Wt

h1. Using HAproxy as a reverse proxy

"HAproxy":http://haproxy.1wt.eu/ has a great feature set when used in conjunction with Wt:

• Uses async I/O and thus handles thousands of connections without any problem. Just like Wt!
• Supports reverse proxying of WebSocket connections (as per draft-76).
• Can be configured to use session affinity without needing cookies.

You need a fairly recent of haproxy for the options 'http-server-close' and 'http-pretend-keepalive' to work, which is needed for reliable load-balancing.

h2. Basic setup

global
log 127.0.0.1 local0
log 127.0.0.1 local1 notice
maxconn 4096
user haproxy
group haproxy
daemon

defaults
log global
mode http
option httplog
option dontlognull
option http-server-close
option http-pretend-keepalive
option forwardfor
option originalto
retries 3
option redispatch
maxconn 2000
contimeout 5000
clitimeout 50000
srvtimeout 50000

listen 0.0.0.0:8181
server srv1 0.0.0.0:9090 check

h2. Using session affinity

All of the built-in mechanisms in HAproxy for session affinity using the @appsession@ option rely on cookies, but cookies are not our preferred method since this does not give an intuitive user experience (e.g. a user cannot open multiple sessions), are not entirely reliable (a user can disable cookies) and a source of security risks (CSRF).

Luckily there is a work-around: using Wt's ability to generate session-id's that have a prefix which identifies the back-end, we can have HAproxy match on this prefix in the request URL and send the requests to the correct server.

Below is an example configuration for two back-end servers.

global
log 127.0.0.1 local0
log 127.0.0.1 local1 notice
maxconn 4096
user haproxy
group haproxy
daemon

defaults
log global
mode http
option httplog
option dontlognull
option http-server-close
option http-pretend-keepalive
option forwardfor
option originalto
retries 3
option redispatch
maxconn 2000
contimeout 5000
clitimeout 50000
srvtimeout 50000

frontend wt
bind 0.0.0.0:80
acl srv1 url_sub wtd=wt1
acl srv2 url_sub wtd=wt2
acl srv1_up nbsrv(bck1) gt 0
acl srv2_up nbsrv(bck2) gt 0
use_backend bck1 if srv1_up srv1
use_backend bck2 if srv2_up srv2
default_backend bck_lb

backend bck_lb
balance roundrobin
server srv1 0.0.0.0:9090 track bck1/srv1
server srv2 0.0.0.0:9091 track bck2/srv2

backend bck1
balance roundrobin
server srv1 0.0.0.0:9090 check

backend bck2
balance roundrobin
server srv2 0.0.0.0:9091 check

And start the two Wt httpd servers using:

$ app.wt --session-id-prefix=wt1 --http-port 9090 ...
$ app.wt --session-id-prefix=wt2 --http-port 9091 ...