The query dependencies tracking provides a method to link subjects to embedded queries and to validate the "freshness" of a query relative to its registered dependencies. In cases where the freshness characteristic can no longer be attributed it will signal to the RejectParserCacheValue hook to evict the parser cache, allowing a page view request to re-parse its content hereby updating query results while otherwise keeping the parser cache intact and untouched when queries are assumed to be fresh.

Requirements

• $smwgEnabledQueryDependencyLinksStore to be enabled to track entities used in queries via the SQLStore::QUERY_LINKS_TABLE table
• $smwgQueryProfiler to be enabled to track embedded queries

Features and limitations

• Dependencies are resolved for properties, categories, concepts (non cached) and hierarchies
• Invalidation of the parser cache happens on a view request (not in advance of any viewing)
• Setting $smwgQueryDependencyPropertyExemptionList contains property keys that are excluded from detection because their update frequency may cause a disruption or have been categorized as unnecessary for a tracking

Exemption rules

• Namespace queries, e.g. <nowiki>[[Help:+]]</nowiki> are not tracked (this would significantly impact update performance for when a single namespace dependency is altered)
• Queries with arbitrary conditions, e.g. <nowiki>[[~Issue/*]]</nowiki> cannot be tracked as they are not distinguishable in terms of an object description (cannot be assigned an ID which is required for a successful tracking)
• Queries with limit "<code>0</code>" (|limit=0) are not tracked (queries return an empty result list and only represent a simple link)
• Queries via Special:Ask are not tracked (those are not embedded)

Technical notes

• SQLStore
  • QueryDependency
    • DependencyLinksTableUpdater responsible for updating the SQLStore::QUERY_LINKS_TABLE table
    • DependencyLinksValidator detect a possible discrepancy (validate the "freshness") and is triggered via the RejectParserCacheValue hook
    • QueryDependencyLinksStore prune, find, and update dependencies
    • QueryReferenceBacklinks to display query back links via Special:Browse
    • QueryResultDependencyListResolver resolve entities used in a QueryResult