[upstream] consistent hash support backup?

[洪志道]

Thank you for your reply.

What I want to solve is that backup servers can be selected when all the
primary servers are unavailable.

Now I use error_page to solve it, but it's not convenient in the case of
multi-servers-locations.
Especially some locations have already config error_page directive.

upstream backends {
    hash $uri consistent;
    server 10.0.0.1;
    server 10.0.0.2;
}

upstream backup {
    server 10.0.0.3;
}

server {
     ...
     location / {
        error_page  502 504 = @fallback;
        proxy_pass  http://backends;
     }

     location @fallback {
        proxy_pass  http://backup;
     }
}

But if hash support backup, it would be handier, such as following.

upstream backends {
    hash $uri consistent;
    server 10.0.0.1;
    server 10.0.0.2;
    server 10.0.0.3  backup; # Unfortunately it's now allowed.
}

server {
     ...
     location / {
        proxy_pass  http://backends;
     }
}

Anyway.

1. Can you share the reason for "backup" option is not allowed combined
with the hash module?
2. Is there any problem if I add the flag 'NGX_HTTP_UPSTREAM_BACKUP'  in
the hash module?
    I know it's not an ideal design.

Thanks again.


On Wed, Nov 8, 2017 at 9:11 PM, Ruslan Ermilov <ru at nginx.com> wrote:

> On Mon, Nov 06, 2017 at 09:35:14PM +0800, 洪志道 wrote:
> >    Hi!
> >    We know that consistent hash upstream improve its selection in the
> latest
> >    version.
> >
> >  -        if (hp->tries >= points->number) {
> >  -            pc->name = hp->rrp.peers->name;
> >  +        if (hp->tries > 20) {
> >               ngx_http_upstream_rr_peers_unlock(hp->rrp.peers);
> >  -            return NGX_BUSY;
> >  +            return hp->get_rr_peer(pc, &hp->rrp);
> >
> >  Does it mean that "backup" option is allowed in the module?
>
> It just means that if, after 20 tries, we weren't able to select
> a peer using the hash algorithm, then we'll continue a selection
> process using the round-robin algorithm.  This is also consistent
> with the ip_hash module.
>
> >                     |NGX_HTTP_UPSTREAM_MAX_CONNS
> >                     |NGX_HTTP_UPSTREAM_MAX_FAILS
> >                     |NGX_HTTP_UPSTREAM_FAIL_TIMEOUT
> >  +                  |NGX_HTTP_UPSTREAM_BACKUP
> >                     |NGX_HTTP_UPSTREAM_DOWN;
>
> We do not support backup servers with ip_hash and hash balancers,
> though there's currently a bypass that allows to have backup
> servers in configurations with hash balancers.  But these backup
> servers will be used only when falling back to round-robin, which
> is unlikely.
>
> >  I wonder how to archive the effect of "backup" in hash like round robin,
> >  even if we don't want to use error_page.
>
> I'm not sure what did you mean here.
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