upstream hash consistent seems to depend on order of DNS entries (Maxim Dounin)

[Robert Paprocki]

> In my case, the number of hosts will change over time, and I can’t update
the nginx config.  So I thought it would make sense to use a hostname that
resolves to many IPs.  This would be a scalable solutioin

In that case, it makes sense to use a templating tool to dynamically
populate the contents of the upstream{} block. Hook it in with your service
discovery/registration system.

On Mon, Feb 3, 2020 at 12:59 PM Ian Morris Nieves <imnieves at gmail.com>
wrote:

> Hi Maxim,
>
> Thank you for your response.
> OK, I understand what you have said.
>
> This seems slightly strange to me.  We both understand the purpose/point
> of hashing (consistent or not consistent).  Why does it then make sense to
> fall-back to round robin?
>
> In my case, the number of hosts will change over time, and I can’t update
> the nginx config.  So I thought it would make sense to use a hostname that
> resolves to many IPs.  This would be a scalable solutioin.
>
> I would think that the more correct solution would be to hash the IP
> addresses that all the names resolve to…. not the names themselves.
>
> I must be missing/mis-understanding something.  Does the current
> implementation/solution make sense to you?
>
> Best,
> Ian
>
> >
> > Message: 2
> > Date: Mon, 3 Feb 2020 16:05:38 +0300
> > From: Maxim Dounin <mdounin at mdounin.ru>
> > To: nginx at nginx.org
> > Subject: Re: upstream hash consistent seems to depend on order of DNS
> >       entries
> > Message-ID: <20200203130538.GR12894 at mdounin.ru>
> > Content-Type: text/plain; charset=utf-8
> >
> > Hello!
> >
> > On Sat, Feb 01, 2020 at 11:48:00AM -0800, Ian Morris Nieves wrote:
> >
> >> Here is the setup:
> >> - I am running nginx in a single docker container and it has an
> >> upstream to a docker service which is composed of 3 docker
> >> containers (which happens to be php-fpm)
> >> - the service is configured to _not_ expose a single virtual ip
> >> address (vip), instead the service exposes the ip addresses of
> >> all 3 containers through docker?s built-in DNS.  When this DNS
> >> is asked for the IP address of the service it will respond with
> >> a list of 3 IP address but the list will rotate in round-robin
> >> fashion each time a lookup is performed.  Thus the first IP in
> >> the list will not be the same for any 2 consecutive lookups.
> >>
> >> My first question is:
> >> Is it the correct behavior that consistent hashing depends on
> >> the order of IP addresses in the DNS query response?  I can
> >> imagine arguments either way, and it is possible that this
> >> critical detail is outside the scope of consistent hashing.  I
> >> will also forward this question to the author of Ketama.
> >
> > Consistent hashing uses the _name_ as written in the
> > configuration, not the IP addresses the name resolves to.
> >
> > If a name resolves to multiple IP addresses, these addresses are
> > considered equal and requests are distributed between them using
> > round-robin balancing.
> >
> > That is, to balance multiple servers (containers) using consistent
> > hashing, you have to configure an upstream block with multiple
> > "server" directives in it.
> >
> > --
> > Maxim Dounin
> > http://mdounin.ru/
> >
>
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