HTTP Status: 100 Continue

UPDATE: This post contains some false information. Particularly, you should not send back a 417 Expectation Failed response as a way of telling the client that the server could not fulfill the response because of data presented in any of the request headers other than the Expect header. I have corrected my statements here in a newer post, and I apologize if I caused any confusion.

As a self-described HTTP status code pedant and an architect of RESTful Web services, I’ve spent a great deal of time researching and thinking about the proper use of HTTP status codes in Web applications. It occurs to me that many people use status codes in their applications in ways that I find improper, so I’ve decided to start up a series of blog posts to talk about select status codes and how they might appropriately fit within your Web applications, specifically in a RESTful manner. Consider these “Ben’s HTTP Status Codes Best Practices.”

I won’t be covering these status codes in any particular order. However, the first code up is 100 Continue merely because I’ve been thinking about it lately.

The 100 Continue status code is an informational response that tells the client application it can continue sending the request. In short, the client can make an abbreviated request of the service to check whether or not it should continue making the full request. The service can then respond either “yes” (100 Continue) or “no” (417 Expectation Failed) with an explanation of why not. In RESTful Web Services, Richardson and Ruby refer to this as a “look-before-you-leap” (LBYL) request.

This is helpful in situations where the client might want to post or put a very large file to the service but wants to ensure that the service will accept it before actually sending the file. Consider the following request:

POST /content/videos HTTP/1.1
Content-Type: video/mp4
Content-Length: 105910000
Authorization: Basic bWFkZTp5b3VfbG9vaw==
Expect: 100-continue

In this case, the client is attempting to post an MPEG-4 video that is 101MB. The client sends this request to your service without the video in the body, and it includes the Expect: 100-continue header. Let’s say your service limits uploads to 100MB. In that case, you would return a 417 Expectation Failed including a reason for the failure in the body of the response.

By now, you can probably see how this can be extended to apply to any of the other headers. For example, a client can try a POST or a PUT request first just to see if its authorization succeeds. If it does, your service would return 100, letting the client know it may proceed with the request. Likewise, the client might want to determine whether the service will accept a video/mp4 file before sending it. The list goes on.

It should be noted that the service shouldn’t require the client to use a LBYL request, but it can be helpful to a client so that it doesn’t attempt to send something that would fail at the service anyway.

I’m considering implementing these kinds of responses in a service I’m building, so I’ll let you know how it goes. :-)