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Andy Seaborne commented on JENA-29:
-----------------------------------
Re: 2) The use case that several groups have is to set an upper cost bound on a
query issued by SPARQL protocol through Fuseki or
Joseki. No server-side application specific behaviour. In fact, if the
application can be involved, the scope for control is greater. In the case
inside Fuseki, there is no application specifc code and the default behaviour
should be most appropriate for multiple use cases.
Suppose there are 100 countries in a database with their populations. The query
is to find the top 10 most populous countries, that is ORDER BY + LIMIT 10. If
the sort engine has seen 20 countries and is stopped, 10 results appear in the
"do your best" case. But if China isn't in the result, then it is going to be a
surprise.
But note also the application maybe doing "top 10", "next 10" in pages in the
application logic - ORDER BY and no LIMIT. Simon - you described the contract
for sort as "in either S or TS, it holds that s1<=s2 according to the given
ordering criteria" which is true. But the contract goes further, item one is
the max or min, the first ten are the top 10.
Choices I see are:
1/ The cancel call can take an optional argument giving the style.
2/ The hasNext/next indicates "end" but can be continued to get remaining stuff.
3/ It's a configuration setting of the execution.
Of the these, 1 or 3 tells the cancellation mechanism what is required of it in
doign the cancellation, not after the cancellation as happen in 2. I prefer 3
as it's the earliest point the execution plan can be informed and there's a
mechanism that's already in place (e.g. union query).
Simon - your clients can get the behaviour you currently provide them by
setting the context for the execution.
> 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,
> queryIterRepeatApply.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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