How to avoid packet loss on server application restart?
To avoid this, have multiple application servers behind a load balancer. Before bringing one down, ensure the load balancer is not sending it new clients. Bring it down, traffic will go to the other applications servers, and when it comes back up traffic will begin getting sent to it again.
If you have only one application server, simply 'buffering' network traffic is a poor solution. When the server comes back up, it has none of the TCP state information anymore and the old incoming connections have nowhere to go anyway.
A typical situation with a server/web application is that the application needs to be shut down and restarted to implement an upgrade.
What are the possible/common schemes (and available software) to avoid losing data that clients sent to the server during the short time the application was gone?
An example scheme that could work is: For a simple web server where the client connects to port 80, rather than the client connecting directly to the web server application, there could be a simple application in between that listens to port 80 and seamlessly forwards/returns data to/from the "Actual" web server application (on some other port).  When the web server needs to be shut down and restarted, the relay app could detect this and buffer all incoming data until the webserver comes back to life.  This way there is always an application listening to port 80 and data is never lost (within buffer-size and time reason, of course).  Does such a simple intermediate buffer-on-recipient-unavailable piece of software exist already?
I'm mostly interested in solutions for a single application instance and not one where there are multiple instances (in which case a clever rolling update scheme could be used), but in the interests of having a full answer set, any response would be great!