## A.30 library(persistency): Provide persistent dynamic predicates

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?

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) :-
            with_mutex(user_db,
                       (  retractall_user_role(Name, _),
                          assert_user_role(Name, Role))).

persistent +Spec
  Declare dynamic database terms. Declarations appear in a directive and have the following format:

      :- persistent
              <callable>,
              <callable>,
              ...

  Each specification is a callable term, following the conventions of `library(record)`, where each argument is of the form

      name:type

  Types are defined by `library(error)`.

[nondet]current_persistent_predicate(:PI)
  True if PI is a predicate that provides access to the persistent database DB.
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:
  sync(+Sync)
    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.

[semidet]db_attached(:File)
  True if the context module attached to the persistent database File.
[det]db_detach
  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.
db_sync(:What)
  Synchronise database with the associated file. What is one of:
  reload
    Database is reloaded from file if the file was modified since loaded.
  update
    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.
  gc
    Database was re-written, deleting all retractall statements. This is the same as `gc(50)`.
  gc(Percentage)
    GC DB if the number of deleted terms is greater than the given percentage of the total number of terms.
  gc(always)
    GC DB without checking the percentage.
  close
    Database stream was closed
  detach
    Remove all registered persistency for the calling module
  nop
    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.

db_sync_all(+What)
  Sync all registered databases.