Continue

1xx Informational

Server perspective

Use 100 as an interim response when a client asked whether it should continue sending the request body.

•Send 100 when the request uses Expect: 100-continue and the server is willing to receive the body
•Use it to avoid rejecting large uploads only after the client has already sent them

•Validate request-line and headers first, then either send 100 or a final error response
•Follow 100 with a normal final response after the full request is processed

•Do not send 100 as the final outcome of the request
•Do not require 100-continue for small bodies when it adds latency without real value

Request:POST /upload with Expect: 100-continue

Response:100 Continue followed later by a final 201 Created

The server first invites the client to send the body, then completes the request normally.

The server is saying, 'Let's keep talking, but switch to a different protocol for the rest of this connection.'

The server is saying, 'I’m still working on this. Don’t assume the request was lost just because it’s taking a while.'

The server is giving the browser a head start: 'You’ll probably need these files, so start getting them while I finish the real response.'