On 2/13/2018 7:49 AM, Andrew Smalley wrote:
> We have had a request and not sure if there is any way to implement this.
> 
> Simply think of two real servers being loadbalanced. one fails all the
> connections are moved to the remaining server overloading it.
> 
> What we want is for the traffic from the failed real server to be
> moved to the remaining real server without overloading it. IE Move a
> few connections at a time so the last server is not overloaded.

The following is my understanding of how things work and what you have
said.  If I have made any errors, I hope somebody will point them out.

As far as I am aware, there is no way to "move" an existing connection
from one backend server to another.  That would require special support
from both haproxy and the back end software.  To my knowledge, that
capability does not exist.  So existing connections at the moment of
failure are going to get closed down and the application (which may be a
browser) will need to try again.

There isn't anything gradual about load shifting in the event of a
failure.  Existing connections will be dropped and new connections will
be sent to whatever servers remain.

When planning your capacity, it's prudent to take failures into account.
 Failures *are* going to happen.  They might be unplanned, such as a
motherboard failure or a datacenter outage, or they may be planned, so
you can upgrade software on the back end.

If one server failing means that there is not enough remaining capacity
to handle the load, then you need more capacity, which may require more
servers.  Ideally the remaining servers would handle the load without
users ever noticing any change, but in many environments it is
acceptable for performance to be a little worse until the failed server
is returned to service.

Thanks,
Shawn

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