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https://issues.apache.org/jira/browse/JENA-29?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12995027#comment-12995027
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Simon Helsen commented on JENA-29:
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I am looking again at the code, so, Andy, to clarify the need for requesting
versus cancelling is primarily that when I *request* a cancel, the iterator
will only move to a cancel state once the nextBinding has executed, the reason
being that at that point, hasNext() is allowed to answer true or false, i.e.
when cancelled, it will return false. However, if you immediately make a cancel
out of request for cancel, you may have a situation where hasNext() will return
false, but nextBinding is going to be called because the async thread requested
the cancel after the executing thread already passed hasNext(). Hope that this
is clear now
Now, I agree that the pattern between cancel() and requestCancel() is a bit
confusing. This has to do with the fact that cancel() actually sets the
"requestCancel" flag, whereas requestCancel() just propagates the
cancellationRequest over the iterator network. That is also why the pattern
between QueryIter1 and QueryIter2 looks different. Perhaps this could all be
made more readable by having each iterator just implement requestCancel() *and*
in that method also set the requestCancellation flag. In that case, there is no
need to call cancel() again and it is clear how the cancellation requests are
propagated.
Finally, and this is a different topic. We were noticing weak cancellation
behavior whenever there are optional clauses and I now notice that there may be
glitch for optional clauses and in fact, any QueryIterRepeatApply. The problem
is that each time one moves to the next stage, the currentStage becomes null
for a short time and thus neither close() nor cancel() will catch in that short
time. Afterwards, close or cancel will be forgotten. I'll provide a patch for
this
> cancellation during query execution
> -----------------------------------
>
> Key: JENA-29
> URL: https://issues.apache.org/jira/browse/JENA-29
> Project: Jena
> Issue Type: Improvement
> Components: ARQ, TDB
> Reporter: Simon Helsen
> Assignee: Andy Seaborne
> Attachments: JENA-29_ARQ_r8489.patch, JENA-29_TDB_r8489.patch,
> JENA-29_tests_ARQ_r8489.patch, jena.patch, jenaAddition.patch
>
>
> The requested improvement and proposed patch is made by Simon Helsen on
> behalf of IBM
> ARQ query execution currently does not have a satisfactory way to cancel a
> running query in a safe way. Moreover, cancel (unlike a hard abort) is
> especially useful if it is able to provide partial result sets (i.e. all the
> results it managed to compute up to when the cancellation was requested).
> Although the exact cancellation behavior depends on the capabilities of the
> underlying triple store, the proposed patch merely relies on the iterators
> used by ARQ.
> Here is a more detailed explanation of the proposed changes:
> 1) the cancel() method in the QueryIterator initiates a cancellation request
> (first boolean flag). In analogy with closeIterator(), it propagates through
> all chained iterators, so the entire calculation is aware that a cancellation
> is requested
> 2) to ensure a thread-safe semantics, the cancelRequest becomes a real cancel
> once nextBinding() has been called. It sets the second boolean which is used
> in hasNext(). This 2-phase approach is critical since the cancel() method can
> be called at any time during a query execution by the external thread. And
> because the behavior of hasNext() is such that it has to return the *same*
> value until next() is called, this is the only way to guarantee semantic
> safety when cancel() is invoked (let me re-phrase this: it is the only way I
> was able to make it actually work)
> 3) cancel() does not close anything since it allows execution to finish
> normally and the client is responsible to call close() just like with a
> regular execution. Note that the client has to call cancel() explicitly
> (typically in another thread) and has to assume that the returning result set
> may be incomplete if this method is called (it is undetermined whether the
> result is _actually_ incomplete)
> 4) in order to deal with order-by and groups, I had to make two more changes.
> First, I had to make QueryIterSort and QueryIterGroup a slightly bit more
> lazy. Currently, the full result set is calculated during plan calculation.
> With my proposed adjustments, this full result set is called on the first
> call to any of its Iterator methods (e.g. hasNext). This change does not
> AFAIK affect the semantics. Second, because the desired behavior of
> cancelling a sort or group query is to make sure everything is sorted/grouped
> even if the total result set is not completed, I added an exception which
> reverses the cancellation request of the encompassing iterator (as an example
> see cancel() in QueryIterSort). This makes sure that the entire subset of
> found and sorted elements is returned, not just the first element. However,
> it also implies in the case of sort that when a query is cancelled, it will
> first sort the partially complete result set before returning to the client.
> the attached patch is based on ARQ 2.8.5 (and a few classes in TDB 0.8.7 ->
> possibly the other triple store implementations need adjustement as well)
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