c) a middle-ground approach where we'll store the aggregated usage per app in separate docs, but maintain the aggregated usage at the upper levels (org, space, resource, plan) in the parent doc linking the app usage docs together, and explore what constrains or limitations that would impose on our ability to trigger real time usage limit alerts at any org, space, resource, plan, app etc level.

As a first step (refer to [1] for more details) to refactor the usage data model using middle-ground approach, we have removed Usage Rating Service from Abacus pipeline (refer to commit at [2]) and moved entire rating implementation from Usage Rating Service to Usage Aggregator (refer to commit at [3])

With these commits, If you are using Abacus, be aware that the Abacus pipeline has become shorter and you have one less application (Usage Rating Service) to manage.

[1] https://github.com/cloudfoundry-incubator/cf-abacus/issues/184
[2] https://github.com/cloudfoundry-incubator/cf-abacus/commit/1488e1ae2e4547a010151ad2245f3a3f1ff2e488
[3] https://github.com/cloudfoundry-incubator/cf-abacus/commit/c661b7bdd35e70e985583570cb9920b90ced44a8