SWI-Prolog -- rdf_attach

Documentation
- Reference manual
- Packages
  - SWI-Prolog Semantic Web Library 3.0
    - Plugin modules for rdf_db
      - library(semweb/rdf_persistency): Providing persistent storage
        
        rdf_attach_db/2
        
        rdf_detach_db/0
        
        rdf_current_db/1
        
        rdf_persistency/2
        
        rdf_flush_journals/1
        
        Enriching the journals

Availability::- use_module(library(semweb/rdf_persistency)).

rdf_attach_db(+Directory, +Options)

Attach Directory as the persistent database. If Directory does not exist it is created. Otherwise all sources defined in the directory are loaded into the RDF database. Loading a source means loading the base state (if any) and replaying the journal (if any). The current implementation does not synchronise triples that are in the store before attaching a database. They are not removed from the database, nor added to the persistent store. Different merging options may be supported through the Options argument later. Currently defined options are:

concurrency(+PosInt): Number of threads used to reload database and journals from the files in Directory. Default is the number of physical CPUs determined by the Prolog flag cpu_count or 1 (one) on systems where this number is unknown. See also concurrent/3.
max_open_journals(+PosInt): The library maintains a pool of open journal files. This option specifies the size of this pool. The default is 10. Raising the option can make sense if many writes occur on many different named graphs. The value can be lowered for scenarios where write operations are very infrequent.
silent(Boolean): If true, suppress loading messages from rdf_attach_db/2.
log_nested_transactions(Boolean): If true, nested log transactions are added to the journal information. By default (false), no log-term is added for nested transactions.

The database is locked against concurrent access using a file lock in Directory. An attempt to attach to a locked database raises a permission_error exception. The error context contains a term rdf_locked(Args), where args is a list containing time(Stamp) and pid(PID). The error can be caught by the application. Otherwise it prints:

ERROR: No permission to lock rdf_db `/home/jan/src/pl/packages/semweb/DB'
ERROR: locked at Wed Jun 27 15:37:35 2007 by process id 1748