Re: Describing the Janus

David Cressey wrote:
> I want to describe an object that I've been imagining for a while. I've
> called it the "Janus".
>
> A Janus has two faces. I can describe each face reasonably simply: one
> face is an SQL client, the other face is an object, interacting with other
> objects in an object world.
>
> In the SQL client face, the Janus simply issues SQL requests to an SQL
> server, and interprets the consequent responses. It's appearance doesn't
> differ markedly from any other SQL client, like Interactive SQL, or SQL
> that was written into (or invoked from) an application program.

SQL is pretty different from Relational Model or Logical Model. It is more related to the client, as you write. One important characteristic of SQL is that the user uses his knowledge when putting a query. And a query can be created by a procedural language or by a non procedural language. I also think that a query should not change a database (there is no writing into the database), otherwise it is not a query.

>
> In the object oriented face, the Janus acts like any other object in an
> object oriented world. It interprets messages, uses encapsulated methods,
> and issues responses. It can also initiate dialogue with other objects, as
> needed. Its behavior is much like that of other objects in the same object
> world, although it seems to encapsulate an awful lot of data.
>
> The other objects in the object world, don't know that the Janus has another
> face. The SQL server (presumably the front end for a relational DBMS with a
> database mounted on it) doesn't know anything other than the SQL client
> face of the Janus.
>
> The Janus doesn't keep very much inside of itself. All it does is transform
> object oriented requests into SQL requests, and transform SQL responses into
> object oriented responses. It's not clear to me when the Janus would ever
> initiate a message to another object, but that's food for further thought.
> I have some inklings, but they are very vague.
>
> There it is. I can describe it, but I have no idea how to build it.
>
> Can you build a generic one, or would you need a different Janus for each
> object world? Could several Januses (Janae?) all interact with the same
> database but exist in otherwise disconnected object worlds?
> Could several Januses exist in the same object world, but be connected to
> different databases?
>
> Is there any use for such a thing? I imagine that it might be an approach
> to solving the impedance mismatch between the (SQL based) relational DBMS
> products and the world of Object Oriented application development.

Vladimir Odrljin Received on Fri Sep 09 2005 - 19:04:13 CEST