[xsl] Re: [saxon] Questions about the `saxon:threads` extension attribute

["Michael Kay mike@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Quite a difficult question to answer accurately because I haven't looked at
the code for a while and the logic isn't all in one place; it also depends on
understanding the behaviour of the underlying Java services that we rely on,
notably the ExecutorService.

Q1. Firstly, calling fn:error() is exactly the same as any other dynamic
error. We throw an XPathException; this is caught by the
MultithreadedContextMappingIterator, this invokes ExecutorService.shutdown()
which prevents new tasks being accepted but allows existing tasks to finish. A
"task" here is the processing of a single item in the for-each selection.

Q2. There are two cases to consider with try/catch: where the try/catch is
local to one thread (within the multi-threading for-each), and where it is
outside the multithreading for-each. In the first case there should be no
effect on other threads. In the second case the xsl:for-each fails as a whole,
and the error is caught as a whole.

Q3. We don't take any account of fn:unordered(); we always respect the
ordering.

This is complicated by the fact that xsl:for-each may be executed in either
pull or push mode. Push mode is generally used when writing to a tree, pull
mode when evaluating (non-document) variables and functions. In push mode you
can never see any results until they are complete. In pull mode, items are
delivered as soon as they are available; so a reference to $var[1] may return
a result even though $var[2] has not yet been computed, and this applies to
multithreaded execution just as much as to single-threaded execution.
Multithreading complicates it, because $var[2] may be computed before $var[1],
but we won't allow you to see $var[2] in that situation; it will be sitting in
a queue somewhere waiting to be added to the result.

Michael Kay

> On 27 Dec 2019, at 05:18, Dimitre Novatchev <dnovatchev@xxxxxxxxx> wrote:
>
> The questions below are essentially for Dr. Kay, though anyone interested
and able to shed light on these is welcome.
>
> I need this information, because it seems not to be available in the Saxon
documentation.
>
>
> General setup:  We have multi-threaded processing specified by
saxon:threads=b2b
>
>
> Q1. What will happen if thread1 calls the fn:error() function? Will the
execution of thread2 be terminated immediately/promptly, or will it continue
executing until the end of its processing?
>
>
> Q2. Same as Q1 above, but the multi-threaded processing is enclosed in
<xsl:try> and the error thrown by the fn:error() function is caught in the
`<xsl:catch>` child of `<xsl:try>`. Then `<xsl:catch>` produces a normal value
(the error is not re-thrown) b will the 2nd thread be suppressed or will it
continue executing until the end of its processing?
>
>
> Q3.  This time there is no error thrown. Can we access the result of just
one of the threads (whichever finishes first) even before the other thread has
finished? For example, if the results of the two threads are in the sequence
constructor of an `<xsl:variable select=bvmultithreadingResultsb>`, is it
possible to access the result of the first finished thread in an expression
like:
>
> fn:unordered($vmultithreadingResults)[1] ?
>
>
> --
> Cheers,
> Dimitre Novatchev
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