Hiding Access to Multiple Databases

From: Shane Mingins (shanemingins_at_yahoo.com.clothes)
Date: 02/25/04 

Date: Thu, 26 Feb 2004 11:57:23 +1300

Hi

I am wondering if there is a pattern/solution to accessing multiple
instances of a database?

As an example:

I have two different databases (A & B) with Customers in. Customer "Xavier
Smith" is in both databases.

In database A Xavier has sales of $1000.00
In database B Xavier has sales of $2000.00

What I want to achieve in the Domain Layer is something like:

DB.getCustomers() and return a list of distinct customers, IOW Xavier will
appear once.

Then with the instance of Customer representing Xavier I wish to be able to
call getTotalSales(DateRange) and return $3000.00

The only possibilities that I can see is either:

One solution needs Customer to "know" which of the databases it came from
(i.e. A or B, or both A and B) to quickly request the sales values;

Or the databases are re-queried when getTotalSales is called to determine
which contain the Customer to then request the sales values;

Or each database is queried for sales values with one of those queries
possibly returning zero.

So are there other possibilities to consider?

Or is it a question of choosing between performance over separation, IOW to
totally isolate the Domain Layer from knowledge of the "where" it came from
there will be a loss in performance?

[In case it is relevant to a reply, I am using Java, the databases are
proprietary with read-only access via JDBC-ODBC]

Thanks
Shane

shanemingins@yahoo.com.clothes
remove clothes before replying

"When everybody actually is out to get you, paranoia is just good
thinking." -- Woody Allan

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