[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Disk-based XPath Processing
Uche Ogbuji wrote: > Tatu Saloranta wrote: > >>Alas, although there is quite a bit of interest, I >>haven't seen solutions where streaming parsers could >>use some suitable subset of XPath to match sub-trees >>(suitable meaning that only some axes were supported, >>parent/grandparent, attribute, children, but not >>sibling). I have been hoping to investigate doing this >>myself in near future, since it would seem to simplify >>some streaming-oriented tasks (like only building >>small sub-trees, or one sub-tree at a time from a >>bigger document). > > > What you describe in the above para is pretty much exactly what Amara's > pushbind and pushdom allow, and the trimxml tool that John L. Clark > mentions, exposes this approach on the command line. They use a subset > of XSLT patterns (which are themselves a subset of XPath, as defined > int he XSLT 1.0 spec) to drive a streamable operation that only loads > into memory one subtree at a time from a larget document. I think it > does still need a little baking, but I've been successful using it for > some pretty heavy-duty work. > hi, I have been working few months ago on XPath filtering on SAX streams ; it support XPath patterns with predicates and forward axes, etc, like this : a[@b] a[not(@b)] a[@b='c'] a[@b='c']/d[@e] /a/b/c[1] a/*[2] a/comment()[3] a/node()[position() < 4] /a/b/c[last()] a/*[count() > 3] a/node()[last()] a[following-sibling::b] a[b] a[*[not(self::b)]] id("foo") id("foo")/child::para[position()=5]/a/b/c[last()] but you should be aware that : -when parsing, if you use an expression that consist on reading the whole tree, the whole tree will be cached, and you should use DOM instead ; that is to say if you do silly things, you'll get them ; if you have a really huge XML file, don't do such things otherwise you'll get an OutOfMemory error -when a node has been discarded, you can't reach it again : revert axes (except ancestor axes) are not available ; the sole thing you can do is to anticipate by storing a part of the tree in a DOM fragment and work with it, then discard it the technique used is described on http://reflex.gforge.inria.fr/saxPatterns.html (this is a preview) the implementation is in Java and is part of the RefleX engine (http://reflex.gforge.inria.fr/) ; unfortunately, I didn't have yet published the last release with all that stuff ; however, you can browse the SVN repository if you are (very very very) curious : https://gforge.inria.fr/plugins/scmsvn/viewcvs.php/root/src/java/org/inria/reflex/xml/filter/?rev=104&root=reflex https://gforge.inria.fr/plugins/scmsvn/viewcvs.php/root/src/java/org/inria/reflex/xml/sax/?root=reflex the new version of RefleX to come will supply a set of tags that allow to filter SAX streams with XPath patterns ; here are common use cases that XSLT users should find easy to understand : <xcl:filter xmlns:xcl="http://www.inria.fr/xml/active-tags/xcl"> <!-- copy --> <xcl:rule pattern="copy"> <xcl:forward> <xcl:apply-rules/> </xcl:forward> </xcl:rule> <!--delete the element and its content--> <xcl:rule pattern="deleteElem"/> <!-- ignore an element, but apply rules on its content --> <xcl:rule pattern="ignoreElem"> <xcl:forward> <insertedBefore/> </xcl:forward> <xcl:apply-rules/> <xcl:forward> <insertedAfter/> </xcl:forward> </xcl:rule> <!--insert a container--> <xcl:rule pattern="content"> <xcl:forward> <insertedContainer> <xcl:apply-rules/> </insertedContainer> </xcl:forward> </xcl:rule> <!--remove an attribute--> <xcl:rule pattern="removeAttr"> <xcl:remove parent="{ . }" referent="{ @bar }"/> <xcl:forward> <xcl:apply-rules/> </xcl:forward> </xcl:rule> <!--remove all attributes--> <xcl:rule pattern="removeAllAttr"> <xcl:remove parent="{ . }" referent="{ @* }"/> <xcl:forward> <xcl:apply-rules/> </xcl:forward> </xcl:rule> <!--change the value of an attribute--> <xcl:rule pattern="changeAttr"> <xcl:attribute referent="{ . }" name="foo" value="foo"/> <xcl:forward> <xcl:apply-rules/> </xcl:forward> </xcl:rule> </xcl:filter> A filter reads entirely one or several inputs, and can produce several outputs. Unlike XSLT, an XCL filter traverses each input tree in its natural order only. More complex processes that require deep structure transformations should be considered with XSLT. XCL filters are suitable when processes are localized on independant chunks of datas, which is advantageous for stream-processing of large inputs, although XCL filters can be also convenient for traversing automatically a DOM tree. By combining other active tags with the small set defined here, it is yet possible to achieve efficient pipeline processes. of course, you can combine these basic structures at will, as long as you use a single <xcl:apply-rules/> element of course, several filters can be connected to a pipeline, including steps that are involving XSLT filtering and XInclude processing of course, this kind of filter will be appliable on SAX streams and DOM trees, at user option -- Cordialement, /// (. .) --------ooO--(_)--Ooo-------- | Philippe Poulard | ----------------------------- http://reflex.gforge.inria.fr/ Have the RefleX !
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