Occasionally when I try to open a site I will see a page saying smth like “This site is offline for maintenance” and then some comments follow on how long it would presumably take. Stack Overflow does that too.
How does it work? I mean if the site is shut down who replies to my HTTP request and serves this page?
There is a trick in asp.net where you place a file called
All requests will go to this, until the page is deleted.
For other environments you can often just change where the server points, or another such plan.
— Edit
A server-agnostic approach is achieved through something like load-balancing.
Under the hood you can send the requests to a given internal server. You may then decide to point all requests to your server ‘a’, which you configure to show the ‘downtime’ page. Then, you make changes to server ‘b’, confirm it as successful, and point all requests to ‘b’. Then you update ‘a’, and let requests go to both.