The HTTP Expect request header indicates that there are expectations that need to be met by the server in order to handle the complete request successfully.
When a request has an Expect: 100-continue header, a server sends a 100 Continue response to indicate that the server is ready or capable of receiving the rest of the request content. Waiting for a 100 response can be helpful if a client anticipates that an error is likely, for example, when sending state-changing operations without previously verified authentication credentials.
A 417 Expectation Failed response is returned if the server cannot meet the expectation, or any other status otherwise (e.g., a 4XX status for a client error, or a 2XX status if the request can be resolved successfully without further processing).
None of the more common browsers send the Expect header, but some clients (command-line tools) do so by default.
There is only one defined expectation:
100-continue
Informs recipients that the client is about to send a (presumably large) message body in this request and wishes to receive a 100 Continue interim response.
A client sends a request with Expect header and waits for the server to respond before sending the message body.
PUT /somewhere/fun HTTP/1.1
Host: origin.example.com
Content-Type: video/h264
Content-Length: 1234567890987
Expect: 100-continue
The server checks the headers and generates the response, where a 100 Continue instructs the client to send the message body:
The client completes the request by sending the actual data:
[Video data as content for PUT request]
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