Meeting with developers is always great. Not only does one get challanged and dragged into interesting discussions, often the result is an actual plan or a concrete idea. In this case my trip to Oslo and the vistit of the Trolltech (ah, sorry, Nokia) offices resulted in at least three results: 1. a lively discussion about the possibilities that the Semantic Desktop could offer (including a promise from a bunch of developers to ad wishes describing these ideas into the KDE bug database); 2. the plan for a Nepomuk developer sprint in the next months (more about that in the next days), 3. a very very fruitful discussion with Tobias König (currently doing an internship at Nokia) about the integration of Nepomuk into Akonadi or vice versa. Here I will present the results of the latter discussion.
The current situation
Currently (meaning: in KDE 4.2) both Akonadi and Nepomuk use their own databases: Akonadi starts its own mysql server and Nepomuk runs a java based RDF backend. Many people have problems with both mysql and java. I personally can understand the latter. In any case this separation of data means that at least parts of Akonadi data needs to be mirrored in Nepomuk. Otherwise we cannot search for PIM data, we cannot link to PIM items, and we cannot create relations between PIM items.
This mirroring of data is an ugly thing since it means that we convert the data in Akonadi (stored as VCards, ICal, or other formats) into RDF graphs. This is currently done by two Akonadi agents which can be found in the kdepim package. Whenever data in Akonadi changes, the data in Nepomuk has to be synced. I personally can see no advantage of this situation.
A possible solution worth discussing
The possible solution Tobias and myself came up with looks as follows:
Akonadi and Nepomuk will share one database – a Virtuoso SQL/RDF server. (For a discussion of the new Virtuoso support in Soprano see my last blog entry.) This is achieved as follows: the current database schema of Akonadi will be kept except for the parts tables which at the moment store the actual mime data of the items. Instead the item table will get a new column which points to an RDF resource in Virtuoso’s own RDF tables. The RDF resource will them represent the item in a real object-oriented form: contacts are encoded using the NCO Ontology, emails are encoded using the NMO Ontology, and so on. Akonadi will then use an RDF encoding (we propose turtle) for all data serialization. The advantages are numerous:
- All PIM data does exist as nicely encoded semantic data in Nepomuk which can directly be used to relate to and from.
- No syncing is necessary.
- Serialization plugins for Akonadi can be created automatically from an ontology using a code generator. Thus, an application developer who wants to store data in Akonadi only needs to create their own ontology. A task that is necessary for Nepomuk support anyway. And let’s face it: this is what each application should include in the future ;).
- Only one database server running: Virtuoso.
- Simpler mapping from database objects to convenience objects in C++: the data in Nepomuk is already represented using an object-oriented approach.
The Virtuoso server could be started using the same “process-singleton” approach libakonadi is currently using to start Akonadi if it is not running. This would keep both Nepomuk and Akonadi free of dependancies on each other.
The one possible problem remaining might be performance: It stays to be tested if reading a vcard from SQL and parsing it is much faster than querying an NCO contact resource. Volker, we need your help. Plus: I see you guys at the KDE-PIM meeting in Berlin. Lots to do. ;)