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|library(semweb/rdf_persistency): Providing persistent storage|
provides reliable persistent storage for the RDF data. The store uses a
directory with files for each source (see rdf_source/1)
present in the database. Each source is represented by two files, one in
binary format (see rdf_save_db/2)
representing the base state and one represented as Prolog terms
representing the changes made since the base state. The latter is called
cpu_countor 1 (one) on systems where this number is unknown. See also concurrent/3.
true, supress loading messages from rdf_attach_db/2.
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
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
false, the journal and snapshot for the database are deleted and further changes to triples associated with DB are not recorded. If Bool is
truea snapshot is created for the current state and further modifications are monitored. Switching persistency does not affect the triples in the in-memory RDF database.
min_size(KB)only journals larger than KB Kbytes are merged with the base state. Flushing a journal takes the following steps, ensuring a stable state can be recovered at any moment.
.newfile over the base state.
Note that journals are not merged automatically for two reasons. First of all, some applications may decide never to merge as the journal contains a complete changelog of the database. Second, merging large databases can be slow and the application may wish to schedule such actions at quiet times or scheduled maintenance periods.
The above predicates suffice for most applications. The predicates in
this section provide access to the journal files and the base state
files and are intented to provide additional services, such as reasoning
about the journals, loaded files, etc.3A
library(rdf_history) is under development
exploiting these features supporting wiki style editing of RDF.
rdf_transaction(Goal, log(Message)), we can add
additional records to enrich the journal of affected databases with Term
and some additional bookkeeping information. Such a transaction adds a
begin(Id, Nest, Time, Message) before the change operations
on each affected database and
end(Id, Nest, Affected) after
the change operations. Here is an example call and content of the
mydb.jrn. A full explanation of the terms that
appear in the journal is in the description of rdf_journal_file/2.
?- rdf_transaction(rdf_assert(s,p,o,mydb), log(by(jan))).
start([time(1183540570)]). begin(1, 0, 1183540570.36, by(jan)). assert(s, p, o). end(1, 0, ). end([time(1183540578)]).
rdf_transaction(Goal, log(Message, DB)), where DB
is an atom denoting a (possibly empty) named graph, the system
guarantees that a non-empty transaction will leave a possibly empty
transaction record in DB. This feature assumes named graphs are named
after the user making the changes. If a user action does not affect the
user's graph, such as deleting a triple from another graph, we still
find record of all actions performed by some user in the journal of that
log(Message). Id is an integer counting the logged transactions to this database. Numbers are increasing and designed for binary search within the journal file. Nest is the nesting level, where `0' is a toplevel transaction. Time is a time-stamp, currently using float notation with two fractional digits. Message is the term provided by the user as argument of the
log(Message). Id and Nest match the begin-term. Others gives a list of other databases affected by this transaction and the Id of these records. The terms in this list have the format DB:Id.
.trpfor the base state and
.jrnfor the journal.