Re: [xml] Deep-copy of xsltStylesheetPtr?

[Rasmus Lerdorf <rasmus lerdorf com>]

On Sat, 18 Sep 2004, Daniel Veillard wrote:
> On Fri, Sep 17, 2004 at 04:46:40PM -0700, Rasmus Lerdorf wrote:
> > Probably a very lame question.  Feel free to RTFM me to the right set of
> > docs or an example somewhere.
> >
> > I am looking for a way to get a deep-copy of an xsltStylesheetPtr the same
> > way I can copy an xmlDocPtr using xmlMemSetup() and xmlCopyDoc().
>
>   Hum, there is no API for this, it was never needed so far.
> It may also be a big problem if you try to decouple memory areas, let me
> explain:
>    - we use string interning for document parsing, using a dictionnary
>      stored in doc->dict.
>    - when compiling a stylesheet we try to use the same set of strings
>      so inherit the doc->dict in the style->dict. In general the stylesheet
>      and the document(s) it was build from are deeply related. There
>      is pointers from the doc compiled elements to the stylesheet and
>      from the stylesheet to the doc. The stylesheet doc is modified
>      in the process.
> Copying the xsltStylesheetPtr would mean duplicating the doc first too.
> I think the simplest is to reparse the stylesheet. Honnestly it's usually
> a really fast operation, I doubt it would much cheaper to try to rebuild
> all the relationship between a copy of the original document and a new
> copy of the original stylesheet structure.
> Some explanations about the compilation are at
>   http://xmlsoft.org/XSLT/internals.html
>
> The dictionnary unification is also used for string interning of the
> processed document, which allows to use pointer comparison for a lot of
> internal operation when  the transformation occurs instead of doing
> string content compare.
> It seems you are using memory pools (the xmlMemSetup() trick) I think
> this makes copy even harder, it becomes impossible to reuse the original
> dictionnary and just increment its use count. Seems in such a situation
> the best is to copy the initial document and recompile the stylesheet.

A few more details on what I am doing.  By using xmlMemSetup()
and xmlCopyDoc() I am providing a way for PHP users to store an xmlDoc
struct in shared memory to be shared across Apache/PHP processes.  You get
some decent performance gains by only parsing XML files for an application
once instead of doing it on every single request.

Now I am trying to do the same for XSL transforms.  A very common thing
people do is fetch XML from some source and apply a stylesheet.  The
stylesheet rarely changes so I figured having it cached and ready to apply
without having to recompile it from disk on every request would speed
things up.  I obviously have no idea if it would be faster yet, and if you
tell me it wouldn't be I guess I should believe you.  ;)

If the stylesheet compile really is fast then I can just cache the raw
stylesheet document in shared memory as you suggested and recompile it
when I need it.  Is there perhaps some middle stage I could cache though?
Like reduce it to a set of tokens and store those tokens to speed up the
recompile?  Or store any other sort of hint that could in some way speed
up the recompile?

-Rasmus