HAProxy with multiple backends without conditions - url-rewriting

Question

I would like to use HAProxy to forward an inbound request to two servers, an old one and a new one, depending on which one is up and running, or randomly if both of them are up.
If the request hits the new server I also need a request rewrite with a special path (hence the http-request below):
frontend myservice
bind:1111
use_backend back-a
use_backend back-b
backend back-a
server back-0 hostname_old:2222 check
backend back-b
http-request set-path /special-path/%[path]
server back-1 hostname_new:443 check-ssl ssl verify none
If the hostname_old is down I would like the request to be forwarded to hostname_new, and viceversa.
However with the configuration above only the first use_backend is used.
So if hostname_old is down I get an error.
If hostname_old is up it works.
Similarly if I swap back-a with back-b I only get a reply when hostname_new is up, otherwise I get an error.
Any ideas how I can get it to forward the request to back-a or back-b depending on which one is up and running?

Answer 1

It looks like A/B testing.
We will need nbsrv, rand, checking server state and probably cookies to stick to the randomly chosen backend with the help of http-request set-var with txn (request and response) scope.
Below i describe config lines relevant to the A/B testing:
acl cookie_backend_a req.cook(backend) a - is there a cookie named backend telling us with value a?
acl backend_up_a nbsrv(backend-a) gt 0 - check number of working servers in backend-a and assume that more than 0 is good enough
acl rand_20 rand(100) ge 20 - roll 0-99 and return true if >= 20, so 80% chance to use this, tweak as desired
http-request set-var(txn.backend) req.cook(backend) - store request cookie for response processing
use_backend backend-a if backend_up_a cookie_backend_a - use backend-a if it is up and we have cookie for it
use_backend backend-a if backend_up_a backend_up_b rand_20 - no cookie, both backends up and we rolled >=20 so go to backend-a
use_backend backend-b if backend_up_a backend_up_b - both backends up, but roll failed, so haproxy goes to next config line and chooses backend-b
use_backend backend-a if backend_up_a - no cookie, only one backend up, so go to working one
default_backend maint - you may want to send clients somewhere else when both backends are down
acl cookie_backend_a var(txn.backend) a - i put this ACL in backends, we check cookie value for our backend, but this is for http-response
http-response add-header set-cookie "backend=a; path=/" unless cookie_backend_a - add set-cookie header, which will bind our client to this backend if they didn't have cookie before and rolled into this backend
All put together below:
frontend foo
bind *:1080
acl cookie_backend_a req.cook(backend) a
acl cookie_backend_b req.cook(backend) b
acl backend_up_a nbsrv(backend-a) gt 0
acl backend_up_b nbsrv(backend-b) gt 0
acl rand_20 rand(100) ge 20
http-request set-var(txn.backend) req.cook(backend)
use_backend backend-a if backend_up_a cookie_backend_a
use_backend backend-b if backend_up_b cookie_backend_b
use_backend backend-a if backend_up_a backend_up_b rand_20
use_backend backend-b if backend_up_a backend_up_b
use_backend backend-a if backend_up_a
use_backend backend-b if backend_up_b
default_backend maint
backend backend-a
acl cookie_backend_a var(txn.backend) a
option httpchk
http-check send meth HEAD uri / ver HTTP/1.1 hdr host localhost
http-response add-header set-cookie "backend=a; path=/" unless cookie_backend_a
server a 127.0.0.1:80 check
backend backend-b
acl cookie_backend_b var(txn.backend) b
option httpchk
http-check send meth HEAD uri / ver HTTP/1.1 hdr host localhost
http-response add-header set-cookie "backend=b; path=/" unless cookie_backend_b
server b 127.0.0.1:81 check
backend maint
(...)
In the backend sections you can tweak request as you did in your question and whatever else you need.
In this config if the client had cookie for backend-a, but it's down, then he goes to backend-b, gets its cookie and stays there until backend-b fails or cookie expires (which i didn't set up in this config). Tweak as needed.