100 Continue - Informational Response Status Code

Example Use

A client sends a PUT request with no body to an AWS S3 bucket. The client adds headers with Expect: 100-continue and Content-Length: 10000 values to see if AWS will accept a file upload of 10,000 bytes before wasting time & bandwidth sending the file.

AWS responds with a 100 Continue header.

The client than sends a PUT request with the body.

Official Definition

Indicates the initial part of a request has been received and has not yet been rejected by the server. The server intends to send a final response after the request has been fully received and acted upon.

When the request contains an Expect header field that includes a 100-continue expectation, the 100 response indicates that the server wishes to receive the request payload body. The client ought to continue sending the request and discard the 100 response.

If the request did not contain an Expect header field containing the 100-continue expectation, the client can simply discard this interim response.

Source: RFC 7231 Section 6.2.1