[Tracker] Database access abstraction

[Jürg Billeter <j bitron ch>]

Hi all,

as a preparation to support decomposed database tables in Tracker, I've
been looking into how we could abstract database access so that the
database schema can change without affecting the code in trackerd and
tracker-indexer.

I'm proposing to introduce an additional (private) library that acts as
a high-level database interface, so it sits between libtracker-db and
trackerd/tracker-indexer. That library, let's call it libtracker-data
for now, is the only place where SQL queries get constructed.

As a first step, we should probably just move relevant functions or
function parts from trackerd and tracker-indexer to the new library and
refactor and extend the library later. Looking at the current code, the
API of libtracker-data would be composed of the following parts:

 * Ontology/Schema API
   These functions don't query the actual data or metadata but only the
   ontology/schema and its mapping to the database layout. They are
   currently part of trackerd/tracker-db.c
        tracker_db_metadata_get_related_names
        tracker_db_metadata_get_table
        tracker_db_get_field_name
        tracker_db_get_metadata_field
        tracker_db_create_array_of_services
        tracker_db_xesam_get_metadata_names
        tracker_db_xesam_get_all_text_metadata_names
        tracker_db_xesam_get_service_names

 * Service/Metadata Query API
   These functions query information about a specific service/resource,
   for example, path to id mapping and metadata retrieval. They are
   currently part of trackerd/tracker-db.c and
   tracker-indexer/tracker-indexer-db.c
        tracker_db_metadata_get
        tracker_db_metadata_get_all
        tracker_db_metadata_get_array
        tracker_db_metadata_get_delimited
        tracker_db_get_all_metadata
        tracker_db_get_parsed_metadata
        tracker_db_get_unparsed_metadata
        tracker_db_get_property_values
        tracker_db_check_service
        tracker_db_get_service_type
        tracker_db_service_get_by_entity
        tracker_db_file_get_id
        tracker_db_file_get_id_as_string

 * Search and General Query API
   These functions perform arbitrary queries on the whole database. They
   are currently part of tracker-db.c, tracker-metadata.c,
   tracker-search.c, tracker-keywords.c, and tracker-files.c in trackerd
        tracker_db_search_text
        tracker_db_search_text_and_mime
        tracker_db_search_text_and_location
        tracker_db_search_text_and_mime_and_location
        tracker_db_live_search_start
        tracker_db_live_search_stop
        tracker_db_live_search_get_all_ids
        tracker_db_live_search_get_new_ids
        tracker_db_live_search_get_deleted_ids
        tracker_db_live_search_get_hit_data
        tracker_db_live_search_get_hit_count
        tracker_db_keywords_get_list
        tracker_db_files_get
        tracker_db_files_get_by_service
        tracker_db_files_get_by_mime
        tracker_db_create_event
        tracker_db_xesam_delete_handled_events
        tracker_data_get_unique_values
        tracker_data_get_sum
        tracker_data_get_count
        tracker_data_get_unique_values_with_count
        tracker_data_get_unique_values_with_count_and_sum
        tracker_data_get_metadata_for_files_in_folder
        tracker_data_keywords_search
        tracker_data_search_query
    The tracker_data_* signify the database access parts of the D-Bus
    service methods. The actual D-Bus method implementations stay at
    their place, of course.

  * Update API
    These functions are used to modify data and metadata, they are only
    executed by the indexer and currently reside in
    tracker-indexer/tracker-indexer-db.c
        tracker_db_get_new_service_id
        tracker_db_create_service
        tracker_db_delete_service
        tracker_db_delete_service_recursively
        tracker_db_move_service
        tracker_db_increment_stats
        tracker_db_decrement_stats
        tracker_db_set_metadata
        tracker_db_delete_all_metadata
        tracker_db_delete_metadata
        tracker_db_set_text
        tracker_db_get_text
        tracker_db_delete_text

We also need to move trackerd/tracker-query-tree.c,
trackerd/tracker-rdf-query.c, trackerd/tracker-xesam-query, and
tracker-indexer/tracker-metadata.c to the new library as they generate
SQL queries or are used by other functions in libtracker-data.

Any comments or suggestions about this, does the grouping seem sensible?
Please note that I don't know Tracker's code base very well yet, so I
might be missing or misunderstanding some things.

Regards,
JÃrg