The HTTP response status code 100 Continue is an informational response from the server, indicating that the HTTP session is proceeding as intended and prompting the client to proceed to the next step.

If the HTTP request is complete, the client can safely disregard this response. The server sends this response only when the client includes the "Expect" header in the HTTP request.

When clients include the Expect HTTP header request field, they seek assurance that the server is ready to accept the message body before transmission.

This process becomes crucial in scenarios where the message body is extensive, doesn't conform to the server's accepted types, or when the user lacks authorization to send files. Upon receiving the 100 Continue status code, the client proceeds to send the message body. However, if the server responds with a 417 Expectation Failed HTTP response, the client must halt further actions.

Utilizing the Expect HTTP header with the 100-continue approach offers the advantage of conserving bandwidth, especially in cases where the server is selective about the content it receives and processes. Nonetheless, it comes with the drawback that HTTP request headers and bodies are sent independently, which might lead to separation issues in certain environments, potentially resulting in errors rather than successful processing of the HTTP request.

Client Request:
PUT /docs HTTP/1.1
Host: www.example.com
Content-Type: application/pdf
Content-Length: 99000
Expect: 100-continue

Server Response:
HTTP/1.1 100 Continue

Follow-up to Request:
< PDF file contents are transmitted as the message body >

Final Server Response:
HTTP/1.1 200 OK

In this sample, the client intends to transfer a 99kb PDF file to the server for processing, clearly denoted in the HTTP request, seeking validation beforehand. Upon receipt, the server promptly acknowledges with a "100 Continue" status code, granting permission for the client to proceed with transmitting the message body. Subsequently, the server culminates the HTTP session by responding with a successful "200 OK" status, signifying a completed operation. Alternatively, if the server encounters an issue, it may return an HTTP status code error, like:

HTTP/1.1 417 Expectation Failed

In such a case, the client refrains from sending the PDF and initiates further communication by making a fresh HTTP request.

In conclusion, the HTTP status code 100 Continue serves as an informational response from the server to the client, indicating that the HTTP session is progressing as expected and the client can continue to the next step.

It is particularly useful when clients include the Expect HTTP header in their requests to ensure that the server is prepared to receive the message body before transmission.

By leveraging this status code, developers can optimize data transmission, conserve bandwidth, and enhance the overall performance of web applications, leading to a more efficient and seamless user experience.