HTTP 1xx Informational

HTTP 100 - Continue

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.

Typically a request-response sequence is straightforward. One single request is made, received, and responded to.

But sometimes a request needs to be broken up into parts. This might occur if the request is too big. It might occur if the requester needs to check if the header is formatted properly, or if the server is actually ready to receive the request.

In these cases, the client (browser) might send the initial request with a header that includes Expect: 100-continue When that occurs, the server will receive the initial request and — if everything is okay — respond with the 100: Continue status. This signals the client to complete the request.

If everything doesn’t work out ok, the server will send back a 417 Expectation Failed.

Example: You're sending a large object to the server using a PUT request, you may include a Except header like this

PUT /media/file.mp4   HTTP/1.1
Host: api.example.org
Content-Length: 1073741824
Except: 100-continue

This tells the server that it should with a 100 Continue Status Code if the server is going to be able to accept the request

HTTP/1.1 Continue

When the client receives this, it tells the client the server will accept the request, and it may start sending the request body

101 : Switching Protocols

102 : Processing