‘q’ Parameter in HTTP ‘Accept’ Header

1. API Clients support multiple formats

A REST API can return the resource representation in many formats – to be more specific MIME-types. A client application or browser can request for any supported MIME type in HTTP Accept header.

Technically, Accept header can have multiple values in form of comma-separated values.

For example, an Accept header requesting for text/html or application/xml formats can be set as:

Accept : text/html,application/xml

2. The ‘q’ Parameter

Sometimes clients may want to set their preferences when requesting multiple MIME types. To set this preference, q parameter (relative quality factor) is used.

Value of q parameter can be from 0 to 1. 0 is the lowest value (i.e. least preferred) and 1 is the highest (i.e. most preferred).

A sample usage can be:

Accept : text/html, application/xml;q=0.9, */*;q=0.8

In the above example, the client is indicating to the server that it will prefer to have the response in text/html format, first.

If the server does not support text/html format for the requested resource then it shall send application/xml format. If none of both formats are available, then send the response in whatever format it support (*/*).

  • One of the benefit of the ‘q’ parameter is to minimize the client-server interactions, which could have happened due to failed content negotiations.
  • It also allows clients to receive content types of which they may not be aware, an asterisk “*” may be used in place of either the second half of MIME type value or both halves.

3. According to HTTP Specs

Here’s how the HTTP spec defines it:

Each media-range MAY be followed by one or more accept-params, beginning with the “q” parameter for indicating a relative quality factor. The first “q” parameter (if any) separates the media-range parameter(s) from the accept-params.

Quality factors allow the user or user agent to indicate the relative degree of preference for that media-range, using the ‘q’ value scale from 0 to 1.

The default value is 1.

If there are two MIME types for the same q value, then a more specific type, between both, wins.

For example, if both application/xml and */* had a preference of 0.9 then application/xml will be served by the server.

If no Accept header field is present, then it is assumed that the client accepts all media types.

If an Accept header field is present, and if the server cannot send a response which is acceptable according to the combined Accept field value, then the server SHOULD send a 406 (not acceptable) response.

Was this article helpful?

Comments

  1. I would suggest to not depend on `Accept` header.
    Headers like `Accept`, `Accept-Language`, `Accept-Charset`, etc do not cache well.
    If you depend on them, then you will need to add them to `Vary`, and that will cause huge cache fragmentation.

    Its better to either just support a single response format, or use separate endpoints to ask specifically for a format that you can parse (eg. separate endpoints for xml and json).
    A decent solution is to use a query parameter for that as well.

    Reply
  2. one question arises in :
    Accept : text/html, application/xml;q=0.9, */*;q=0.8
    what is the meaning of the last q parameter (q=0.8) in this Accept header?

    Reply
    • I’m also confused by this; It seems to me that;

      Accept : text/html, application/xml;q=0.9, */*

      would suffice.

      I believe that server-side, that first “q” parameter would be overwritten by the second. The server would see the value 0.8 only. Unless you’re parsing the query string directly I suppose.

      Other than that, I’m finding this site to be a fantastic resource.

      Reply
      • ‘q’ parameter is parsed by the framework for content negotiation. They do not overwrite each other. They simply represent the preference of one media type over other.

        Reply
  3. Perhaps I’m missing something; what’s the point of ranking your Accept header items when they’re already ordered in a comma-separated value?

    Reply
    • [java]Accept : text/html;q=0.8, application/xml;q=0.5, */*;q=0.4[/java]

      Any sequence is possible for ‘Accept’ header media types. Assigning ‘q’ value safegaurds the application from accidental mistakes.

      Reply

Leave a Comment