Re: HAProxy - Frontend / Backend transparency

Hello,

On Fri, Feb 26, 2010 at 03:11:07PM -0300, Ariel Moreno wrote:
> Dear Sir.
> We are testing HAProxy in our network to balance some proxy
> servers and I was wandering about how the front-end and back-end interact to
> make the solution reliable.
> In this particular case, I have simulated a full back-end proxy fail in
> order to know if there will be any change in the front-end listening, but
> nothing happened.

This is expected because a frontend can connect to multiple backends.

> Question: Does the HAProxy have any setting to disable the frontend when
> there is a failure of all back-end servers?

Yes, there is a solution. For this you need to use "monitor-uri" and "monitor fail". For instance, let's say that an external equipment checks haproxy every second on URI "/haproxy-health". You want haproxy to report that it's unusable if there are less than two servers in either of the two backend connected to the frontend :

frontend www

	mode http
	bind :80
	acl bk1_dead nbsrv(bk1) lt 2
	acl bk2_dead nbsrv(bk2) lt 2
	monitor-uri /haproxy-health
	monitor fail if bk1_dead or bk2_dead
	...
	use_backend bk1 if ...
	use_backend bk2 if ...

backend bk1
	...

backend bk2
	...

But you must keep in mind that the frontend will always respond to the port, so you must really perform an HTTP check there. There are 2 reasons to that :

• if you just perform a TCP connection, you're not testing connectivity to haproxy but to the OS which queues the connection request into the TCP backlog. So haproxy's status would not necessarily be reflected outside.
• no most OSes, if you unbind stop listening on a port without unbinding, you can't listen again afterwards. In fact I've only found Linux to support that. And if you unbind, you can't rebind after you have dropped privileges.

Anyway that's not specific to haproxy, as a simple TCP connect does not reflect services availability in general. That's the reason why the "monitor-uri" was implemented.

Hoping this helps,
Willy Received on 2010/02/26 21:56