HTTP status code 100 Continue

The HTTP status code 100 Continue is part of the HTTP/1.1 protocol, indicating that a client’s request has been received by the server and is acceptable so far. This code is used to allow the client to continue sending the request body if needed. Here’s how it works and why it’s used:

How 100 Continue Works

1. Initial Request with Expect Header: The client sends an HTTP request with the Expect: 100-continue header, usually when the request has a large payload (such as a file upload).
2. Server Checks Validity: The server checks the headers and request details to ensure they’re valid and that it can process the request.
3. Server Responds with 100 Continue: If everything looks good, the server responds with a 100 Continue status code, indicating that the client can proceed to send the request body (payload).
4. Client Sends Request Body: After receiving 100 Continue, the client transmits the remaining data, knowing that the server is ready to accept it.

Why 100 Continue is Useful

• Prevents Unnecessary Data Transmission: If the request is likely to fail due to headers or authentication issues, the server can respond with an error before the client sends a large payload, saving bandwidth.
• Improves Performance for Large Requests: By confirming the initial headers, this mechanism helps optimize resources and allows servers to reject requests early if they won’t be processed.

When 100 Continue is Used

This code is typically used in POST, PUT, or PATCH requests where large files or data uploads are involved. It is especially helpful in environments with limited bandwidth or high latency, where unnecessary data transmission could lead to delays or resource constraints.

Example of a 100 Continue Request and Response

Client Request :

POST /upload HTTP/1.1

Host: example.com

Content-Length: 1048576

Expect: 100-continue

Server Response:

HTTP/1.1 100 Continue

Client Sends Request Body:

{data payload}

If the server cannot process the request, it might respond with a different status code (like 417 Expectation Failed), and the client would know not to send the body.