Plack::Middleware::Negotiate - Apply HTTP content negotiation as Plack middleware


    builder {
        enable 'Negotiate',
            formats => {
                xml  => { 
                    type    => 'application/xml',
                    charset => 'utf-8',
                html => { type => 'text/html', language => 'en' },
                _    => { size => 0 }  # default values for all formats           
            parameter => 'format', # e.g.
            extension => 'strip';  # e.g.


Plack::Middleware::Negotiate applies HTTP content negotiation to a PSGI request. In addition to normal content negotiation from a list of defined formats one may enable explicit format selection with a path extension or query parameter. In summary, the following methods are tried in this order to negotiate a known format:

HTTP GET/POST format parameter (if enabled with option parameter)

e.g. format is set to xml for if format xml has been defined.

URL path extension (if enabled with option extension)

e.g. format is set to xml for if format xml has been defined.

HTTP Accept Header (unless disabled with option explicit)

e.g. format is set to xml if format xml has been defined with content type application/xml for Accept header value "application/xml".

The PSGI environment key negotiate.format is set to the chosen format name after negotiation. The PSGI response is enriched with corresponding HTTP headers Content-Type and Content-Language unless these headers already exist.

If used as pure application, this middleware returns a HTTP status code 406 if no format could be negotiated.


new( formats => { ... } [, %options ] )

Creates a new negotiation middleware with a given set of formats.

negotiate( $env )

Chooses a format based on a PSGI request. The request is first checked for explicit format selection via parameter and extension (if configured) and then passed to HTTP::Negotiate. On success the method returns a format name. The method may modify the PSGI request environment keys PATH_INFO and SCRIPT_NAME if format was selected by extension set to strip, and strips the format HTTP GET query parameter from QUERY_STRING if parameter is set to a format. If format was selected by HTTP POST body parameter, the parameter it is not stripped from the request.

known( $format )

Tells whether a format name is known. By default this is the case if the format name exists in the list of formats.

about( $format )

If the format was specified, this method returns a hash with quality, type, encoding, charset, and language. Missing values are set to the default.


Returns a list of content variants to be used in HTTP::Negotiate. The return value is an array reference of array references, each with seven elements: format name, source quality, type, encoding, charset, language, and size. The size is always zero.

add_headers( \@headers, $format )

Add apropriate HTTP response headers for a format unless the headers are already given.



A list of formats to choose among. Each format can be defined with type, quality (defaults to 1), encoding, charset, and language. The special format name _ (underscore) is reserved to define default values for all formats.

Formats can also be used to directly route the request to a PSGI application:

    my $app = Plack::Middleware::Negotiate->new(
        formats => {
            json => { 
                type => 'application/json',
                app  => $json_app,
            html => {
                type => 'text/html',
                app  => $html_app,

Enables explicit format selection with a query paramater, for instance format. Both HTTP GET and HTTP POST body parameters are supported.


Enables explicit format selection with a virtual file extension. The value strip strips a known format name from the request path. The value keep keeps the format name extension after format selection.

The middleware takes care for rewriting and restoring PATH_INFO if it is configured to detect and strip a format extension.


Disables content negotiation based on HTTP headers.


Plack::Middleware::Negotiate uses Log::Contextual to emit a logging message during content negotiation on logging level trace. Just set:



The Content-Encoding HTTP response header is not automatically set on a response and content negotiation based on size is not supported. Feel free to comment on whether and how this middleware should support both.


Jakob Voß, Christopher A. Kirke


Copyright 2014- Jakob Voß

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


Content negotiation in this module is based on HTTP::Negotiate. See HTTP::Headers::ActionPack::ContentNegotiation for an alternative approach. This module has some overlap with Plack::Middleware::SetAccept.