I just want to point to an interresting resource on load balancing. GitHub recently released GLB: GitHub's open source load balancer (GitHub Load Balancer Director and supporting tooling Repo), which is the actual implementation of what was descibed back in September 2016 Introducing the GitHub Load Balancer. It guides the reader through the challanges GitHub faces in the space of load balancing, and how they came to their solution, and what didn't work for them.
The load balancer at GitHub needs to be increadibly reliable. We all use it when cloning or pushing to a repo, which both could take a while. Cutting the connection for maintenance purposes would affect many people, especially those, working with bad internet access.
Even if you are not GitHub, this might be interresting. For instance, if you run OpenStack and depend on the Image-API (Glance) for VM snapshots, a restart of the load balancer results into a corrupt snapshot, which might be unnoticed until there is an urge to use the snapshot. For sure. One should verify if the snapshot works, but not everyone does. And even if one does, it is quite annoying to wait let's say an our for the upload, just to discover the time has been wasted.