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Accepted
Christophe Drevet
Asked: 2015-07-16 08:39:20 +0800 CST

haproxy: how to prepare a server maintenance without kicking app sessions?

  • 772

The issue

I'm using haproxy to load balance web servers. I use session persistence with additional cookies as some applications use session files and these are not synchronized between servers.

I want to disable a server for maintenance, but without breaking sessions. So I would like to allow existing clients to continue their application session, but not accept new clients.

haproxy behavior

  • I set a server to "going to maintenance"
  • if a client has the cookie set, use the server even if marked as "going into maintenance"
  • if a new client (without cookie) come, it is directed to another server
  • after all clients end their application sessions, no more client would have the cookie set to this particular server, and I'd be good to shut it down without user disruption.

Do you think this is achievable with some haproxy configuration? Or is there a clever way to do it?

Other ways

Non exhaustive list of other ways to achieve this need:

  • synchronize session files between servers (needs a way to synchronize files between several servers, or a common single mountpoint)
  • use database to store session information (needs to change application behavior)

More details

I use this kind of configuration:

frontend https-in
   bind xxx.xxx.xxx.xxx:443 ssl crt /etc/haproxy/ssl/_default.pem crt /etc/haproxy/ssl
   reqadd X-Forwarded-Proto:\ https

   acl APP1 hdr(host) -i APP1.atac.local
   use_backend APP1 if APP1

   default_backend _default

backend APP1
   redirect scheme https if !{ ssl_fc }
   mode http
   balance roundrobin
   cookie HAPROXY_SESSION insert indirect
   option httpchk HEAD /haproxy_test_page.php HTTP/1.0\nUser-Agent:\ HAProxy
   server SRV1 SRV1_IP:PORT cookie SRV1 check
   server SRV2 SRV2_IP:PORT cookie SRV2 check

If I just disable SRV1 (with haproxy cli command), I think all application sessions opened on SRV1 will break after the end of the current HTTP "session". Is that right?

haproxy

4 Answers

  1. If you use socat for communicate with your haproxy configurations, you can put a server in drain state of following way:

    echo "set server backend/serv state drain" | sudo socat stdio /etc/haproxy/haproxysock

    More commands here! For install socat in Ubuntu go to this answer

    I test that with haproxy 1.6.3 version :)

    • 16
  2. Best Answer

    Put the server in drain mode using the web management interface. That provides the exact functionality you're looking for.

    For details on the web management interface - https://github.com/Aidaho12/haproxy-wi

    • 6

  3. For the future reader that wants to drain a backend from a HAProxy container...

    docker exec $(docker ps | grep haproxy | awk '{print $1}') bash -c 'echo "set server backend_name/svc_name state drain" | nc -U /haproxy.sock'

    Note:
    The /haproxy.sock path is the one you have set in the haproxy.cfg file [eg stats socket /haproxy.sock mode 660 level admin]

    • 2

  4. Other ways

    synchronize session files between servers (needs a way to synchronize files between several servers, or a common single mountpoint)

    If your backend servers use PHP for the application(s) you can use Memcache to sync the sessions between them.

    Also Couchbase-Server can do memcache replication out of the box.
    Of course it's kind of overkill to use couchbase-server just for sessions replication :)

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