Graceful Restarts and Health Checks

A service must replace its old process each time a new version is deployed, and the old process is still answering requests at that moment. If it exits at once, every request in progress fails.

The safe order has four steps. The process first asks for no new traffic, and it then stops accepting connections. It finishes the requests still in progress, and it ends any request that takes too long. Stopping a server in that order is called a graceful shutdown: it stops accepting new work, finishes the current work, and then exits