What is an Upgrade header?
Asks to switch the current connection to a different protocol, such as from HTTP/1.1 to WebSocket.
The Upgrade header is how a plain HTTP connection to a website becomes something else entirely, most commonly a WebSocket for real-time features like chat or live dashboards. Because the upgrade either succeeds cleanly or the whole connection breaks, a monitor that specifically exercises the upgrade handshake is far more useful here than one that only checks the initial HTTP response looks healthy.
Common use case
A chat or live-dashboard client sends Upgrade: websocket to convert a plain HTTP connection into a persistent, bidirectional WebSocket for real-time updates.
Example
Upgrade: websocketHistory
Part of the original HTTP/1.1 specification, RFC 2068 (1997), later central to the WebSocket handshake defined in RFC 6455 (2011).
Did you know?
HTTP/2 forbids this header entirely - connections that need to negotiate a protocol switch under HTTP/2 use a different, frame-based mechanism instead.