Application, database and schema?

From: JMF (jfavaro_at_tin.it)
Date: 01/11/05 

Date: Tue, 11 Jan 2005 11:36:21 GMT

While we're sort of on the topic of O-O and databases: a colleague has been
drawing a class diagram for a system where there's also an RDB (I think it's
MySQL) that does something simple like store the log, the list of events.
But the way he depicted this situation I found puzzling.

           <<protocol>>
Application-----------> Schema
    | / \
    | ___
    | |
    V |
  MySQL ---------->ApplicationDatabase

Since that probably doesn't print well, here's how he described it: "The
application has an association with the MySQL RDB. It in turn has an
association to the actual database containing the data."

That part makes sense to me. But then he shows the ApplicationDatabase
(containing the user's data) realizing an interface, which is its schema. He
explained this saying "The way I see it, a database's schema is effectively
its interface, because that's how you access it."

Then he put a dependency from the Application to the Schema, stereotyped
with <<protocol>>. His explanation: "The Application depends on the Schema
of my database, because if I change the schema, that affects my application.
The protocol is SQL."

Well, that part about the schema being the interface to the database doesn't
make much sense to me. Yet I do get his point about how the application does
have some kind of dependency on the schema.

But I'm not sure how to get this untangled. How would you express all this?

John

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