A status code of 100 indicates that (usually the first) part of a request has been received without any problems, and that the rest of the request should now be sent.

The HTTP status code 100 occurs if the request body is large, sending it to a server when a request has already been rejected based upon inappropriate headers is inefficient. To have a server check if the request could be accepted based on the request's headers alone a client must send "Expect: 100-continue" as a header in its initial request[2] and check if a 100 Continue status code is received in response before continuing.

The client SHOULD continue with its request. This interim response is used to inform the client that the initial part of the request has been received and has not yet been rejected by the server. The client SHOULD continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The server MUST send a final response after the request has been completed. If you monitor your website through 100 pulse, we will intimate you through mail or short message service whenever you encounter 100 status code.