The HTTP status code 226 IM Used is a lesser-known status code, but it serves an important purpose in certain scenarios. This article will provide an in-depth explanation of the 226 IM Used status code, its use cases, and examples of requests and responses.
Overview of the 226 IM Used Status Code
The 226 IM Used status code is part of the HTTP/1.1 protocol and is defined in RFC 3229, “Delta encoding in HTTP.” It indicates that the server has fulfilled a request for the resource and the response is a representation of the result of applying one or more instance-manipulations to the current instance corresponding to the requested resource.
In simpler terms, the 226 IM Used status code is used when the server sends a delta-encoded response, which is a compact representation of the differences between the current and previous versions of a resource. This is particularly useful in situations where bandwidth is limited, as it allows clients to receive only the changes made to a resource since their last request, rather than downloading the entire resource again.
When to Use the 226 IM Used Status Code
The 226 IM Used status code is primarily used in scenarios where delta encoding is employed. It is most commonly seen in applications that involve synchronizing data between a client and server, where only the differences between the current and previous versions of a resource need to be transferred.
To use delta encoding, the client must include an A-IM header in its request, which specifies the instance-manipulation methods it supports. The server will respond with a 226 IM Used status code if it successfully applies one or more of the requested instance-manipulations.
Example Request and Response
Here’s an example of a request and response using the 226 IM Used status code:
Request
GET /example-resource HTTP/1.1
Host: example.com
A-IM: vcdiff
If-None-Match: "1234567890"In this example, the client requests the /example-resource resource from the example.com server. The client includes the A-IM: vcdiff header, indicating that it supports the vcdiff delta encoding method. The If-None-Match header is also included, which allows the server to determine if the client’s current version of the resource is up to date.
Response
HTTP/1.1 226 IM Used
Date: Wed, 15 Sep 2022 10:00:00 GMT
IM: vcdiff
ETag: "0987654321"
Content-Type: application/vcdiff
Content-Length: 100
<delta-encoded data>In the response, the server indicates that it has successfully applied the vcdiff instance-manipulation method by including the IM: vcdiff header. The ETag header provides the entity tag for the new version of the resource, and the Content-Type header specifies that the response body contains vcdiff delta-encoded data. The Content-Length header specifies the size of the delta-encoded data.
Summary
The 226 IM Used HTTP status code is an important yet lesser-known status code that is primarily used in scenarios involving delta encoding. It allows clients and servers to synchronize data more efficiently by transferring only the differences between the current and previous versions of a resource.
