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All about virtual resources, virtual collections and searching. So far mostly a brain dump of ideas and requirements so we can come up with a nice design for all this. It is based on previous discussions for this topic, those are linked in the last section for reference.


  • Storage collection: An Akonadi collections that represents a storage location for items.
  • Virtual collection: An Akonadi collection that does represent an arbitrary set of items stored in any storage collection. Virtual collections are implemented by "linking" to existing items, comparable to symlinks in a file system.
  • Virtual resource: An Akonadi resource that creates virtual collections instead of storage collections.


Assumptions all the following is based on.

  • An Akonadi collection can either be a storage collection or a virtual collection, i.e. it can either contain "real" items or links to items stored elsewhere, never both.
  • An Akonadi resource can either contain storage collections or virtual collections, never both.

Use-Cases for Virtual Collections

  • Representing the results of a persistent search:
    • in a per-session scope, e.g. the currently displayed week in KOrganizer
    • in a per-application scope, e.g. user-defined persistent search folders in KMail
    • in a global scope, e.g. user-defined persistent search folders shared between KMail, Mailody and LionMail.
  • The Nepomuk tag resource
    • Representing the entire tag-tree

Ideas on the Search Infrastructure

  • Delegation to resources that support search in the back-ends
    • Requires query transformation
    • Requires management for live searches
    • Requires the ability to report search results (needs protocol extension)
  • Query language still undecided, current options: XESAM, SPARQL
  • Back-end still undecided, current options: XESAM, Nepomuk

Akonadi Search Infrastructure (concept)

Virtual Resources

A resource that creates virtual collections instead of storage collections.

A virtual resource can exist in two forms:

  • Physical virtual resources: I.e. there is a conventional agent instance for that resource that operates like any other resource but instead creates and maintains virtual collections.
  • Virtual virtual resources: These have no agent instance responsible for their collections, but are used for scoping search folders. The Akonadi server itself is responsible for maintaining those, while the applications decide about their lifetime.

We definitely need better terms for these...

Virtual Collections

The following lists properties of "normal" Akonadi::Collection objects and maps them to the corresponding semantics of virtual collections:

  • UID: stays the same
  • RID: for application use (currently it holds the query, but that's a dirty hack) (search folders only, still used for its original purpose by the tag resource)
  • Name: stays the same, for display purposes only
  • Resource Id: see virtual resources
  • Content mime-types: good question actually...
  • ACL: extend by link/unlink operations
    • Item creation: not allowed, would lack a storage location
    • Item modification: in theory possible if the storage collection allows it
    • Item deletion: in theory possible if the storage location allows it
    • Collection creation: depends on resources, useful for the Nepomuk tag resource, not for persistent search folders
    • Collection modification
      • Name: no problem
      • RID: not allowed once there is a actual resource behind it, not a problem when used by the application
      • Query: requires special support in the server, but no principal problem
      • Parent (that is moving): tricky for the tag resource, not really a problem for search folders, moving to non-virtual resources needs to be prevented.
      • Any other attribute: no problem
    • Collection deletion: no effects on stored items/collections, so no problem
  • Query: for search folders only, should be moved into an dedicated attribute.

Note that virtual collections so far are not explicitly marked as such, to keep them as transparent as possible for the clients. However, since the interaction with them differs from that with storage collections, we need ways to distinguish them at least implicitly. All the below for example is covered by the additional ACL properties for linking/unlinking.

Interaction with Virtual Collections

This is about possible user interaction patterns with virtual collections.

Linking/Unlinking of Items

  • Copy/DnD a "real" item into a virtual collection: That would mean "linking" the item there.
    • Useful for the tag resource
    • Makes no sense for search folders
  • Move/Copy/DnD a "link" out of a virtual collection into a storage one: That would mean moving/copying the item to a different storage location, not changing anything regarding the actual linking.
  • Moving/Copying/DnDing links between two virtual collections
    • Makes no sense for search folders, they are read-only for the clients
    • useful for the tag resource though

Modification/Deletion of Items

That requires accessing the ACL of the actual storage collection.

Operations on the Virtual Collection

  • Modifications
    • Renaming
      • Only for display purposes anyway for search folders
      • Rename the tag in case of the tag resource?
  • Moving
    • Only for display purposes for search folders
    • Change the tag hierarchy?
  • Deletion
    • Delete persistent search
    • Delete tag

Open Questions

  • RID usage in virtual collections
  • How does the ETM hide virtual collections by default?
  • What are the content mime-types of a virtual collection?


Past discussions about this topic:

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