Haproxy redirection based on semaphore detection

Help!
1

I would like to redirect requests to a “sorry page” during maintenance downtimes. Is it possible to base redirection on whether, or not, a semaphore file is present on my haproxy host?

That is:

If file is present, redirect to sorry page.

Thank you

sean.r

2

I’m not aware of any way to interact with the file-system from HAProxy.

However there are multiple ways to achieve what you are searching for (from simplest to most complex):

  • use backend server checks for an path like /up, which if it is served (say by NGinx) from the server’s filesystem you can just remove it to mark the server down for HAProxy; (then use nbsrv sample to identify if there are any servers left in a backend, and route requests to the “maintainance” website server;)
  • use the HAProxy stats page with admin rights enabled (or a tool like hatop, or directly by using HAProxy management commands) to manually mark the server down or for maintainance; (also use the nbsrv sample as above;) (see set server health) (you can even automate this with socat as described in the linked documentation;)
  • if you really need to use that semaphore file on the HAProxy server, you could use the following trick: install webfs (an extremly simple webserver), make it listen only on 127.0.0.1 and use that as a backend server check target (use the addr and port server options, which although don’t have the “check” in their name, they refer to that;)
  • or alternatively you can use the agent-check directive that allows you not only to designate a server up or down, but also its weight, etc.;
  • you can even come up with something like: have a path like /admin/up/<random-token> and /admin/down/<random-token> which you can use to set a process variable to decide the routing by using http-request set-var(proc.down) 1 if { path /admin/.... } (and similar for up); then route traffic to the “maintainance” webserver based on this variable with ACL’s;

I hope that I gave you enough (and diverse) ideas to solve your issue. If you settle on a particular option and want more info, just let me know.