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https://issues.apache.org/jira/browse/JENA-29?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12995014#comment-12995014
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Simon Helsen commented on JENA-29:
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Q1: We have concrete use-cases to get partial results, so I really have to
stress we need that support very strongly. As for what exactly a truncated
answer is, of course, you need to define what it means. But in the case of
sorting, it is accepted by our clients that the returned result set S is a
subset of TS (where TS is the complete result set if it was allowed to finish)
and for all s1,s2 in either S or TS, it holds that s1<=s2 according to the
given ordering criteria. My patch does exactly that and does a good job at
making S as large as possible until the cancel() is invoked. Without the
changes in the sorting iterator, I would only ever see 1 result, which is
useless for us
Q2: yes and no. I need to keep repeating that the current version of abort() is
not thread-safe. If you call it from a secondary thread, it will cause
exceptions in the main execution thread. If you can live with that, then, yes,
you could use abort or even close since either way, you need to catch all
exceptions in both the thread which performs the cancel as well as the thread
which executes. The way the cancel() patch has been implemented, this is not
needed. It is safe being called from an outside thread to then safely finish
the cancelled query and return as many results as was possible up to that point
While I cannot speak for the requirements of other stakeholders, we need the
partial results, which is why I made all the changes you see in the patches.
Moreover, if you want partial results, you want a graceful completion of the
execution, making either abort or close unusable
> 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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