Dear list!
My name is Andrii. I work for Adyen. We are using haproxy as our main
software loadbalancer at quite large scale.
Af of now our main use-case for backends routing based on
server-template and dynamic dns from consul as service discovery. Below
is the example of simple backend configuration:
```
backend example
balance roundrobin
server-template application 10 _application._tcp.consul resolvers
someresolvers init-addr last,libc,none resolve-opts allow-dup-ip
resolve-prefer ipv4 check ssl verify none
```
and in global configuration
```
resolvers someresolvers
nameserver ns1 10.10.10.10:53
nameserver ns2 10.10.10.11:53
```
As we see haproxy will create internal table for backends with some
be_id and be_name=application and allocate 10 records for each server
with se_id from 1 to 10. Then those records get populated and updated
with the data from resolvers.
I would like to understand couple of things with regards to this
structure and how it works, which I could not figure out myself from the
source code:
1) In tcpdump for dns queries we see that haproxy asynchronously polls
all the nameservers simultaneously. For instance:
```
11:06:17.587798 eth2 Out ifindex 4 aa:aa:aa:aa:aa:aa ethertype IPv4
(0x0800), length 108: 10.10.10.50.24050 > 10.10.10.10.53: 34307+ [1au]
SRV? _application._tcp.consul. (60)
11:06:17.587802 eth2 Out ifindex 4 aa:aa:aa:aa:aa:aa ethertype IPv4
(0x0800), length 108: 10.10.10.50.63155 > 10.10.10.11.53: 34307+ [1au]
SRV? _application._tcp.consul. (60)
11:06:17.588097 eth2 In ifindex 4 ff:ff:ff:ff:ff:ff ethertype IPv4
(0x0800), length 205: 10.10.10.10.53 > 10.10.10.50.24050: 2194 2/0/1 SRV
0a5099e5.addr.consul.:25340 1 1, SRV 0a509934.addr.consul.:26010 1 1 (157)
11:06:17.588097 eth2 In ifindex 4 ff:ff:ff:ff:ff:ff ethertype IPv4
(0x0800), length 205: 10.10.10.11.53 > 10.10.10.50.63155: 2194 2/0/1 SRV
0a5099e5.addr.consul.:25340 1 1, SRV 0a509934.addr.consul.:26010 1 1 (157)
```
Both nameservers reply with the same response. But what if they are out
of sync? Let's say one says: server1, server2 and the second one says
server2, server3? So far testing this locally - I see sometimes the
reply overrides the table, but sometimes it seems to just gets merged
with the rest.
2) Each entry from SRV reply will be placed into the table under
specific se_id. Most of the times that placement won't change. So, for
the example above the most likely 0a5099e5.addr.consul. and
0a509934.addr.consul. will have se_id 1 and 2 respectively. However
sometimes we have the following scenario:
1. We admistratively disable the server (drain traffic) with the next
command:
```
echo "set server example/application1 state maint" | nc -U
/var/lib/haproxy/stats
```
the MAINT flag will be added to the record with se_id 1
2. Instance of application goes down and gets de-registered from consul,
so also evicted from srv replies and out of discovery of haproxy.
3. Instance of application goes up and gets registered by consul and
discovered by haproxy, but haproxy allocates different se_id for it.
Haproxy healthchecks will control the traffic to it in this case.
4. We will still have se_id 1 with MAINT flag and application instance
dns record placed into different se_id.
The problem comes that any new discovered record which get placed into
se_id 1 will never be active until either command:
```
echo "set server example/application1 state ready" | nc -U
/var/lib/haproxy/stats
```
executed or haproxy gets reloaded without state file. With this pattern
we basically have persistent "records pollution" due to operations made
directly with control socket.
I am not sure is there anything to do about this. Maybe, if haproxy
could cache the state not only of se_id but also associated record with
that and then if that gets changed - re-schedule healtchecks. Or instead
of integer ids use some hashed ids based on dns/ip-addresses of
discovered records, in this case binding will happen exactly in the same
slot.
Thanks in advance!
--
Best regards,
Andrii Ustymenko
