[boost] persistence

From: Scott Woods (scottw_at_[hidden])
Date: 2003-05-25 18:04:27

Hi,
Is anyone interested in a persistence mechanism? Rather than dumping a bunch
of doc+code and finding that its just inappropriate, here is some summary
stuff.

Technique uses info available in a variant to manage the type-safe transfer
of
data between an application and some external storage. It can be
characterized
as a replacement for the windows registry or unix profiles. The technique
has
following properties;

* generic - load and save operations are templates
* portable - library and data are not platform dependent
* standard types - bool, integer, float, string, etc
* complex types - fixed dimension arrays and variable dimension vectors
* user-defined types - records (a set of members of any type)

Simplest usage involves "hand-forming" the variants. This is a bit
long-winded
(and error-prone) so a "compiler" has been developed. This accepts a
"schema"
conforming to a grammer and outputs a set of C++ classes (cpp and header).
The application can then derive application classes.

Given an application class "alarm_monitor" the schema might look like;

record sensor_signal {   // Generated by raising the signal on an alarm
sensor
 time when     // Instant of detection
 string line     // Expanded text
 void detail[]
}

record monitor_store {   // Persisted image of a monitored sensor
 integer number    // Unique id
 integer status    // Currently
 sensor_signal log[]   // Rolling log of signals
 integer ring_count   // Signal count since last ringing transition
}

The application would derive "alarm_monitor" from "monitor_store" and a
minimal
ctor might look like;

alarm_monitor::alarm_monitor( const alarm_sensor &sensor ) :
{
    try
    {
        // Recover the previously saved image of "this"

        typed_load_file file( sensor.name );

        load( file.begin(), *this );
    }
    catch( typed_memory_not_found & )
    {
        // No previous image. Set the members of "this"
        // to reasonable defaults and save an image

        number = sensor.number;
        status = monitor_ready;
        ring_count = 0;
        log.clear();

        typed_save_file file( sensor.name );

       save( file.begin(), *this );
    }
    // Resume normal operation
}

Hope this is enough info. Code is currently getting a good workout (part of
beta
in a 24x7, server-type product). Example above is "para-phrased" to keep it
short and to remove some confusing coding standards.

Thanks,
Scott