/usr/lib/swi-prolog/doc/Manual/persistency.html

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<h2 id="sec:persistency"><a id="sec:A.21"><span class="sec-nr">A.21</span> <span class="sec-title">library(persistency): 
Provide persistent dynamic predicates</span></a></h2>

<p><a id="sec:persistency"></a>

<dl class="tags">
<dt class="mtag">To be done</dt>
<dd>
- Provide type safety while loading <br>
- 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. <br>
- Transaction management? <br>
- Should assert_<var>&lt;</var>name<var>&gt;</var> only assert if the 
database does not contain a variant?
</dd>
</dl>

<p>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 <a class="pred" href="persistency.html#persistent/1">persistent/1</a>.

<p>The <a class="pred" href="persistency.html#persistent/1">persistent/1</a> 
expands each declaration into four predicates:

<p>
<ul class="compact">
<li><code>name(Arg, ...)</code>
<li><code>assert_name(Arg, ...)</code>
<li><code>retract_name(Arg, ...)</code>
<li><code>retractall_name(Arg, ...)</code>
</ul>

<p>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.

<p>Below is a simple example:

<pre class="code">
:- 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))).
</pre>

<dl class="latex">
<dt class="pubdef"><a id="persistent/1"><strong>persistent</strong> <var>+Spec</var></a></dt>
<dd class="defbody">
Declare dynamic database terms. Declarations appear in a directive and 
have the following format:

<pre class="code">
:- persistent
        &lt;callable&gt;,
        &lt;callable&gt;,
        ...
</pre>

<p>Each specification is a callable term, following the conventions of <code>library(record)</code>, 
where each argument is of the form

<pre class="code">
name:type
</pre>

<p>Types are defined by <code>library(error)</code>.</dd>
<dt class="pubdef"><span class="pred-tag">[nondet]</span><a id="current_persistent_predicate/1"><strong>current_persistent_predicate</strong>(<var>:PI</var>)</a></dt>
<dd class="defbody">
True if <var>PI</var> is a predicate that provides access to the 
persistent database DB.</dd>
<dt class="pubdef"><a id="db_attach/2"><strong>db_attach</strong>(<var>:File, 
+Options</var>)</a></dt>
<dd class="defbody">
Use <var>File</var> as persistent database for the calling module. The 
calling module must defined <a class="pred" href="persistency.html#persistent/1">persistent/1</a> 
to declare the database terms. Defined options:

<dl class="latex">
<dt><strong>sync</strong>(<var>+Sync</var>)</dt>
<dd class="defbody">
One of <code>close</code> (close journal after write), <code>flush</code> 
(default, flush journal after write) or <code>none</code> (handle as 
fully buffered stream).
</dd>
</dl>

</dd>
<dt class="pubdef"><a id="db_sync/1"><strong>db_sync</strong>(<var>:What</var>)</a></dt>
<dd class="defbody">
Synchronise database with the associated file. <var>What</var> is one 
of:

<dl class="latex">
<dt><strong>reload</strong></dt>
<dd class="defbody">
Database is reloaded from file
</dd>
<dt><strong>gc</strong></dt>
<dd class="defbody">
Database was re-written, deleting all retractall statements. This is the 
same as <code>gc(50)</code>.
</dd>
<dt><strong>gc</strong>(<var>Percentage</var>)</dt>
<dd class="defbody">
GC DB if the number of deleted terms is the given percentage of the 
total number of terms.
</dd>
<dt><strong>close</strong></dt>
<dd class="defbody">
Database stream was closed
</dd>
<dt><strong>detach</strong></dt>
<dd class="defbody">
Remove all registered persistency for the calling module
</dd>
<dt><strong>nop</strong></dt>
<dd class="defbody">
No-operation performed
</dd>
</dl>

<p>With unbound <var>What</var>, <a class="pred" href="persistency.html#db_sync/1">db_sync/1</a> 
reloads the database if it was modified on disk, gc it if it is dirty 
and close it if it is opened.</dd>
<dt class="pubdef"><a id="db_sync_all/1"><strong>db_sync_all</strong>(<var>+What</var>)</a></dt>
<dd class="defbody">
Sync all registered databases.
</dd>
</dl>

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swi-prolog-nox 7.2.3-2 / usr / lib / swi-prolog / doc / Manual / persistency.html
