100 Continue: An HTTP Status Code Indicating That the Initial Request Has Been Accepted

About 100 Continue

Overview of 100 Continue

Optimizing the Preparation StageHTTP Status Code

100 Continue Overview 100 Continue is an HTTP status code indicating that the client’s request is acceptable, and the client can proceed with sending the request body. This code is typically returned by the server when the request header includes Expect: 100-continue.

Meaning The server has received the initial part of the request and instructed the client to continue.

When is 100 Continue Returned?

• When the client sends a request with the Expect: 100-continue header
• When the client wants to ensure server approval before sending the request body
• When used to avoid unnecessary data transfer for large requests

Examples of 100 Continue

Confirming the Initial Part of a Request

POST /upload HTTP/1.1  
Host: example.com  
Content-Length: 1048576  
Expect: 100-continue  

Response example:

HTTP/1.1 100 Continue  

Explanation: When the client sends a request with the Expect: 100-continue header, the server returns the 100 status code to indicate readiness to accept the request. The client can then proceed to send the request body (e.g., large data).

Alternative Response in Case of Error

POST /upload HTTP/1.1  
Host: example.com  
Content-Length: 1048576  
Expect: 100-continue  

Response example:

HTTP/1.1 417 Expectation Failed  

Explanation: If the server is not ready to accept the request, it may return 417 Expectation Failed instead of the 100 status code.

Points to Note

Considerations when returning 100 Continue:

• Return before receiving the request body
  The 100 status code must be sent before the request body is received.
• Handle client expectations properly
  Ignoring Expect: 100-continue can lead to inefficiencies in client-server communication.

Comparison with Related HTTP Status Codes

Explanation of HTTP status codes related to 100 Continue:

• 200 OK: Used when the request is fully successful. 100 indicates an intermediate stage.
• 417 Expectation Failed: Returned when the Expect: 100-continue expectation is not met.

Understanding these differences ensures the appropriate use of the 100 status code.