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persistency.pl -- Provide persistent dynamic predicates
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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:

  • name(Arg, ...)
  • assert_name(Arg, ...)
  • retract_name(Arg, ...)
  • retractall_name(Arg, ...)

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) :-
                   (  retractall_user_role(Name, _),
                      assert_user_role(Name, Role))).
To be done
- Provide type safety while loading
- Thread safety must now be provided at the user-level. Can we provide generic thread safety? Basically, this means that we must wrap all exported predicates. That might better be done outside this library.
- Transaction management?
- Should assert_<name> only assert if the database does not contain a variant?
Source persistent(+Spec)
Declare dynamic database terms. Declarations appear in a directive and have the following format:
:- persistent

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).

Source current_persistent_predicate(:PI) is nondet
True if PI is a predicate that provides access to the persistent database DB.
Source db_attach(:File, +Options)
Use File as persistent database for the calling module. The calling module must defined persistent/1 to declare the database terms. Defined options:
One of 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 sync behaviour.

Source db_size(+Module, -Terms) is det[private]
Terms is the total number of terms in the DB for Module.
Source db_attached(:File) is semidet
True if the context module attached to the persistent database File.
Source db_assert(:Term) is det[private]
Assert Term into the database and record it for persistency. Note that if the on-disk file has been modified it is first reloaded.
Source db_detach is det
Detach persistency from the calling module and delete all persistent clauses from the Prolog database. Note that the file is not affected. After this operation another file may be attached, providing it satisfies the same persistency declaration.
Source sync(+Module, +Stream) is det[private]
Synchronise journal after a write. Using close, the journal file is closed, making it easier to edit the file externally. Using flush flushes the stream but does not close it. This provides better performance. Using none, the stream is not even flushed. This makes the journal sensitive to crashes, but much faster.
Source db_retractall(:Term) is det[private]
Retract all matching facts and do the same in the database. If Term is unbound, persistent/1 from the calling module is used as generator.
Source db_retract(:Term) is nondet[private]
Retract terms from the database one-by-one.
Source db_sync(:What)
Synchronise database with the associated file. What is one of:
Database is reloaded from file if the file was modified since loaded.
As 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.
Database was re-written, deleting all retractall statements. This is the same as gc(50).
GC DB if the number of deleted terms is greater than the given percentage of the total number of terms.
GC DB without checking the percentage.
Database stream was closed
Remove all registered persistency for the calling module
No-operation performed

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.

Source db_sync_all(+What)
Sync all registered databases.