+----- WEB01 -----+ +----- WEB02 -----+
|                 | |                 |
| Apache + PHP    | | Apache + PHP    |
| Apc             | | Apc             |
| mongos          | | mongos          |
+-----------------+ +-----------------+


+----- DB01 -----+  +----- DB02 -----+
|                |  |                |
| replica set 1  |  | replica set 1  |
| replica set 2  |  | replica set 2  |
| replica set 3  |  | replica set 3  |
| mongo config   |  | mongo config   |
+----------------+  +----------------+