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From Chris Mattmann <>
Subject Re: [Nutch-cvs] [Nutch Wiki] Update of "ParserFactoryImprovementProposal" by ChrisMattmann
Date Thu, 15 Sep 2005 18:20:00 GMT
Hi Otis,

On 9/15/05 10:14 AM, "" <>

> Quick comment about order="N" and the paragraph that describes how to
> deal with cases where people mess things up and enter multiple plugins
> for the same content type and the same order:
> - Why is the order attribute even needed?  It looks like a redundant
> piece of information - why not derive order from the order of plugin
> definitions in the XML file?

Well, yes and no. Having the "order=" attribute explicitly forces the user
to decide up front what the order should be. By having order be semantically
derived from the order in which the tags are parsed, you remove that
information from the user up front, and assume they know that order matters.

However, in the interest of time and brevity, we decided that it would be
nice to support both options, that is, if they specify an order, then go
ahead and accept that, otherwise, if they don't specify one, then the
ordering which you suggest is applied.

We think that such a solution affords both types of users: users that like
to explicitly spell out options in their XML, and on the other hand, users
which like the short hand of just doing the xml attributes in order.
We are open to suggestions on this though...

Thanks for your comments Otis, and for reading the proposal!


> For instance:
> Instead of this:
>   <mimeType name="*">
>       <plugin id=”parse-text” order=”1”/>
>       <plugin id=”another-one-default-parser” order=”2”/>
>      ....
>   </mimeType>
> We have this:
>   <mimeType name="*">
>       <plugin id=”parse-text”/>
>       <plugin id=”another-one-default-parser”/>
>      ....
>   </mimeType>
> parse-text first, another-one-default-parser second.  Less typing, and
> we avoid the case of equal ordering all together.
> Otis
> --- Apache Wiki <> wrote:
>> Dear Wiki user,
>> You have subscribed to a wiki page or wiki category on "Nutch Wiki"
>> for change notification.
>> The following page has been changed by ChrisMattmann:
>> The comment on the change is:
>> Initial Draft of ParserFactoryImprovementProposal
>> New page:
>> = Parser Factory Improvement Proposal =
>> == Summary of Issue ==
>> Currently Nutch provides a plugin mechanism wherein plugins register
>> certain metadata about themselves, including their id, classname, and
>> so forth. In particular, the set of parsing plugins register which
>> contentTypes and file suffixes they can support with a
>> PluginRepository.
>> One “adopted practice� in current Nutch parsing plugins
>> (committed in Subversion, e.g., see parse-pdf, parse-rss, etc.) has
>> also been to verify that the content type passed to it during a fetch
>> is indeed one of the contentTypes that it supports (be it
>> application/xml, or application/pdf, etc.). This practice is
>> cumbersome for a few reasons:
>>  *Any updates to supported content types for a parsing plugin will
>> require a recompilation of the plugin code
>>  *Checking for “hard coded� content types within the parsing
>> plugin is a duplication of information that already exists in the
>> plugin’s descriptor file, plugin.xml
>>  *By the time that content gets to a parsing plugin, (e.g., the
>> parsing plugin is returned by the ParserFactory, and provided content
>> during a fetch), the ParsingFactory should have already ensured that
>> the appropriate plugin is getting called for a particular
>> contentType.
>> In addition to this problem is the fact that several parsing plugins
>> may all support many of the same content types. For instance, the
>> parse-js plugin may be the only well suited parsing plugin for
>> javascript, but perhaps it may also provided a good enough heuristic
>> parser for plain text as well, and so it may support both types.
>> However, there may be a parsing plugin for text (which there is!),
>> parse-text, whose primary purpose is to parse plain text as well.
>> == Suggested Remedy ==
>> To deal with ensuring the desired parsing plugin is called for the
>> appropriate content type, and to in effect, “kill two birds with
>> one stone�, we propose that there be a parsing plugin preference
>> list for each content type that Nutch knows how to handle, i.e., each
