hope this data will help another newbie like me, here is the sequence.

1. User sends REST API request to Quantum, it is routed to the corresponding Extension and Plugin.
2. Plugin performs validation of request (schema conformance, values and references check, etc). If validation succeed, HTTP 202 is returned, if not - one of 40x codes (depending on reason).
3. DB object is updated and object is moved to PENDING_UPDATE state. Request is transformed into task and pushed into queue. The device corresponding to object is locked.
4. The worker scans queue and picks any task for unlocked device. The task is transmitted to Agent
5. Agent with help of Driver changes configuration of LB appliance.
6. If some other user request arrives to the Plugin it is processed a usual way, except of rejecting attempts to update object in PENDING_UPDATE state. If request succeeds Plugin pushes it into queue. If the request is for the locked device, it remain in the queue until device is unlocked.
7. Once Agent completes work, it replies Plugin with the status. DB is updated and object is moved into ACTIVE or ERROR state. Device is unlocked.

source https://wiki.openstack.org/wiki/Neutr...