What Is a 100 Status Code?

The initial part of a request has been received and has not yet been rejected by the server. The server intends to send a final response after the request has been fully received and acted upon.

When the request contains an Expect header field that includes a 100-continue expectation, the 100 response indicates that the server wishes to receive the request payload body¹. The client ought to continue sending the request and discard the 100 response.

If the request did not contain an Expect header field containing the 100-continue expectation, the client can simply discard this interim response.

• ¹ Expect RFC7231 Section 5.1.1
• Source: RFC7231 Section 6.1.1

100 CODE REFERENCES

Rails HTTP Status Symbol :continue

Go HTTP Status Constant http.StatusContinue

Symfony HTTP Status Constant Response::HTTP_CONTINUE

Python2 HTTP Status Constant httplib.CONTINUE

Python3+ HTTP Status Constant http.client.CONTINUE

Python3.5+ HTTP Status Constant http.HTTPStatus.CONTINUE

.NET HttpStatusCode.Continue

Rust http::StatusCode::CONTINUE

Go http.StatusContinue

Symfony Response::HTTP_CONTINUE

Apache HttpComponents Core org.apache.hc.core5.http.HttpStatus.SC_CONTINUE

Angular @angular/common/http/HttpStatusCode.Continue

How does the 100 status code work?

When a client sends a request, it expects a response from the server.

If the server needs additional information before processing the request, it sends a 100 status code. The client then sends the remainder of the request. Once the server receives the request, the server processes the request and sends another response code. This response code indicates a success or failure.

You can think of the 100 continue status code like a game of catch. As you (the client) prepare to throw the ball (the request), your friend (the server) asks you to wait a minute. After your friend gets their mitt on, they give you the go-ahead to throw the ball.

What is an example of a 100 continue status code?

Here is an example of a 100 continue status code:

Client request

The client requests to upload a large image file:

POST /upload HTTP/1.1
Host: example.com
Content-Length: 1000000
Content-Type: image/jpeg

Server response

The server responds with a 100 continue status code:

HTTP/1.1 100 Continue

Client request

The client proceeds with sending the file:

POST /upload HTTP/1.1
Host: example.com
Content-Length: 1000000
Content-Type: image/jpeg

<binary data>

Server response

The server responds with a 200 status code — the upload is a success:

HTTP/1.1 200 OK

5 best practices for using 100 continue status codes

For the best results with 100 continue status codes, follow these best practices:

1. Use the 100 status code only when necessary

Use the 100 status code when the server needs more information before it can process a request. If the server can immediately process the request without additional information, skip using a 100 status code.

2. Respond with a 100 status code immediately

If the server needs more information before processing a request, it should send a 100 status code right away to let the client know: (1.) It’s received the initial request and (2.) Is waiting for more information.

3. Use the appropriate headers

The server should include appropriate headers in the 100 status code response to share what additional information it needs and how the client should send it.

4. Avoid sending multiple 100 responses for the same request

If the server has already sent a 100 response for a request, it should not send another 100 response for the same request. The only time a server should send an additional response for the same request is when it needs information that wasn’t included in the previous request.

5. Include a final status code to indicate success or failure

The 100 status code is a temporary response — it indicates the server needs more information before it can process the request. Once the server has received the entire request and processed it, it should send a final status code to share whether the request was a success or failure.

Help! My final status code following a 100 status code failed.

A final status code can fail for multiple reasons, including:

• Formatting
• Client authorization
• Requested resource location
• Processing error
• And more

You can troubleshoot this final status code by:

• Reviewing your request’s formatting
• Confirming the requested resource exists on the server
• Checking the client has access to the resource

Additional resources

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