This UML Diagram uses DBI as an example Resource. Other Resources are handled in the same way.

             +---------------------------------+
             | ResourcePool::LoadBalancer      |
             +---------------------------------+
             +---------------------------------+
             | +new(key, options)              |
             | +add_pool(pool, options)        |
             +---------------------------------+
                               |
                               |
                               V      +---------------+
             +------------------------+ PlainResource |
             | ResourcePool           +--------+------+
             +---------------------------------+
             | -factory: ResourcePool::Factory +--+
          +--+ -pool: ResourcePool::Resource   |  |
          |  +---------------------------------+  |
          |  | +new(fatory,options)            |  |
          |  | +get():PlainResource            |  |
          |  | +free(plainresource)            |  |
          |  | +fail(plainresource)            |  |
          |  +---------------------------------+  |
          |                    |                  |
          |                   / \                 |
          |  +---------------------------------+  |
          |  | ResourcePool::Singleton         |  |
          |  +---------------------------------+  |
          |  +---------------------------------+  |
          |  +---------------------------------+  |
          V                                       V
+-----------------------------+  +-----------------------------+
| ResourcePool::Resource::DBI |  | ResourcePool::Factory::DBI  |
+-----------------------------+  +-----------------------------+
| #dbh                        |  | -ds                         |
+-----------------------------+  | -user                       |
| +new(ds, user, auth, attr)  |  | -auth                       |
| +get_plain_resource()       |  | -attr                       |
| +close()                    |  +-----------------------------+
| +fail_close()               |  | +new(ds, user, auth, attr)  |
| +precheck()                 |  | +create_resource()          |
| +postcheck()                |  +-----------------------------+
+-----------------------------+                 |
               |                                |
              / \                              / \
+-----------------------------+  +-----------------------------+
| ResourcePool::Resource      |  | ResourcePool::Factory       |
+-----------------------------+  +-----------------------------+
+-----------------------------+  +-----------------------------+
| +new()                      |  | +new(key)                   |
| +get_plain_resource()       |  | +create_Resource()          |
| +close()                    |  +-----------------------------+
| +fail_close()               |                 |
| +precheck()                 |                 |
| +postcheck()                |                / \
+-----------------------------+  +-----------------------------+
                                 | ResourcePool::Singleton     |
                                 +-----------------------------+
                                 +-----------------------------+
                                 +-----------------------------+