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Hey Sergey--

Apologies for the delay in my response. I'm still wrapping my head around
your option 2 suggestion and the implications it might have for the code
base moving forward. I think, though, that I'm against your option 2
proposal and in favor of option 1 (which, yes, is more work initially) for
the following reasons:

A. We have a precedent in the code tree with how the stand-alone and
active-standby topologies are currently being handled. Yes, this does
entail various conditionals and branches in tasks and flows-- which is not
really that ideal, as it means the controller worker needs to have more
specific information on how topologies work than I think any of us would
like, and this adds some rigidity to the implementation (meaning 3rd party
vendors may have more trouble interfacing at that level)...  but it's
actually "not that bad" in many ways, especially given we don't anticipate
supporting a large or variable number of topologies. (stand-alone,
active-standby, active-active... and then what? We've been doing this for a
number of years and nobody has mentioned any radically new topologies they
would like in their load balancing. Things like auto-scale are just a
specific case of active-active).

B. If anything Option 2 builds more less-obvious rigidity into the
implementation than option 1. For example, it makes the assumption that the
distributor is necessarily an amphora or service VM, whereas we have
already heard that some will implement the distributor as a pure network
routing function that isn't going to be managed the same way other amphorae
are.

C. Option 2 seems like it's going to have a lot more permutations that
would need testing to ensure that code changes don't break existing /
potentially supported functionality. Option 1 keeps the distributor and
amphorae management code separate, which means tests should be more
straight-forward, and any breaking changes which slip through potentially
break less stuff. Make sense?

Stephen


On Sun, May 29, 2016 at 7:12 AM, Sergey Guenender <GUENEN at il.ibm.com> wrote:

> 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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