+------------+          +------------+
|            |    5     |            |
| App Client +----------> App Server |
|            |          |            |
+-+--------^-+          +------------+
  |1       |4
  |        |
+-v--------+-+  3       +---------------+
|            <----------+               |
| User Agent |          | Push Service  |
|            +---------->               |
+------------+  2       +---------------+