The 200 range of HTTP status codes represents successful requests. I’ve already covered 201 Created and 202 Accepted and 206 Partial Content. Today, I’ll wrap up my discussion of the 200 range by talking about 204 No Content and 205 Reset Content. The 200 OK response is probably the status with which most are familiar, and I’ll discuss it later when covering the HTTP verbs.
The 204 No Content response is useful in a web service when you want to return a success message but do not want to return a message in the body or do not have a body to return. In my personal experience, I use this when DELETE requests are sent to an Atom web service. If the resource is successfully deleted, the service returns a 204 No Content status message. This tells the client that the deletion was successful, and that’s really all the client needs to know. There’s nothing to return because it was deleted.
However, even outside of the web services realm, the 204 No Content status actually means something to a user agent (browser). The HTTP specification (RFC 2616) states the following about the 204 No Content status code:
If the client is a user agent, it SHOULD NOT change its document view from that which caused the request to be sent. This response is primarily intended to allow input for actions to take place without causing a change to the user agent’s active document view, although any new or updated metainformation SHOULD be applied to the document currently in the user agent’s active view.
This means that, if I have an HTML form and submit it, then, if the server returns a 204 status code, the browser should not refresh the form or take me to another page. The document view does not change, and I remain at the form. All of the data I entered remains unchanged. All browsers I tested support this, but, in practice, it is not very useful since there’s no indication to a user that anything happened on the server side.
However, consider the use of 204 No Content in an Ajax application. If the Ajax library used simply looks for a success message of 204 in response to the requests it sends, then it can present a success message to the user without changing any of the UI elements, as this response intends. This is also useful because the service doesn’t need to send back any data; it needs only tell the client that the request was successful.
On the other hand, with a 205 Reset Content response, the intent is to tell the client to clear the content from the form or to refresh the UI. That is, I could fill out a form, click the submit button, and the form I am currently working on would refresh to its default values if the client receives a 205 response. According to the HTTP spec, a 205 response means:
The server has fulfilled the request and the user agent SHOULD reset the document view which caused the request to be sent. This response is primarily intended to allow input for actions to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate another input action.
The problem is that I have not found a browser yet that supports this behavior. Browsers either treat a 205 like it’s a 204, or they simply treat it as a 200. This does not mean, though, that Ajax applications cannot properly respect a 205 response. In an Ajax application, if the client receives a 205 response (instead of a 204 response), then the UI elements should change back to their default state.
In RESTful Web Services, Richardson and Ruby make this distinction between 204 and 205:
In data entry terms: 204 is good for making a series of edits to a single record; 205 is good for entering a series of records in succession.
In my simple browser test to see what browsers supported 204 and 205 on forms (without using any Ajax), here’s what I found:
|Browser||204 No Content||205 Reset Content|
|Firefox 18.104.22.168||supported||treated like 204|
|Safari 3.1.1||supported||treated like 200|
|Opera 9.27||supported||treated like 200|
|Internet Explorer 7 (7.0.5730.13)||supported||treated like 200|
|Internet Explorer 8 beta 1 (8.0.6001.17184)||supported||treated like 200|
|Firefox 3 beta 5||supported||treated like 204|
Since I know I’ll be asked this question, to send back a 204 or 205 response with PHP, see the PHP manual on the use of the header() function.