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Re: Load balancing and load determination

On Mon, Nov 5, 2018 at 7:48 AM jean-frederic clere <jfclere@xxxxxxxxx> wrote:
On 30/10/2018 13:53, Jim Jagielski wrote:
> As some of you know, one of my passions and area of focus is
> on the use of Apache httpd as a reverse proxy and, as such, load
> balancing, failover, etc are of vital interest to me.
> One topic which I have mulling over, off and on, has been the
> idea of some sort of universal load number, that could be used
> and agreed upon by web servers. Right now, the reverse proxy
> "guesses" the load on the backend servers which is OK, and
> works well enough, but it would be great if it actually "knew"
> the current loads on those servers. I already have code that
> shares basic architectural info, such as number of CPUs, available
> memory, loadavg, etc which can help, of course, but again, all
> this info can be used to *infer* the current status of those backend
> servers; it doesn't really provide what the current load actually
> *is*.
> So I was thinking maybe some sort of small, simple and "fast"
> benchmark which could be run by the backends as part of their
> "status" update to the front-end reverse proxy server... something
> that shows general capability at that point in time, like Hanoi or
> something similar. Or maybe some hash function. Some simple code
> that could be used to create that "universal" load number.
> Thoughts? Ideas? Comments? Suggestions? :)

having the back-ends to provide the load they are able to handle
lbfactor (via w_lf or somethere similar. That requires the back-ends to
be able to send request to httpd balancer-manager handler.

Not really. I'd suggest a response header, travelling with each response
back to the balancer, which can be composed quickly enough to share
a play-by-play snapshot of the availability of that backend. This adds
next to no traffic and minimal cpu drain if composed cleanly. And it can
optionally be axed by the balancer in the response to the client.

The last thing we want are the routing headaches of contacting an
ever-changing list one-or-many potential balancers. And we can't
rely on a dying lbmember to "check in" that it isn't functional. Since
the balancer must already start requests to the backend, having that
backend supplement the responses with its health status is simple.