-------------------------------
  |  application component      |          
-----    --------    --------   |
|   |    |  bo  |    |  bo  |   |
-----    |------|    |------|   |
  |      |      |    |      |   |
-----    |      |    |      |   |
|   |    --------    --------   |
-----                           |
  |                             |
  |                             |
  |                             |
  -------------------------------