[Opensim-dev] further db thoughts

[Sean Dague]

On Fri, Jan 11, 2008 at 09:46:21PM +0100, Stefan Andersson wrote:
> Sean,
>  
> I know you're going to hate me for this, but before I post code, I just wanted to explain whats here 'on my own hard-disk' to see if it's anything like what you want to do:
>  
> * I have a DbConnectionPool that acts as a programmatic manager of connections (Subclassed into MySqlDbConnectionPool)
> * this is passed to a TableMapper which exposes it as a protected method WithConnection(delegate) for subclasses to access; they never access the connectionpool directly; the delegate approach lets the TableMapper lock, allocate, pre- and postprocess the connection securely and encapsulated. (kind of like a using clause)
> * The TableMapper is subclassed into a TableMapper<TRowMapper> that implements a CRUD interface by creating dynamic sql queries (the param prefix is pulled from the ConnectionPool) and populating them from a 'Schema'
> * A 'Schema' is a collection of FieldMapper<T>
> * FieldMapper<TField> is initialized with a field/param name (same for laziness sake), TField is the RowMapper field value type, a get delegate and a set delegate for the TField type object. It also does all the db/field type casting as well as expands things like Vector and Quaternion into several db fields.
> * The delegate approach keeps everything field related together, as opposed to a FromReader/ToReader method approach, which helps dodge inconsistencies.* The TRowMapper is mostly of type RowMapper<TRowObject> which just creates a class that has a member of TRowObject; this lets you subclass TRowMapper<TRowObject> and add db-specific extra-fields to it in order to manage hierarchies.
> * Upon operations, you create your rowmapper object (or just a plain object if there is no aux data) and either do FillObject( Schema, rowObject) or extract stuff by overrifing the FromReader(rowObject, DbReader) - the latter I use to populate a group from a part table (the select on the key field returns multiple rows, so i just PartMapper.FromReader() on each one to build the group.
>  
> - it does NOT auto-create tables, as I'm not interested in that approach as you know, but you already have the code for it and the data in the 'Schema' so it should be a piece of pie.
>  
> All in all, everything is explicit and compile-time; no cacheing, synchronization or double-buffering, no reflected and emitted code, but it works, and is quite efficient. It takes a while to get into the flow of how the various components work together, but after that I feel it's very conveniens and trustworthy.
>  
> Of course, it needs more work to get it going on all different database interfaces, but it's created from the start to be subclassed into differing databases.
>  
> If this sounds like something we'd want, I'd be happy to make it
> available as a third-party BSD Licence lib within the project, my only
> demand is that it is called TribalMedia.Framework.Data and that the
> BSD header remains intact.

I think it is definitely a promissing approach.  If you wanted to check
it in under TribalMedia/Framework/Data I'll definitely check it out.  If
it turns out to not work out, we can just remove it down the road.

   	 -Sean

-- 
__________________________________________________________________

Sean Dague                                       Mid-Hudson Valley
sean at dague dot net                            Linux Users Group
http://dague.net                                 http://mhvlug.org

There is no silver bullet.  Plus, werewolves make better neighbors
than zombies, and they tend to keep the vampire population down.
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