Intelligent RPC routing for multiple conductors

Prerequisites
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1. Given Ironic set up with one or more API services and multiple conductor services running.
2. Different conductors might run different vendor drivers.
3. The environment is heterogeneous.
4. Some of the conductors exclusively locked several nodes.

Problems
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1. As some operations on nodes are interruptible, a user might want to send request to interrupt an operation on a locked node. In that case the request must be dispatched to the same conductor that acquired a lock on the node in order to be able to perform the operation.

2. In a heterogenous environment different nodes might require different drivers to perform power management and image deployment. However, it’s not possible to guarantee that all conductor nodes run drivers for every node vendor. In order to perform deployment of any node it’s necessary to detect which of the conductors runs a required driver and dispatch an RPC call to it.

Solutions
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1. The problem [1] is solvable by tracking the information about locks (which is already done) and forwarding RPC calls to specific conductor.

2. There are several ways to solve the problem [2]:

2.1 Ironic polls all conductors about their drivers periodically and stores that to the DB. The solution is not practical because Ironic has to know about all the conductors.

2.2 Each conductor sends its heartbeat to the API server. The heartbeat contains some meta information about the conductor which is stored to the DB. This solution is more practical because Ironic does not have to know about conductors in order to get the information. However, using heartbeats might cause certain problems.

2.3 In order to get rid of heartbeats it's possible to create different routing topics for different drivers.
Conductors then can subscribe to certain topics depending on what drivers they run. This is the preferred solution for now.