I couldn't find this behavior described in the documentation, indeed.
I am not sure to what extent documentation should cover this. For us it is important to understand how it works, so we can build more predictable/reliable setup of the load-balancing layer.
Also, if the behavior has changed/is going to be changed in new versions, maybe, indeed some words would be nice to have in the documentation as well.
On 08/04/2024 10:45, Илья Шипицин wrote:
am I right that you consider that as a documentation bug ? пн, 8 апр. 2024 г. в 10:44, Andrii Ustymenko <andrii.ustyme...@adyen.com>: Yes, for the 1) question indeed. Basically I have tested with local "out of sync" custom nameserver. And I was observing some inconsistent results of the backend-servers table. That led to this question. Most of the time I was seeing the state of only from the local nameserver. However sometimes I have seen the "merged" state where all the replies existed together in the table. It was also observed that amount of the requests made by haproxy to all nameservers is the same even though the local one normally replies faster. And sorry, forgot to mention we are running haproxy version 2.8.7 On 08/04/2024 10:31, Илья Шипицин wrote:--and particularly your question is "does HAProxy merge all responses or pick the first one or use some other approach" ? пн, 8 апр. 2024 г. в 10:23, Andrii Ustymenko <andrii.ustyme...@adyen.com>: I guess indeed it is not a case of consul-template specifically, but more of rendered templates and how haproxy maintains it. On 06/04/2024 20:15, Илья Шипицин wrote:--Consul template is something done by consul itself, after that haproxy.conf is rendered Do you mean "how haproxy deals with rendered template"? On Fri, Apr 5, 2024, 15:02 Andrii Ustymenko <andrii.ustyme...@adyen.com> wrote: 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 <http://10.10.10.10:53> nameserver ns2 10.10.10.11:53 <http://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 UstymenkoAndrii Ustymenko Platform Reliability Engineer office +31 20 240 12 40 Adyen Headquarters Simon Carmiggeltstraat 6-50, 5th floor 1011 DJ Amsterdam, The Netherlands Adyen <https://www.adyen.com>Andrii Ustymenko Platform Reliability Engineer office +31 20 240 12 40 Adyen Headquarters Simon Carmiggeltstraat 6-50, 5th floor 1011 DJ Amsterdam, The Netherlands Adyen <https://www.adyen.com>
-- Andrii Ustymenko Platform Reliability Engineer office +31 20 240 12 40 Adyen Headquarters Simon Carmiggeltstraat 6-50, 5th floor 1011 DJ Amsterdam, The Netherlands Adyen <https://www.adyen.com>
