Re: realtime switch to another backend if got 5xx error?

From: Maciej Bogucki <macbogucki#gmail.com>
Date: Mon, 15 Jun 2009 08:50:09 +0200

Dawid Sieradzki / Gadu-Gadu S.A. pisze:
> Hi.
>
> The problem is how to silent switch to another backend in realtime if
> got 500 answer from backend, without http_client knowledge
> Yes i know, httpchk, but the error 500 is 10 per hour, we don't know
> when and why.
> So, it is a race who get 500 first - httpchk or http_client.
>
> If You don't know what i mean:
>
> example config:
>
> ----8<--------
>
> frontend
> (..)
> default_backend back_1
>
> backend back_1
> option httpchk GET /index.php HTTP/1.1\r\nHost:\ test.pl
> mode http
> retries 10
> balance roundrobin
>
> server chk1 127.0.0.1:81 weight 1 check
> server chk2 127.0.0.1:82 weight 1 check
> server chk3 127.0.0.1:83 weight 1 check backup
>
> ---->8----------
>
> http_client -> haproxy -> (backend1|backend2|backend3)
>
> let's go inside request:
>
> A. haproxy recived request from http_client
> B. haproxy sent request from http_client to backend1
> C. backend1 said 500 internal server error
>
> I want: :-)
> D. haproxy sent request from_http to backend2 (or backup backend or
> another one, or one more time to backend1)
>
> I have: :-(
> D. haproxy sent 500 internal server error to http_client from backend1
> E. haproxy will mark backend1 as down if got 2 > errror 500 from backend1
>
>
> It is possible to do that?
>
>
>

Hello,

Yes it is possible but it could be dengerous for some kinde of application fe. billing system ;)
Here is an example how to do it. I know that it is the hack but it works good ;P

frontend fr1

      default_backend back_1
      rspirep         ^HTTP/...\ [23]0..* \0\nSet-Cookie:\ 
cookiexxx=0;path=/;domain=.yourdomain.com
      rspirep         ^(HTTP/...)\ 5[0-9][0-9].* \1\ 202\ Again\ 

Please\nSet-Cookie:\
cookiexxx=1;path=/;domain=.yourdomain.com\nRefresh:\ 6\nContent-Length:\ Lenght_xxx\nContent-Type:\ text/html\n\n<FRAMESET\ cols="100%"><FRAME\ src="http://www.yourdomain.com/redispatch.pl">

backend back_1

       cookie          cookiexxx
       server chk1 127.0.0.1:81 weight 1 check
       server chk2 127.0.0.1:82 weight 1 check
       server chk3 127.0.0.1:83 weight 1 check cookie 1 backup

Remember to set Lenght_xxx properly.

Best Regards
Maciej Bogucki Received on 2009/06/15 08:50