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AOLserver

XML

XML support in AOLserver

cvs -d:pserver:[email protected]:/cvsroot/aolserver login
cvs -z3 -d:pserver:[email protected]:/cvsroot/aolserver co nsxml

As of Dec. 2000 Linux distributions usually come with version 1.x of libxml library so chances are that you'll need to install 2.x by yourself (this will change in the future since everyone is migrating to 2.x). To install nsxml module go into nsxml directory, optionally edit a path in Makefile to point into AOLserver source directory. Then run make. You should get nsxml.so module that should be placed in AOLserver bin directory (the same that has main nsd executable). Add the following to your nsd.tcl config file:

ns_section "ns/server/${servername}/modules"
ns_param   nsxml           ${bindir}/ns_xml.so

tail -f $AOLSERVERDIR/log/server.log

XML Quick reference

A simple example

• use ns_xml parse $xml_doc to parse XML document in string $xml_doc and get its document id
• use ns_xml doc root $doc_id to get the id of a root node
• use ns_xml node children $node_id to traverse document tree and ns_xml node ... commands to get node content and attributes

In code it could look like this:

proc dump_node {node_id level} {
    set name [ns_xml node name $node_id]
    set type [ns_xml node type $node_id]
    set content [ns_xml node getcontent $node_id]
    ns_write "<li>"
    ns_write "node id=$node_id name=$name type=$type"
    if { [string compare $type "attribute"] != 0 } {
        ns_write " content=$content\n"
    }
}

proc dump_tree_rec {children} {
    ns_write "<ul>\n"
    foreach child_id $children {
        dump_node $child_id
        set new_children [ns_xml node children $child_id]
        if { [llength $new_children] > 0 } {
            dump_tree_rec $new_children
        }
    }
}

proc dump_tree {node_id} {
    dump_tree_rec [list $node_id] 0
}

proc dump_doc {doc_id} {
    ns_write "doc id=$doc_id<br>\n"
    set root_id [ns_xml doc root $doc_id]
    dump_tree $root_id
}

set xml_doc "<test version="1.0">this is a
<blind>test</blind> of xml</test>"
set doc_id [ns_xml parse $xml_doc]
dump_doc $doc_id    

if { [catch {set doc_id [ns_xml parse $xml_doc]} err] } {
    ns_write "There was an error parsing the following XML document: "
    ns_write [ns_quotehtml $xml_doc]
    ns_write "Error message is:"
    ns_write [ns_quotehtml $err]
    ns_write "\n"
    return
}

See how the code works in practice [external site running AOLserver] and get the full source [included in Linux Gazette]. It's a bit more complex than the above snippet. You can see the structure of an arbitrary XML document by typing it in the provided text area. The script also shows how to parse form data and has more robust error handling.

Real life example

In the past it could've been done in a rather distasteful way by grabbing the whole HTML page and trying to extract relevant information. It would be hard to program and fragile (a change in the way HTML page is generated would most likely break such parsing).

Today the site that wants to provide headlines for others can publish this data in an easily to parse XML format under some URL. In our case the data are provided at http://www.linuxtoday.com/backend/linuxtoday.xml. See the format of this file (using previously developed script).

As you can see XML document represent headlines on LinuxToday site. It is a set of stories, each story having title, url, author etc. We know that after parsing the XML document we would like to have a way to easily extract the information. Let's use a "wishful-thinking" (in other words top-down) method of writing the code advocated in a Structure and interpretation of computer programs (a truly great CS book). Let's assume that we've converted XML representation into an object. To build an HTML table showing the data we need the following procedures:

• get total number of stories: headlines_get_stories_count $headlines
• get n-th story: headlines_get_story $headline $story_no
• get URL of a given story: story_get_url $story
• get title of a given story: story_get_title $story

Having those procedures we can generate the simplest (but rather ugly) table:

proc story_to_html_table_row { story } {
    set url [story_get_url $story]
    set title [story_get_title $story]
    return "- <a href=\"$url\"><font color=#000000>$title</font></a><br>\n"
}

# given headlines generate HTML code of the table with this data
proc headlines_to_html_table { headlines } {
    set to_return "<table border=0 cellspacing=1 cellpadding=3>"
    append to_return "<tr><td><small>"

    set stories_count [headlines_get_stories_count $headlines]
    for {set i 0} {$i < $stories_count} {incr i} {
        set story [headlines_get_story $headlines $i]
        append to_return [story_to_html_table_row $story]
    }

    append to_return "</td></tr></table>\n"
    return $to_return
}

proc headlines_get_stories_count { headlines } {
    return [llength $headlines]
}

proc headlines_get_story { headlines story_no } {
    return [lindex $headlines $story_no]
}

proc story_get_url { story } {
    return [lindex $story 0]
}

proc story_get_title { story } {
    return [lindex $story 1]
}

set stories_count [headlines_get_stories_count $headlines]
for {set i 0} {$i < $stories_count} {incr i} {
    set story [headlines_get_story $headlines $i]
    append to_return [story_to_html_table_row $story]
}

foreach story $headlines {
    append to_return [story_to_html_table_row $story]
}

