[SMW-devel] New feature: Query control and query permissions

[Markus K. <ma...@se...>]

SMW 1.2 extends the options for controlling which queries users are allowed to 
ask, and it has a mechanism that can be used to let some users ask more than 
others ("permissions").

The detailed query control settings are achieved by the new configuration 
parameter $smwgQFeatures. This parameter defines which query features are 
allowed in #ask. The default is the following:

$smwgQFeatures = SMW_PROPERTY_QUERY | SMW_CATEGORY_QUERY | SMW_CONCEPT_QUERY |
          SMW_NAMESPACE_QUERY | SMW_CONJUNCTION_QUERY | SMW_DISJUNCTION_QUERY;

This allows all existing features to be used. Other possible settings could 
be:

only category intersections: 
  $smwgQFeatures = SMW_CATEGORY_QUERY | SMW_CONJUNCTION_QUERY;

only single concepts: (see earlier mail on concepts)
  $smwgQFeatures = SMW_CONCEPT_QUERY;

anything but disjunctions:  
  $smwgQFeatures = SMW_ANY_QUERY & ~SMW_DISJUNCTION_QUERY;

So you can very clearly customise SMW queries. This affects only the #ask 
queries. Queries stored as concepts have their own setting, called 
$smwgQConceptFeatures. For example, concepts currently are not allowed to 
refer to other concepts (recursion is not handled properly by SMW yet).

This gives you a way to restrict query creation to some user group of the 
wiki:

* set $smwgQFeatures = SMW_CONCEPT_QUERY;
* configure MediaWiki to have the Concept: namespace being editable only by 
some user groups (there are extensions for that, I have no further details 
here, search the web).

Then #ask queries can only use single concepts and not be used to invent any 
new queries. So you can have very complex queries, but have responsible 
people creating them and making sure they are fine and fast in the wiki. You 
can also set $smwgQFeatures = SMW_CONCEPT_QUERY | SMW_CONJUNCTION_QUERY; to 
allow for conjunctions of multiple concepts, if desired, or you can further 
include categories. This appears to be a good option especially for very 
large sites that cannot afford users toying around with arbitrary queries for 
reasons of performance. This restriction was not tested thoroughly yet (so do 
not rely on this performance safety).

Restricting complex queries to concept will become even more useful as soon as 
SMW 1.3 offers concept-specific caching (e.g. some concepts being precomputed 
in the database so they can be recalled like categories). Category caching 
was originally planned for SMW 1.2 already, but we decided to first introduce 
concepts and then have a general caching mechanism for them. In this way, all 
queries will be cacheable and scale to any site that is able to display 
categories. Wikipedia comes to mind. But one more release is needed for that 
to be done properly.


Cheers,

Markus

-- 
Markus Krötzsch
Semantic MediaWiki    http://semantic-mediawiki.org
http://korrekt.org    ma...@se...