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From: Raymond Burkholder (ray_at_[hidden])
Date: 2020-04-21 19:12:31

On 2020-04-21 10:52 a.m., Dominique Devienne via Boost-users wrote:
> I didn't equate routing traffic from 1 server to another as Load
> Balancing,
> but I guess it makes sense. My server is already multi-user and
> multi-threaded,
> and not expected to have traffic that justifies a Load Balancer. Other
> people in
> the company are going crazy with Kubernetes and Docker, but I'm trying
> to keep things
> simple and make a good server fast and robust enough to avoid all that
> complexity.
Well, your premise seems to be that you want to upgrade a service. From
a programming perspective, you have a few choices:

a) as someone else has mentioned, make use of the SO_REUSEPORT socket
option:  write your own little packet forwarder, and encapsulate your
code in a reloadable module of some sort, and when you want upgrade,
load the module, and migrate the socket.  Taking care of what ever state
management is necessary for migrating traffic.

b) use an external service (call it a load balancer or whatever), yes it
needs to 'know' the old and new service.  Send a signal that new traffic
goes to the new instance.  It should be smart enough to forward packets
of old sessions to the old instance.  When no sessions need the old
instance, the old instance can be removed. This can be scaled as
necessary.  The least cost could be done with iptables/nftables: 
existing connections remain in place until torn down.

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