[Kea-users] Kea 1.4 HA questions

[Jason Guy]

After reading the HA page http://kea.isc.org/wiki/HADesign, I realized that
the mode for Load-balancing does not state if a separate device (load
balancer) is used to direct the traffic to the servers. It would be good to
indicate how the traffic is distributed across the servers.

There is a growing trend in networking, to utilize IP equal-cost multipath
(ECMP) forwarding to reach services, rather than maintain a separate device
(like an F5 or Linux HAproxy). This does not work for all services
obviously, but I think it could work fine for DHCP, provided it is fully
HA-aware.

How will the current HA design work in the scenario where the clients are
reaching the load-balanced DHCP servers via dhcp-relay?
1) Client sends the DHCPDISCOVER, which is relayed and arrives on Server1
2) Server1 sends DHCPOFFER to the Client via the relay.
3) Client sends DHCPREQUEST via the relay, but arrives on server2.
4) Server2 should send a DHCPACK, but will it?
Is the state between the servers synchronized at each point in the overall
transaction, such that server2 can complete the lease?

I think the answer is no, because it appears that the subnet definitions in
the Kea configurations have to be manually partitioned, as explained in the
section Subnets and Pools Configuration for HA. This implies that each
server is still basically autonomous in the subnets they can allocate,
while both servers are alive.

This seems like a cumbersome way to implement HA from the user point of
view. It is much more intuitive to have a matching configuration (enforced
when the HA communication is established), and either server can perform
operations on the same resources. In openstack this is all handled via a
message queue, so redundant nodes know the active state of transactions.

Anyhow, I think this is a really cool addition to Kea. I am curious how it
will work for various data center environments I see every day, where the
microservices concept is way things are moving. It may be I misunderstood
the HAdesign page, but I would like to understand this better as I plan to
test this as soon as possible.

Cheers,
Jason
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