I'm working with the IBM team implementing the Active-Active N+1 topology [1].
I've been commissioned with the task to help integrate the code supporting the new topology while a) making as few code changes and b) reusing as much code as possible. To make sure the changes to existing code are future-proof, I'd like to implement them outside AA N+1, submit them on their own and let the AA N+1 base itself on top of it. --TL;DR-- what follows is a description of the challenges I'm facing and the way I propose to solve them. Please skip down to the end of the email to see the actual questions. --The details-- I've been studying the code for a few weeks now to see where the best places for minimal changes might be. Currently I see two options: 1. introduce a new kind of entity (the distributor) and make sure it's being handled on any of the 6 levels of controller worker code (endpoint, controller worker, *_flows, *_tasks, *_driver) 2. leave most of the code layers intact by building on the fact that distributor will inherit most of the controller worker logic of amphora In Active-Active topology, very much like in Active/StandBy: * top level of distributors will have to run VRRP * the distributors will have a Neutron port made on the VIP network * the distributors' neutron ports on VIP network will need the same security groups * the amphorae facing the pool member networks still require * ports on the pool member networks * "peers" HAProxy configuration for real-time state exchange * VIP network connections with the right security groups The fact that existing topologies lack the notion of distributor and inspecting the 30-or-so existing references to amphorae clusters, swayed me towards the second option. The easiest way to make use of existing code seems to be by splitting load-balancer's amphorae into three overlapping sets: 1. The front-facing - those connected to the VIP network 2. The back-facing - subset of front-facing amphorae, also connected to the pool members' networks 3. The VRRP-running - subset of front-facing amphorae, making sure the VIP routing remains highly available At the code-changes level * the three sets can be simply added as properties of common.data_model.LoadBalancer * the existing amphorae cluster references would switch to using one of these properties, for example * the VRRP sub-flow would loop over only the VRRP amphorae * the network driver, when plugging the VIP, would loop over the front-facing amphorae * when connecting to the pool members' networks, network_tasks.CalculateDelta would only loop over the back-facing amphorae In terms of backwards compatibility, Active-StandBy topology would have the 3 sets equal and contain both of its amphorae. An even more future-proof approach might be to implement the sets-getters as selector methods, supporting operation on subsets of each kind of amphorae. For instance when growing/shrinking back-facing amphorae cluster, only the added/removed ones will need to be processed. Finally (thank you for your patience, dear reader), my question is: if any of the above makes sense, and to facilitate the design/code review, what would be the best way to move forward? Should I create a mini-blueprint describing the changes and implement it? Should I just open a bug for it and supply a fix? Thanks, -Sergey. [1] https://review.openstack.org/#/c/234639
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