Ip_hash in active_active nginx setup

Hi all,

we want to use a F5 loadbalancer in front of two nginx instances, which
balance the load to our app-nodes.

The F5 loadbalancer distributes incoming requests via round-robin
algorithm
to the two nginx instances.

The nginx instances should be configured to use ip_hash to distribute
the
requests to the addnodes ensuring sticky sessions. Specific customer
requirements make the use of cookies to ensure sticky sessions
impossible.

I understand, that all nginx instances use the same function to hash the
IP
adress. But i am not sure if the distribution of the hash values to the
appnodes happens in a consistent way.

I came across the following discussion on github which indicates that
the
same IP address may be forwarded to different appnodes in the scenario
described above:

http://serverfault.com/questions/511763/do-multiple-nginx-servers-load-balance-the-same-ip-address-to-the-same-backend-w

Long story short / tl;dr:

Will different nginx instances forward requests from the same IP always
to
the same appnode when using ip_hash as a load balancing method?

Thanks,

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,259046,259046#msg-259046

s_n Wrote:

Hi all,

we want to use a F5 loadbalancer in front of two nginx instances,
which balance the load to our app-nodes.

Why not cut out the F5, use stream to balance between the same
instance(s)
(loop back) with either a single server block and loadbalanced upstream
or
split up, which ever works best / more consistent in load distribution.

nginx is by default a better loadbalancer then a F5, but that’s a
personal
opinion.

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,259046,259049#msg-259049

Hello!

On Thu, May 21, 2015 at 12:14:36PM -0400, s_n wrote:

requirements make the use of cookies to ensure sticky sessions impossible.

Long story short / tl;dr:

Will different nginx instances forward requests from the same IP always to
the same appnode when using ip_hash as a load balancing method?

As long as you use identical lists of servers in the upstream{}
blocks on your nginx instances, balancing on instances should be
identical.

Note though, that if an upstream server is considered down due to
errors, nginx will re-route requests to other servers, see
http://nginx.org/en/docs/http/ngx_http_upstream_module.html#max_fails.
And since different instances (and even different worker processes)
may see different errors, this re-routing may be different.

That is, ip_hash balancing method does not guarantee that all
requests from a specific IP address will be routed to a given
upstream server. Rather, it’s a method to minimize (but not to
eliminate) migration of users between upstream servers. Obviously
enough, it’s not possible to completely eliminate migration as
long as upstream servers may fail.


Maxim D.
http://nginx.org/

Hi,

based on your information we implemented the described setup and did not
encounter any stickiness problems so far.

Cutting out F5 is sadly a decision that is not under my control. I agree
that it would be a simpler setup with just nginx loadbalancing.

Thanks all.

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,259046,259217#msg-259217

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