[Reference: http://blogs.cisco.com/tag/opflex/ , Policy Repository, The policy repository (PR) is a logically centralized entity containing the definition of all policies governing the behavior of the system. In Cisco ACI, this function is performed by the Cisco APIC or by the leaf nodes of the network fabric. The policy authority handles policy resolution requests from each policy element., Policy Element (Policy Agent), A policy element (PE) is a logical abstraction for a physical or virtual device that implements and enforces policy. This is where the Policy Agent describe in detail herein resides. Policy elements are responsible for requesting portions of the policy from the policy authority as new endpoints connect, disconnect, or change. Additionally, policy elements are responsible for rendering that policy from an abstract form into a concrete form that maps to their internal capabilities. This process is a local operation and can function differently on each device as long as the semantics of the policy are honored., Endpoint Registry, The endpoint registry (ER) stores the current operation state (identity, location, etc.) of each endpoint (EP) in the system. The endpoint registry receives information about each endpoint from the local policy element and then can share it with other policy elements in the system. The endpoint registry may be physically co-located with the policy authority, but it may also be distributed in the network fabric itself. In Cisco's ACI solution, the endpoint registry actually lives in a distributed database within the network itself to provide additional performance and resiliency., ]

[Reference: http://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/unified-fabric/white-paper-c11-730021.html (effect of replication on reliablity), , ]

[Reference: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/aci/apic/sw/1-x/troubleshooting/b_APIC_Troubleshooting/b_APIC_Troubleshooting_chapter_01.html (About atomic counters), , ]