# does a name of the node identified by $node_id equals $name
proc is_node_name_p { node_id name } {
    set node_name [ns_xml node name $node_id]
    if { [string_equal_p $name $node_name] } {
        return 1
    } else {
        return 0
    }
}

# does a type of the node identified by $node_id equals $type
proc is_node_type_p { node_id type } {
    set node_type [ns_xml node type $node_id]
    if { [string_equal_p $type $node_type] } {
        return 1
    } else {
        return 0
    }
}

# is this an node of type "attribute"?
proc is_attribute_node_p { node_id } {
    return [is_node_type_p $node_id "attribute"]
}

# raise an error if node name is different than $name
proc error_if_node_name_not {node_id name} {
    if { ![is_node_name_p $node_id $name] } {
        set node_name [ns_xml node name $node_id]
        error "node name should be $name and not $node_name"
    }
}

# raise an error if node type is different than $type
proc error_if_node_type_not {node_id type} {
    if { ![is_node_type_p $node_id $type] } {
        set node_type [ns_xml node type $node_id]
        error "node type should be $type and not $node_type"
    }
}

# given url and title construct a story object with
# those attributes
proc define_story { url title } {
    return [list $url $title]
}

# convert a node of name "story" into an object
# that represents story
proc story_node_to_story {node_id} {
    set url ""
    set title ""
    # go through all children and extract content of url and title nodes
    set children [ns_xml node children $node_id]
    foreach node_id $children {
        # we're only interested in nodes whose name is "url" or "title"
        if { [is_attribute_node_p $node_id]} {
            if { [is_node_name_p $node_id "url"] || [is_node_name_p $node_id "title"]} {
                set node_children [ns_xml node children $node_id]
                # those should only have one children node with
                # the name "text" and type "cdata_section"
                if { [llength $node_children] != 1 } {
                    set name [ns_xml node name $node_id]
                    error "$name node should only have 1 child"
                }
                set one_node_id [lindex $node_children 0]
                error_if_node_type_not $one_node_id "cdata_section"
                error_if_node_name_not $one_node_id "text"
                set txt [ns_xml node getcontent $one_node_id]
                if { [is_node_name_p $node_id "url"] } {
                    set url $txt
                }
                if { [is_node_name_p $node_id "title"]} {
                    set title $txt
                }
            }
        }
    }
    return [define_story $url $title]
}

# convert XML doc to headlines object
proc xml_to_headlines { doc_id } {
    set headlines [list]
    set root_id [ns_xml doc root $doc_id]
    # root node should be named "linuxtoday" and of type "attribute"
    error_if_node_name_not $root_id "linuxtoday"
    error_if_node_type_not $root_id "attribute"
    set children [ns_xml node children $root_id]
    foreach node_id $children {
        # only interested in attribute type nodes whose name is "story"
        if { [is_node_name_p $node_id "story"] && [is_attribute_node_p $node_id]} {
            set story [story_node_to_story $node_id]
            lappend headlines $story
        }
    }
    return $headlines
}

Having an intermediate representation of the data might look like an excess given that it costs us more code and some performance but there are very good reasons to have it. We could have written a proc xml_to_html_table that would create HTML table directly from XML document but such code would be more complex, more buggy and harder to modify. Separation that we've made provides an abstraction that reduces complexity, which is always good. It also gives us more flexibility: we can easily imagine writing another headlines_to_html_table procedure that gives us slightly different table.

See how it works in practice [external site running AOLserver] and get the source [included in Linux Gazette]. It should produce something like this:

One thing missing in this code is caching. As it is, it will grab the XML file from other people's server everytime it is invoked. This is not nice. It would be fairly easy to add a logic to cache XML file (or its in-memory representation) and only fetch a new version if, say, 1 hour passed since it was last retrieved.

Conclusion about XML as a data exchange language

Conclusion about XML parsing in AOLserver

ns_xml module provides basics of XML processing. Although you can do quite a bit with it one could wish to do more. Things that are obviously missing:

• SAX API (it's already present in libxml so this would only require extending ns_xml)
• support for XSLT (support for XSLT, although planned, is not yet present in libxml)

• use PyWx, a Python interpreter embedded inside AOLserver and standard PyXML Python module
• write another module wrapping some other XML parsing library
• use pure Tcl parser

Links

• to find out more about AOLserver read intro in December 2000 issue of LG or part one and part two of another intro
• AOLserver home page
• Philip and Alex's Guide to Web Publishing, a book that will make you a better web programmer
• Structure and Interpretation of Computer Programs, a book that will make you a better programmer
• Tcl for Web Nerds, a handy book on Tcl
• everybody has a web page and this one is mine

Copyright © 2001, Irving Washington.
Copying license http://www.linuxgazette.com/copying.html
Published in Issue 63 of Linux Gazette, Mid-February (EXTRA) 2001