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This module provides simple persistent storage for one or more dynamic predicates. A database is always associated with a module. A module that wishes to maintain a database must declare the terms that can be placed in the database using the directive persistent/1.
The persistent/1 expands each declaration into four predicates:
As mentioned, a database can only be accessed from within a single module. This limitation is on purpose, forcing the user to provide a proper API for accessing the shared persistent data.
Below is a simple example:
:- module(user_db, [ attach_user_db/1, % +File current_user_role/2, % ?User, ?Role add_user/2, % +User, +Role set_user_role/2 % +User, +Role ]). :- use_module(library(persistency)). :- persistent user_role(name:atom, role:oneof([user,administrator])). attach_user_db(File) :- db_attach(File, ). %% current_user_role(+Name, -Role) is semidet. current_user_role(Name, Role) :- with_mutex(user_db, user_role(Name, Role)). add_user(Name, Role) :- assert_user_role(Name, Role). set_user_role(Name, Role) :- user_role(Name, Role), !. set_user_role(Name, Role) :- with_mutex(user_db, ( retractall_user_role(Name, _), assert_user_role(Name, Role))).
:- persistent <callable>, <callable>, ...
Each specification is a callable term, following the conventions of library(record), where each argument is of the form
Types are defined by library(error).
close(close journal after write),
flush(default, flush journal after write) or
none(handle as fully buffered stream).
If File is already attached this operation may change the
reload, but use incremental loading if possible. This allows for two processes to examine the same database file, where one writes the database and the other periodycally calls
db_sync(update)to follow the modified data.
With unbound What, db_sync/1 reloads the database if it was modified on disk, gc it if it is dirty and close it if it is opened.