The other day I got a challenge: I had to 1) do a basic setup of haproxy so one frontend would round-robin on 3 backends (simple task), but 2) verify that it was actually round-robin'ing, without using a web browser (to access the stats page that haproxy can be
configured to serve, eg see “HAProxy Stats Page” on https://www.haproxy.com/blog/introduction-to-haproxy-logging/) or syslog.
I tried curl which technically is not a web browser, but html was horribly formatted and didn't seem to have the backend status up/down etc; and converting to markdown and plain text with pandoc did not help.
This won't be news to those who work with haproxy all the time, but for those starting:
Turns out the haproxy makes status available through a unix socket that can be configured in the global section of /etc/haproxy/haproxy.cfg. On my centOS it defaults to /var/lib/haproxy/stats, whereas on ubuntu it defaults to /run/haproxy/admin.sock.
After a bit of research I found it was sufficient to send the show stat command to the socket, and then process the output. Even better,
watch 'echo "show stat" | sudo nc -U /var/lib/haproxy/stats | cut -d "," -f 1,2,8-10,18 | column -s, -t'
on the load balancer host (LB_HOST) to get the stats. Then in a separate terminal (same or different host), I ran
while true; do curl LB_HOST:HAPROXY_PORT; sleep 1; done
to generate http requests on the load balancer, and in a third terminal I could add/remove backend servers and observe the load-balancer status up/down. Example output of the watched command:
$ echo "show stat" | sudo nc -U /run/haproxy/admin.sock | cut -d "," -f 1,2,8-10,18 | column -s, -t
# pxname svname stot bin bout status
test_frontend FRONTEND 275 21725 67755 OPEN
test_backend server1 158 12482 39026 UP
test_backend server2 93 6399 19871 DOWN
test_backend server3 39 2844 8858 DOWN
test_backend BACKEND 275 21725 67755 UP
Pretty cool!
You can even have a "prompt" where you can do various things other than stats, like take down one of the backend servers for maintenance, debug certificates/SSL, etc.
Eg with only servers 1 and 2 up (ie 3 is down), I disabled 1 and 2 in haproxy for maintenance, by doing this:
$ sudo nc -U /run/haproxy/admin.sock
prompt
> disable server test_backend/server1
> disable server test_backend/server2
then the watch automatically updated to show this:
# pxname svname stot bin bout status
test_frontend FRONTEND 674 53246 164642 OPEN
test_backend server1 472 37288 116584 MAINT
test_backend server2 148 11692 36556 MAINT
test_backend server3 0 0 0 DOWN
test_backend BACKEND 674 53246 164642 DOWN
Notice now the whole BACKEND is down too, because of server 3. Start server 3 and observe its state going from DOWN to UP, and then that of BACKEND does same.
Enjoy!
A couple of useful refs:
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