Re: connecting to PostgreSQL database

From: Christian Holm Christensen <cholm_at_nbi.dk>
Date: Wed, 19 Jul 2006 00:58:41 +0200

Hi Roger,

On Tue, 2006-07-18 at 09:46 -0230, Roger Mason wrote:
> Hi Christian,
>
> Christian Holm Christensen <cholm_at_nbi.dk> writes:
>

>>> I have seen the examples in the tutorials but they are for MySQL and


> >> Oracle.

> >

> > The SQL database interface of ROOT uses the plug-in architecture.

> > Hence, if you have the appropriate plug-in and setting in your rootrc
> > file, all you need to do is
>
> I'm not sure what you mean here. Could you expand?

In many real-life examples, what you want to do is something like

• `I want to open a connection to a database'
• `I want to read a file of this format'
• `I want to get access to this remote file server'
• `I want to make a simulation of my detector'
• `I want to do a discrete fast fourier transform of may spectrum'

and last but not least

• `I want to fit my data to this function'

For these cases, and many others, there may exist competing implementation out on the information super high-way - most of them not compatible with each other. Object-Oriented Programming offers a nice way to abstract the tasks mention above into common interface via inheritance. For example

        struct Simulation
        {
           enum Type
           {
             kGeant3,
             kGeant4,
             ...
           };
           static Simulation* Create(Type type)
           {
              switch (type) {
              case kGeant3:  return new Geant3;
              case kGeant4:  return new Geant4;
              ...
              }
              return 0;
           }
           bool Run();
           bool Event();
           bool MakePrimary();
           bool PropagatePrimary();
           ...
        };
        
        struct Geant3 : public Simulation { ... };
        struct Geant4 : public Simulation { ... };

There's nothing fancy about this. It's just the power of OOP :-)

ROOT also provides the facilities to load object code dynamically via calls to say `dlopen' or `OpenLibrary'. This means that we could load the concrete implementation of `Simulation' using some sort of look-up or the like:

   Simulation*
   Simulation::Create(const std::string& name)    {

     Library* lib = LibraryLoader::Instance()->Load(name);
     if (!lib) return 0;
      
     Factory* factory = lib->GetFactory("Simulation");
     if (!factory) return 0;

     void* object = factory->Create();
     if (!object) return 0;

     return reinterpretor_cast<Simulation*>(object);

   }

Combine that with the reflection provided with CINT, which provides the `Factory' part above, and one can load object code on the fly as needed, provided that the code exists, and that you have some sort of mapping from `name' to code. The mapping is by default done in `etc/root/system.rootrc' via simple user editable entries.

This is the `plug-in architecture' or ROOT. The abstract interface, dynamic loading of code based on a mapping look-up.

> > TSQLServer* dbms = TSQLServer::Connect("<dbms>://<host>[:port][/<database>]");

For database connections, the look-up key in the plug-in mapping, is the `<dbms>' part of the connection URI.

> >
> > In your case, using PostgreSQL you need to specify `pgsql' for `<dbms>', that is
> >
> > TSQLServer* dbms = TSQLServer::Connect("pgsql://127.0.0.1/test");
> >
>
> Notwithstanding my comment above, this works. I thought I'd tried
> pgsql://... before posting.

Good chance that you wrote `pqsql' instead of `pgsql' (note that the `g' is miss-typed as a `q') - I know I've done that a thousand times :-)

Yours,

-- 
 ___  |  Christian Holm Christensen 
  |_| |  -------------------------------------------------------------
    | |  Address: Sankt Hansgade 23, 1. th.  Phone:  (+45) 35 35 96 91
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    _|            Denmark                    Office: (+45) 353  25 404
 ____|   Email:   cholm_at_nbi.dk               Web:    www.nbi.dk/~cholm
 | |

Received on Wed Jul 19 2006 - 01:02:33 MEST