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|Enriching the journals|
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.