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# Wednesday, 21 April 2010

When time has come to process big xml log files we've decided to implement streamable tree in saxon the very same way it was implemented in .net eight years ago (see How would we approach to streaming facility in xslt).

It's interesting enough that the implementation is similar to one of composable tree. There a node never stores a reference to a parent, while in the streamed tree no references to children are stored. This way only a limited subview of tree is available at any time. Implementation does not support preceding and preceding-sibling axes. Also, one cannot navigate to a node that is out of scope.

Implementation is external (there are no changes to saxon itself). To use it one needs to create an instance of DocumentInfo, which pulls data from XMLStreamReader, and to pass it as an input to a transformation:

Controller controller = (Controller)transformer;
XMLInputFactory factory = XMLInputFactory.newInstance();
StreamSource inputSource = new StreamSource(new File(input));
XMLStreamReader reader = factory.createXMLStreamReader(inputSource);
StaxBridge bridge = new StaxBridge();

bridge.setPipelineConfiguration(
  controller.makePipelineConfiguration());
bridge.setXMLStreamReader(reader);
inputSource = new DocumentImpl(bridge);

transformer.transform(inputSource, new StreamResult(output));

This helped us to format an xml log file of arbitrary size. An xslt like this can do the work:

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns="http://www.w3.org/1999/xhtml"
  exclude-result-prefixes="xs">

  <xsl:template match="/log">
    <html>
      <head>
        <title>Log</title>
      </head>
      <body>
        <xsl:apply-templates/>
      </body>
    </html>
  </xsl:template>

  <xsl:template match="message">
   ...
  </xsl:template>

  <xsl:template match="message[@error]">
    ...
  </xsl:template>

  ...

</xsl:stylesheet>

Implementation can be found at: streamedtree.zip

Wednesday, 21 April 2010 07:10:34 UTC  #    Comments [0] -
Announce | Thinking aloud | xslt
# Thursday, 15 April 2010

jxom else if (google search)

Google helps with many things but with retrospective support.

Probably guy's trying to build a nested if then else jxom elements.

We expected this and have defined a function t:generate-if-statement() in java-optimizer.xslt.

Its signature:

<!--
  Generates if/then/else if ... statements.
    $closure - a series of conditions and blocks.
    $index - current index.
    $result - collected result.
    Returns if/then/else if ... statements.
-->
<xsl:function name="t:generate-if-statement" as="element()">
  <xsl:param name="closure" as="element()*"/>
  <xsl:param name="index" as="xs:integer"/>
  <xsl:param name="result" as="element()?"/>

Usage is like this:

<!-- Generate a sequence of pairs: (condition, scope). -->
<xsl:variable name="branches" as="element()+">
  <xsl:for-each select="...">
    <!-- Generate condition. -->
    <scope>
        <!-- Generate statemetns.  -->
    </scope>
  </xsl:for-each>
</xsl:variable>

<xsl:variable name="else" as="element()?">
  <!-- Generate final else, if any. -->
</xsl:variable>

<!-- This generates if statement. -->
<xsl:sequence
  select="t:generate-if-statement($branches, count($branches) - 1, $else)"/>

P.S. By the way, we like that someone is looking into jxom.

Thursday, 15 April 2010 06:59:01 UTC  #    Comments [0] -
Tips and tricks | xslt
# Friday, 09 April 2010

By the generator we assume a function that produces an infinitive output sequence for a particular input.

That's a rather theoretical question, as xslt does not allow infinitive sequence, but look at the example:

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:t="http://www.nesterovsky-bros.com/xslt"
  exclude-result-prefixes="xs t">

  <xsl:template match="/">
    <xsl:variable name="value" as="xs:string" select="'10100101'"/>

    <xsl:variable name="values" as="xs:integer+" select="t:generate(1)"/>

    <!--<xsl:variable name="values" as="xs:integer+">
      <xsl:call-template name="t:generate">
        <xsl:with-param name="value" select="1"/>
      </xsl:call-template>
    </xsl:variable>-->

    <xsl:variable name="integer" as="xs:integer" select="
      sum
      (
        for $index in 1 to string-length($value) return
          $values[$index][substring($value, $index, 1) = '1']
      )"/>

    <xsl:message select="$integer"/>
  </xsl:template>

  <xsl:function name="t:generate" as="xs:integer*">
    <xsl:param name="value" as="xs:integer"/>

    <xsl:sequence select="$value"/>
    <xsl:sequence select="t:generate($value * 2)"/>
  </xsl:function>

  <!--<xsl:template name="t:generate" as="xs:integer*">
    <xsl:param name="value" as="xs:integer"/>

    <xsl:sequence select="$value"/>

    <xsl:call-template name="t:generate">
      <xsl:with-param name="value" select="$value * 2"/>
    </xsl:call-template>
  </xsl:template>-->

</xsl:stylesheet>

Here the logic uses such a generator and decides by itself where to break.

