General talk about software patterns

["souravm" <SOURAVM AT infosys.com>]

Hi All,

I'm looking for a design pattern for accessing data across multiple
subsystems.

To elaborate -

1. Let us assume that there are 3 functionally independent subsystems in
a system being designed.
2. The interaction happens between these subsystems through messages at
the business layer of each subsystem.
In this way no subsystem accesses the table(s) of other subsystem
directly.
3. However, for performance optimization, it might be needed that for
certain cases one subsystem need to access tables of other subsystems
through JOINs.

In this situation I'll like to know if there is any specific pattern
used for implementing a performance trade off described in point 3 ? 

My aim is to achieve the above mentioned performance goal as well as
retaining the modularity of the subsystems as much as possible.

Any pointer/insight/information will be really appreciated.

Regards,
Sourav