My use case is that I have a non-backwards compatible database migration to
run, so there is a brief period of time where I do not want any requests to
be serviced by frontends. So I would like HAProxy to temporarily queue all
traffic while the database migration runs, buffering it until I bring the
frontend back up. I found an interesting article[1] that explains how to
do this that I am trying to replicate for my own purposes.
Basically the article explains dynamically setting maxconn to 0 while the
migration runs, then setting it back to the original value when done. This
works perfectly when I test it with curl or with a browser that doesn't
have a pre-existing session with the frontend server. But when I do have a
pre-existing session, I am still let in to the frontend with maxconn 0.
And if I disable the frontend server, then I see an HTTP 503 error with
"No server is available to handle this request".
So basically I'm wondering if there is a way to "expire" these pre-existing
sessions or connections or somehow force them to behave like a new one so
that they will queue up in HAProxy?
Here is my haproxy.cfg; I installed from the Ubuntu vbernat/haproxy-1.5 PPA
and have left the global and defaults sections unchanged, only adding the
frontend/backend sections:
global
log /dev/log local0
log /dev/log local1 notice
chroot /var/lib/haproxy
stats socket /run/haproxy/admin.sock mode 660 level admin
stats timeout 30s
user haproxy
group haproxy
daemon
# Default SSL material locations
ca-base /etc/ssl/certs
crt-base /etc/ssl/private
# Default ciphers to use on SSL-enabled listening sockets.
# For more information, see ciphers(1SSL).
ssl-default-bind-ciphers
kEECDH+aRSA+AES:kRSA+AES:+AES256:RC4-SHA:!kEDH:!LOW:!EXP:!MD5:!aNULL:!eNULL
defaults
log global
mode http
option httplog
option dontlognull
timeout connect 5000
timeout client 50000
timeout server 50000
errorfile 400 /etc/haproxy/errors/400.http
errorfile 403 /etc/haproxy/errors/403.http
errorfile 408 /etc/haproxy/errors/408.http
errorfile 500 /etc/haproxy/errors/500.http
errorfile 502 /etc/haproxy/errors/502.http
errorfile 503 /etc/haproxy/errors/503.http
errorfile 504 /etc/haproxy/errors/504.http
frontend localnodes
bind *:8081
mode http
default_backend nodes
backend nodes
mode http
balance roundrobin
option forwardfor
http-request set-header X-Forwarded-Port %[dst_port]
option httpchk HEAD / HTTP/1.1\r\nHost:localhost
server web01 127.0.0.1:3000 check
These are the commands I'm running to test:
echo "set maxconn frontend localnodes 0" | socat stdio
/run/haproxy/admin.sock
echo "disable server localnodes/web01" | socat stdio
/run/haproxy/admin.sock
echo "set maxconn frontend localnodes 100" | socat stdio
/run/haproxy/admin.sock
echo "enable server nodes/web01" | socat stdio /run/haproxy/admin.sock
Thanks,
Abe
[1]:
http://blog.balancedpayments.com/payments-infrastructure-suspending-traffic-zero-downtime-migrations/
