100 Continue HTTP Status Code

Description

HTTP status code 100 is known as "Continue". It is an informational response code indicating that the initial part of a request has been received and has not yet been rejected by the server. The server sends this response to inform the client that it can continue with its request. This is typically used in situations where the request is made with an Expect: 100-continue header, which asks the server to confirm that the request is acceptable before sending the body of the request.

In practice, when a client sends a request with Expect: 100-continue, it pauses for a moment to see if the server responds with a 100 Continue status code, or if it rejects the request with an error (like 417 Expectation Failed). If the client receives the 100 Continue response, it proceeds to send the body of the request. This mechanism is useful for optimizing network usage and improving performance when sending large payloads, as it avoids sending large amounts of data over the network only to have the request rejected based on the headers.

Status

HTTP

100 Continue

Common Causes

Here are the main causes or scenarios where you might encounter a 100 Continue status code:

Expect Header: The primary use case for a 100 Continue response is in conjunction with the Expect: 100-continue header in a client's HTTP request. When a client includes this header in its request, it is asking the server if it is okay to send the body of the request (which is typically large in such cases). The server responds with a 100 Continue status if it is willing to accept the body of the request. This mechanism allows clients to avoid sending large payloads over the network if the server is going to reject the request based on the headers alone (e.g., because of authentication failure).
Optimizing Network Usage: By using the 100 Continue status, clients and servers can make more efficient use of network resources. It prevents the unnecessary transfer of large data payloads over the network if the request is going to be rejected based on the preliminary headers.
Preemptive Response Handling: In some cases, servers may use the 100 Continue status code to start processing parts of a request while waiting for the remainder of the request's content. This can lead to more efficient request processing in situations where the server can make certain assumptions about the incoming request based on initial headers or data.

It's important to note that not all HTTP/1.1 clients use the Expect: 100-continue mechanism, and not all servers support it. Additionally, the presence of a 100 Continue status code is typically transparent to end-users, as it is mainly a protocol-level interaction between HTTP clients and servers.