koifman
> Priority: Blocker
> Attachments: HIVE-13369.1.patch, HIVE-13369.2.patch,
> HIVE-13369.3.patch
>
>
> The JavaDoc on getAcidState() reads, in part:
> "Note that because major compactions don't
> preserve the history, we can't use a base directory that includes a
> transaction id that we must exclude."
> which is correct but there is nothing in the code that does this.
> And if we detect a situation where txn X must be excluded but and there are
> deltas that contain X, we'll have to abort the txn. This can't (reasonably)
> happen with auto commit mode, but with multi statement txns it's possible.
> Suppose some long running txn starts and lock in snapshot at 17 (HWM). An
> hour later it decides to access some partition for which all txns < 20 (for
> example) have already been compacted (i.e. GC'd).
> ==========================================================
> Here is a more concrete example. Let's say the file for table A are as
> follows and created in the order listed.
> delta_4_4
> delta_5_5
> delta_4_5
> base_5
> delta_16_16
> delta_17_17
> base_17 (for example user ran major compaction)
> let's say getAcidState() is called with ValidTxnList(20:16), i.e. with HWM=20
> and ExceptionList=<16>
> Assume that all txns <= 20 commit.
> Reader can't use base_17 because it has result of txn16. So it should chose
> base_5 "TxnBase bestBase" in _getChildState()_.
> Then the reset of the logic in _getAcidState()_ should choose delta_16_16 and
> delta_17_17 in _Directory_ object. This would represent acceptable snapshot
> for such reader.
> The issue is if at the same time the Cleaner process is running. It will see
> everything with txnid<17 as obsolete. Then it will check lock manger state
> and decide to delete (as there may not be any locks in LM for table A). The
> order in which the files are deleted is undefined right now. It may delete
> delta_16_16 and delta_17_17 first and right at this moment the read request
> with ValidTxnList(20:16) arrives (such snapshot may have bee locked in by
> some multi-stmt txn that started some time ago. It acquires locks after the
> Cleaner checks LM state and calls getAcidState(). This request will choose
> base_5 but it won't see delta_16_16 and delta_17_17 and thus return the
> snapshot w/o modifications made by those txns.
> [This is not possible currently since we only support autoCommit=true. The
> reason is the a query (0) opens txn (if appropriate), (1) acquires locks, (2)
> locks in the snapshot. The cleaner won't delete anything for a given
> compaction (partition) if there are locks on it. Thus for duration of the
> transaction, nothing will be deleted so it's safe to use base_5]
> This is a subtle race condition but possible.
> 1. So the safest thing to do to ensure correctness is to use the latest
> base_x as the "best" and check against exceptions in ValidTxnList and throw
> an exception if there is an exception <=x.
> 2. A better option is to keep 2 exception lists: aborted and open and only
> throw if there is an open txn <=x. Compaction throws away data from aborted
> txns and thus there is no harm using base with aborted txns in its range.
> 3. You could make each txn record the lowest open txn id at its start and
> prevent the cleaner from cleaning anything delta with id range that includes
> this open txn id for any txn that is still running. This has a drawback of
> potentially delaying GC of old files for arbitrarily long periods. So this
> should be a user config choice. The implementation is not trivial.
> I would go with 1 now and do 2/3 together with multi-statement txn work.
> Side note: if 2 deltas have overlapping ID range, then 1 must be a subset of
> the other
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