Should such code be valid?

From the algorithmic perspective example would better to work, as separation of generator logic and its use are two different things.

Friday, 09 April 2010 14:38:34 UTC  #    Comments [0] -
Thinking aloud | xslt

Lately, after playing a little with saxon tree models, we thought that design would be more cleaner and implementation faster if NamePool were implemented differently.

Now, saxon is very pessimistic about java objects thus it prefers to encode qualified names with integers. The encoding and decoding is done in the NamePool. Other parts of code use these integer values.

Operations done over these integers are:

  • equality comparision of two such integers in order to check whether to qualified or extended names are equal;
  • get different parts of qualified name from NamePool.

We would design this differently. We would:

  1. create a QualifiedName class to store all name parts.
  2. declare NamePool to create and cache QualifiedName instances.

This way:

  • equality comparision would be a reference comparision of two instances;
  • different parts of qualified name would become a trivial getter;
  • contention of such name pool would be lower.

That's the implementation we would propose: QualifiedName.java, NameCache.java

Friday, 09 April 2010 13:05:30 UTC  #    Comments [0] -
Thinking aloud | xslt
# Thursday, 08 April 2010

Earlier, in the entry "Inline functions in xslt 2.1" we've described an implementation of xml tree model that may share subtrees among different trees.

This way, in a code:

<xsl:variable name="elements" as="element()*" select="..."/>

<xsl:variable name="result" as="element()">
  <result>
    <xsl:sequence select="$elements"/>
  </result>
</xsl:variable>

the implementation shares internal representation among $elements and subtree of $result. From the perspective of xslt it looks as completely different subtrees with different node identities, which is in the accordance with its view of the world.

After a short study we've decided to create a research implementation of this tree model in saxon. It's took only a couple of days to introduce a minimal changes to engine, to refactor linked tree into a new composable tree, and to perform some tests.

In many cases saxon has benefited immediately from this new tree model, in some other cases more tunings are required.

Our tests've showed that this new tree performed better than linked tree, but a little bit worser than tiny tree. On the other hand, it's obvious that conventional code patterns avoid subtree copying, assuming it's expensive operation, thus one should rethink some code practices to benefit from composable tree.

Implementation can be downloaded at: saxon.composabletree.zip

Thursday, 08 April 2010 06:26:02 UTC  #    Comments [0] -
Announce | Thinking aloud | xslt
# Sunday, 04 April 2010

From the web we know that xslt WG is thinking now on how to make xslt more friendly to a huge documents. They will probably introduce some xslt syntax to allow implementation to be ready for a such processing.

They will probably introduce an indicator marking a specific mode for streaming. XPath in this mode will probably be restricted to a some its subset.

The funny part is that we have implemented similar feature back in 2002 in .net. It was called XPathForwardOnlyNavigator.

Implementation stored only several nodes at a time (context node and its ancestors), and read data from XmlReader perforce. Thus one could navigate to ancestor elements, to children, and to the following siblings, but never to any previous node. When one tried to reach a node that was already not available we threw an exception.

It was simple, not perfect (too restrictive) but it was pluggable in .net's xslt, and allowed to process files of any size.

That old implementation looks very attractive even now in 2010. We expect that WG with their decisions will not rule out such or similar solutions, and will not force implementers to write alternative engine for xslt streaming.

Sunday, 04 April 2010 20:53:27 UTC  #    Comments [0] -
Thinking aloud | xslt
# Friday, 02 April 2010

Xslt 1.0 has been designed based on the best intentions. Xslt 2.0 got a legacy baggage.

If you're not entirely concentrated during translation of your algorithms into xslt 2.0 you can get into trap, as we did.

Consider a code snapshot:

<xsl:variable name="elements" as="element()+">
  <xsl:apply-templates/>
</xsl:variable>

<xsl:variable name="converted-elements" as="element()+"
  select="$elements/t:convert(.)"/>

Looks simple, isn't it?

Our intention was to get converted elements, which result from some xsl:apply-templates logic.

Well, this code works... but rather sporadically, as results are often in wrong order! This bug is very close to what is called a Heisenbug.

So, where is the problem?

