[xep-support] XEP 371 multi-threading

[Nikolai Grigoriev]

David,

> I use xep to transform an xml file to xsl-fo and then render the pdf, 
> so i use xep to perform 2 steps. I was wondering if performance 
> gains could be achieved by separating out the transform step?

For XEP 3.7.x, there is a chance to increase performance 
if you do the transformation in a slightly different manner:

- make your transformer an XMLReader (e.g. by using
  SAXTransformerFactory.newTransformerHandler(...);
- specify it as parser to create a SAXSource, and then pass
  the result to FormatterImpl.render().

(This advice is applicable only to 3.7: we expect that XEP 3.8
will have the identical performance when called either way).

Additional tips:

1. Try not to call cleanup() too often. This method cleans up
internal caches  for fonts and images, and subsequent calls
to the formatter will have to re-parse the fonts (which actually
takes quite a  bit of time). If you are using a closed set of
images and fonts, than it is advisable to call cleanup()
only once, when the FormatterImpl object is disposed of.

2. FormatterImpl is a costly object, but it's thread safe. 
Unless you need several different configurations simultaneously, 
you can consider creating a single static instance, and access it from 
several threads. You can easily create many FOTransformer 
objects from a single Formatter: they don't interfere with each other. 

(Please note that FOTransformer is reusable but _not_ thread safe: 
it can transform several documents in sequence, but not in 
parallel. This limitation is due to JAXP, and is outside our control). 

> Perhaps I could use Xalan or Saxon to do the transform and 
> pass xep the xslfo to render? I beleive xep uses JAXP...and
> maybe using another transformation engine (with freedom 
> to optimise the engine) woudl help. 

JAXP is just an interface, implemented by both Saxon and Xalan.
You can control which engine is picked by setting a system
property, javax.xml.transform.TransformerFactory, to the
class name of the desired implementation. Please also note
that FOTransformer can be constructed from a JAXP Transformer
object; you can pass an explicitly created Xalan or Saxon 
implementation there. 

Regards,
Nikolai Grigoriev
RenderX


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