>> content type available via the mimeType system. Therefore, during a
>> fetch, once the appropriate mimeType has been determined for content,
>> and the ParserFactory is tasked with returning a parsing plugin, the
>> ParserFactory should consult a preference list for that contentType,
>> allowing it to determine which plugin has the highest preference for
>> the contentType. That parsing plugin should be returned via the
>> ParserFactory to the fetcher. If there is any problem using the
>> initial returned parsing plugin for a particular contentType (i.e.,
>> if a ParseException is throw during the parser, or a null ParseStatus
>> is returned), then the ParserFactory should be called again, this
>> time asking for the “next highest ranked
>>  � plugin for that contentType. Such a process should repeat on and
>> on until the parse is successful.
>> We propose that the “plugin preference list� should be a separate
>> file that lives in $NUTCH_HOME/conf called “parse-plugins.xml�.
>> The format of the file (full DTD to be developed during coding)
>> should be something like: {{{
>> <parse-plugins>
>>   <default pluginname=�parse-text�/>
>>   <fileType name=�powerpoint�>
>>    <mimeTypes>
>>     <mimeType name=�application/pdf� />
>>     <mimeType name=�application/x-pdf� />
>>     …
>>    </mimeTypes>
>>    <plugins>
>>       <plugin name=�parse-pdf� order=�1�/>
>>       <plugin name=�parse-pdf-worse� order=�2�/>
>>      …
>>    </plugins>
>>   </fileType>
>>     …
>> </parse-plugins>
>> }}}
>> One of the main impacts of having a file like parse-plugins.xml is
>> that no longer should the pathSuffix="" be part of the plugin.xml
>> descriptor. We propose to move that out of plugin.xml and into the
>> mime-types.xml file.
>> == Architectural Impact ==
>> === Components ===
>>  *Fetcher
>>  *PluginSystem
>>  *ParserFactory
>> === Impact on current releases of Nutch ===
>> ''Incompatibilities''
>> By moving the contentType and pathSuffix out of the plugin.xml file,
>> this would create an updated version of the plugin.xml descriptor
>> schema for each plugin. To lessen the effect on previous and
>> near-term releases of Nutch this information could be left as an
>> option in the plugin.xml schema, but marked as “deprecated� to
>> let people know that this functionality isn’t part of the parse
>> plugin identification process anymore, but it is left in the schema
>> so as not to create incompatibilities with the plugin.xml files that
>> people have already wrote. However, ultimately in future releases of
>> Nutch, we propose that the contentType and pathSuffix attributes
>> should be removed from the plugin.xml schema.
>> Other than the plugin.xml file schema change, this capability
>> addition will simply control the order in which parsing plugins get
>> called during fetching activities. It won’t directly impact the
>> segments stored, or the webapp, or any of the main components of
>> Nutch.
>> ''Issues''
>> The proposed new capabilities should be first tested on local
>> systems, and if successful, uploaded to JIRA, and verified against
>> the latest SVNs.
>> Unit tests should be written to verify appropriate plugin parsing
>> order.
>> Users will need to be notified in the Nutch tutorial and instruction
>> lists about how to set up the parsing plugin preferences prior to
>> performing a fetch.
>> == Personnel ==
>>  *Jerome Charron
>>  *Sébastien Le Callonnec
>>  *Chris A. Mattmann
>> == Timeframe ==
>>  *Begin work the weekend of 9/9
>>  *Complete first prototype patches to JIRA by end of week, 9/18
>>  *Test against latest SVNs of Nutch, by 9/25
>>  *Delivery of operational capability, by 10/1
>> == Affected files ==
>>  *
>>  *
>>  *
>>  *plugin.xml descriptor files
>>  *files in package {{{org.apache.nutch.util.mime}}}
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Chris A. Mattmann
Staff Member
Modeling and Data Management Systems Section (387)
Data Management Systems and Technologies Group
Jet Propulsion Laboratory            Pasadena, CA
Office: 171-266B                        Mailstop:  171-246
Phone:  818-354-8810
Disclaimer:  The opinions presented within are my own and do not reflect
those of either NASA, JPL, or the California Institute of Technology.

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