FUNCTIONAL UNITS vs BLACK BOX

Time for some ASCII art. Here’s a typical system divided into Functional Units.

FU-A                 FU-B              FU-C              FU-D
Client PC            Web Server        App Server        Database Server

We can treat FU-D as a black box.

FU-A                 FU-B              FU-C            +--- Black Box ---+
Client PC            Web Server        App Server      | Database Server |
                                                       +-----------------+

We can treat FU-C and FU-D as a black box

FU-A                 FU-B            +------------ Black Box ------------+
Client PC            Web Server      | App Server        Database Server |
                                     +-----------------------------------+
 

We can treat FU-B, FU-C and FU-D as a black box.

FU-A               +--------------------- Black Box ---------------------+
Client PC          | Web Server        App Server        Database Server |
                   +-----------------------------------------------------+
 

We can’t treat FU-B as a standalone black box.

We can’t treat FU-C as a standalone black box.