Elementary, my dear Watson:

  1. xsl:apply-templates constructs a sequence of rootless elements.
  2. $elements/t:convert(.) converts elements and orders them in document order.

Here is a tricky part:

The relative order of nodes in distinct trees is stable but implementation-dependent...

Clearly each rootless element belongs to a unique tree.

After that we have realized what the problem is, code has been immediately rewritten as:

<xsl:variable name="elements" as="element()+">
  <xsl:apply-templates/>
</xsl:variable>

<xsl:variable name="converted-elements" as="element()+" select="
  for $element in $elements return
    t:convert($element)"/>

P.S. Taking into an accout a size of our xslt code base, it took a half an hour to localize the problem. Now, we're at position to review all uses of slashes in xslt. As you like it?

Friday, 02 April 2010 17:53:18 UTC  #    Comments [0] -
Thinking aloud | xslt
# Saturday, 27 March 2010

Opinions on xml namespaces

olegtk: @srivatsn Originally the idea was that namespace URI would point to some schema definition. Long abandoned idea.

Not so long ago, I've seen a good reasoning about the same subject:

Saturday, 27 March 2010 09:49:45 UTC  #    Comments [0] -
xslt
# Monday, 22 March 2010

Inline functions in xslt 2.1 look often as a some strange aberration. Sure, there are very usefull cases when they are delegates of program logic (e.g. comparators, and filters), but often (probably more often) we can see that it's use is to model data structures.

As an example, suppose you want to model a structure with three properties say a, b, and c. You implement this creating functions that wrap and unwrap the data:

function make-data($a as item(), $b as item(), $c as item()) as function() as item()+
{
  function() { $a, $b, $c }
}

function a($data as function() as item()+) as item()
{
  $data()[1]
}

function b($data as function() as item()+) as item()
{
  $data()[2]
}

function c($data as function() as item()+) as item()
{
  $data()[3]
}

Clever?

Sure, it is! Here, we have modeled structrue with the help of sequence, which we have wrapped into a function item.

Alas, clever is not always good (often it's a sign of a bad). We just wanted to define a simple structure. What it has in common with function?

There is a distance between what we want to express, designing an algorithm, and what we see looking at the code. The greater the distance, the more efforts are required to write, and to read the code.

It would be so good to have simpler way to express such concept as a structure. Let's dream a little. Suppose you already have a structure, and just want to access its members. An idea we can think about is an xpath like access method:

$data/a, $data/b, $data/c

But wait a second, doesn't $data looks very like an xml element, and its accessors are just node tests? That's correct, so data constructor may coincide with element constructor.

Then what pros and cons of using of xml elements to model structures?

Pros are: existing xml type system, and sensibly looking code (you just understand that here we're constructing a structure).

Cons are: xml trees are implemented the way that does not assume fast (from the perfromace perspective) composition, as when you construct a structure a copy of data is made.

But here we observe that "implemented" is very important word in this context. If xml tree implementation would not store reference to the parent node then subtrees could be composed very efficiently (note that tree is assumed to be immutable). Parent node could be available through a tree navigator, which would contain reference to a node itself and to a parent tree navigator (or to store child parent map somewhere near the root).

Such tree structure would probably help not only in this particular case but also with other conventional xslt code patterns.

P.S. Saxon probably could implement its NodeInfo this way.

Update: see also Custom tree model.

Monday, 22 March 2010 11:02:07 UTC  #    Comments [0] -
Thinking aloud | xslt
# Monday, 15 March 2010

A while ago we have proposed to introduce maps as built-in types in xpath/xquery type system: Tuples and maps.

The suggestion has been declined (probably our arguments were not convincing). We, however, still think that maps along with sequences are primitives, required to build sensible (not obscure one) algorithms. To see that map is at times is the best way to resolve the problems we shall refer to an utility function to allocate names in scope. Its signature looks like this:

<!--
Allocates unique names in the form $prefix{number}?.
Note that prefixes may coincide.
$prefixes - a name prefixes.
$names - allocated names pool.
$name-max-length - a longest allowable name length.
Returns unique names.
-->
<xsl:function name="t:allocate-names" as="xs:string*">
  <xsl:param name="prefixes" as="xs:string*"/>
  <xsl:param name="names" as="xs:string*"/>
  <xsl:param name="name-max-length" as="xs:integer?"/>

Just try to program it and you will find yourselves coding something like one defined at cobolxom.

