/ 
XML-Comma-1.08
River stage zero No dependents
/ XML::Comma

NAME

XML::Comma FAQ - frequently asked questions about XML::Comma

DESCRIPTION

XML::Comma is an information management platform. It was designed to be used as a core tool for developing Very Large Websites. Comma specifies an XML-based document definition format, encourages Perl code to be embedded in these definitions, and specifies an API for manipulating documents and document collections. Comma includes functionality to store sets of documents in ordered, extensible ways, and integrates with a relational database to index, sort and retrieve collections of documents.

BASIC INFORMATION

How is XML::Comma licensed and distributed?

XML::Comma is Free Software released under the GNU General Public License. For more information about the license, please see http://www.fsf.org/licenses/gpl.html

Comma is distributed as a Perl module. The most recent version is always available at http://xml-comma.org/download/

Is there an XML::Comma web site?

Yes. http://xml-comma.org/

Is there an XML::Comma mailing list?

Yes. Please see http://xml-comma.org/mailing-list.html

What documentation is available?

The XML::Comma User's Guide describes the Comma API in detail. It is available in HTML and PDF. The Guide can be found in the Comma/docs directory of the distribution, or at:

http://xml-comma.org/guide-filter.html
http://xml-comma.org/guide.pdf

CODE QUESTIONS

What's the difference between $foo->bar() and $foo->element('bar')->get()?

This is really a question about Comma's "shortcut" syntax, which defines a mapping between method calls and the elements of a given container.

There is a section on the shortcut syntax in the guide that goes into more detail, but here is a quick list of the seven possible ways a shortcut can can be resolved.

$x->foo ( [@args] )                  # becomes:
$x->method('foo, @args)              # if there is a method "foo"
$x->element('foo')                   # singular, nested
$x->elements('foo')                  # plural, nested
$x->element('foo')->get()            # singular, non-nested, no @args 
$x->element('foo')->set(@args)       # singular, non-nested, with @args
$x->elements_group_get('foo')        # plural, nested, no @args
$x->elements_group_set('foo', @args) # plural, nested, with @args

How do I tell how many instances of a given plural element exist?

Because the elements() method always tries to return a "list" of elements -- which means that it returns an array in list context and a reference to an array in scalar context -- you have to do a bit of extra work to determine how many instances of a plural element exist. The most concise (and the recommended) way to do this is:

my $how_many_foos = scalar ( @{$doc->elements('foo')} );

What does the BAD_CONTENT error that talks about '& found that isn't part of an entity reference' mean?

You have a problem that boils down to something along these lines:

$doc->element('some_element')->set ( "foo & bar" );

Element content must be legal XML -- so no <, >, or & characters are allowed. These special characters must be "escaped" by replacing them with their entity codes (respectively <, >, or &). The Comma::Util::XML_basic_escape() and Comma::Util::XML_basic_unescape() methods are available, as are shortcut flags for the element set() and get() methods:

$doc->element('some_element()->set ( "foo & bar", escape=>1 );
$doc->element('some_element')->get ( unescape=>1 );
$doc->some_element ( "foo & bar", escape=>1 );

Note that there is no way to pass the unescape flag in the shortcut-get syntax (so there are three examples above, rather than four). It is fair to construe this as a problem with the API.

What does an error that ends 'sh: /tmp/log.comma: Permission denied' mean?

Comma writes a line about all un-caught errors to a log file. The location of the log file is controlled by a setting in Comma.pm -- the default is /tmp/log.comma/. This file probably needs to be writable by any processes that use the Comma framework. In most installations, the file is made world-writable (which should tell you that the Comma log system isn't intended to be used as part of any security auditing or similar framework -- you should write additional code to handle any secure reporting that an application might need.)

PERFORMANCE

Is XML::Comma fast?

Sure. We don't know of any faster way to develop (or to add new features to) large-scale applications that manipulate collections of hundreds of thousands of pieces of messy-but-structured information. We use it every day, and so do many, many people who access the web sites we build.

Oh, wait: you meant, "does it run fast?" Well, that's in the eye of the beholder. Comma's bottleneck is the parsing and object-ifying of XML files. The power and flexibility that the API gives you comes at some cost -- a hand-coded, special-purpose implementation could well be faster for any single usage.

However, we've worked hard to make Comma fast enough to be really, really, useful. For example, Comma's "Inline" parser is about twice as fast as the general-use XML parsers against which we've benchmarked it (because Comma documents aren't allowed to make use of all parts of the XML specification). An experienced designer of large-scale internet systems will easily be able to structure and tune a Comma-based system to serve hundreds of thousands of dynamic pages a day on mid-range x86 boxes.