High performance XML parser

[Piotr Szturmaj]

Tomek Sowiński wrote:
> I am now intensely accumulating information on how to go about creating a high-performance parser as it quickly became clear that my old one won't deliver. And if anything is clear is that memory is the key.
>
> One way is the slicing approach mentioned on this NG, notably used by RapidXML. I already contacted Marcin (the author) to ensure that using solutions inspired by his lib is OK with him; it is. But I don't think I'll go this way. One reason is, surprisingly, performance. RapidXML cannot start parsing until the entire document is loaded and ready as a random-access string. Then it's blazingly fast but the time for I/O has already elapsed. Besides, as Marcin himself said, we need a 100% W3C-compliant implementation and RapidXML isn't one.
>
> I think a much more fertile approach is to operate on a forward range, perhaps assuming bufferized input. That way I can start parsing as soon as the first buffer gets filled. Not to mention that the end result will use much less memory. Plenty of the XML data stream is indents, spaces, and markup -- there's no reason to copy all this into memory.
>
> To sum up, I belive memory and overlapping I/O latencies with parsing effort are pivotal.
>
> Please comment on this.
>

Few years ago I needed to write my own parser in Delphi for my XMPP 
(Jabber) client and server. In XMPP you get socket-streamed XML document 
with xml elements as protocol messages ("XMPP stanzas").
The problem I had was no Delphi parser had hybrid support of SAX/DOM, 
i.e. I wanted to parse xml nodes like SAX, but when I received whole 
message then I wanted to return it as XML Element (like in DOM). This 
way I could easily process incoming messages - now it's accomplishable 
with pull parsers.

I think std needs both SAX/pull and DOM parsers. For DOM, if whole 
document is in memory, maybe this approach could be advantageous:

http://en.wikipedia.org/wiki/VTD-XML
http://vtd-xml.sourceforge.net/