[Chronos]

It still requires explicit action. However, the old way had a little dance between the old process and the new process: the new process tells the old process to start shutting down, the old process stops listening for new connections, then the new process starts listening for new connections. That left a gap where connections got rejected.

The new technique is for the old process to use a Unix socket to seamlessly transfer ownership of the listening sockets to the new process. At no point are the listening sockets closed, so no connections are rejected.

It's still a (potentially) new haproxy binary starting up and parsing the (potentially) changed haproxy config because the user requested a graceful restart.

[vbernat]

The new process listen to connections before the old process stop listening. The problem is that the old process can still have new connections queued up. They are lost when its sockets are closed.