To be efficient such maps should provide, at worst, a logarithmic operation complexity:

  • Access to the map through a key (and/or by index) - complexity is LnN;
  • Creation of a new map with added or removed item - complexity is LnN;
  • Construction of the map from ordered items - complexity is O(N);
  • Construction of the map from unordered items - complexity is O(N*LnN);
  • Total enumeration of all items - complexity is O(N*LnN);

These performance metrics are found in many functional and procedural implementations of the maps. Typical RB and AVL tree based maps satisfy these restrictions.

What we suggest is to introduce map implementation into the exslt2 (assuming inline functions are present). As a sketch we have implemented pure AVL Tree in Xslt 2.0:

We do not assume that implementation like this should be used, but rather think of some extension function(s) that provides a better performance.

What do you think?

Monday, 15 March 2010 13:59:19 UTC  #    Comments [1] -
Thinking aloud | xslt
# Sunday, 28 February 2010

The story about immutable tree would not be complete without xslt implementation. To make it possible one needs something to approxomate tree nodes. You cannot implement such consruct efficiently in pure xslt 2.0 (it would be either unefficient or not pure).

To isolate the problem we have used tuple interface:

  • tuple:ref($items as item()*) as item() - to wrap items into a tuple;
  • tuple:deref($tuple as item()?) as item()* - to unwrap items from a tuple;
  • tuple:is-same($first as item(), $second as item()) as xs:boolean - to test whether two tuples are the same.

and defined inefficient implementation based on xml elements. Every other part of code is a regular AVL algorithm implementation.

We want to stress again that even assuming that there is a good tuple implementation we would prefer built-in associative container implementation. Why the heck you need to include about 1000 lines of code just to use a map?

Source code is:

Sunday, 28 February 2010 19:28:07 UTC  #    Comments [0] -
Thinking aloud | xslt

We like Visual Studio very much, and try to adopt new version earlier.

For the last time our VS's use pattern is centered around xml and xslt. In our opinion VS 2008 is the best xslt 2 editor we have ever seen even with lack of support of xslt 2.0 debugging.

Unfortunatelly, that is still a true when VS 2010 is almost out. VS 2008 is just 2 - 3 times faster. You can observe this working with xslt files like those in languages-xom.zip (1000 - 2000 rows). Things just become slow.

We still hope that VS 2010 will make a final effort to outdo what VS 2008 has already delivered.

Sunday, 28 February 2010 18:37:58 UTC  #    Comments [0] -
Thinking aloud | xslt
# Thursday, 25 February 2010

While bemoaning about lack of associative containers in xpath type system, we have came up with a good implementation of t:allocate-names(). Implementation can be seen at location cobol-names.xslt.

It is based on recursion and on the use of xsl:for-each-group. Alogrithmic worst case complexity is O(N*LogN*LogL), where N is number of names, and L is a length of a longest name.

This does not invalidate the idea that associative containers are very wishful, as blessed one who naturally types such implementation. For us, it went the hard way, and has taken three days to realize that original algorithm is problematic, and to work out the better one.

In practice this means 2 seconds for the new implementation against 25 minutes for the old one.

Thursday, 25 February 2010 07:19:06 UTC  #    Comments [0] -
xslt
# Wednesday, 24 February 2010

Ladies and gentlemen of the jury...

Xml Editor in VS 2010

Wednesday, 24 February 2010 21:00:36 UTC  #    Comments [7] -

Why do we return to this theme again?

Well, it's itching!

In cobolxom there is an utility function to allocate names in scope. Its signature looks like this:

<!--
  Allocates unique names in the form $prefix{number}?.
  Note that prefixes may coincide.
    $prefixes - a name prefixes.
    $names - allocated names pool.
    $name-max-length - a longest allowable name length.
    Returns unique names.
-->
<xsl:function name="t:allocate-names" as="xs:string*">
  <xsl:param name="prefixes" as="xs:string*"/>
  <xsl:param name="names" as="xs:string*"/>
  <xsl:param name="name-max-length" as="xs:integer?"/>

We have created several different implementations (all use recursion). Every implementation works fair for relatively small input sequences, say N < 100, but we have cases when there are about 10000 items on input. Algorithm's worst case complexity, in absence of associative containers, is O(N*N), and be sure it's an O-o-o-oh... due to xslt engine implementation.

If there were associative containers with efficient access (complexity is O(LogN)), and construction of updated container (complexity is also O(LogN)) then implementation would be straightforward and had complexity O(N*LogN).

Wednesday, 24 February 2010 07:34:07 UTC  #    Comments [0] -
Thinking aloud | xslt
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