DOI Content Negotiation

1 Introduction

DOIs provide a persistent link to content. They identify many types of work, from journal articles to research data sets. Typically, someone interacting with DOIs will be a researcher, who will resolve DOIs found in scholarly references to content using a DOI resolver. Such researchers may not even realise they are using DOIs and a DOI resolver since they may follow links with embedded DOIs.

Yet DOIs can provide more than a permanent, indirect link to content. DOI registration agencies such as Crossref, DataCite and mEDRA collect bibliographic metadata about the works they link to. This metadata can be retrieved from a DOI resolver too, using content negotiation to request a particular representation of the metadata.

For some DOIs content negotiation can be used to retrieve different representations of a work. For example, some DataCite DOIs identify data sets that may be available in a number of data formats and container formats.

2 Redirection

The DOI resolver at doi.org will normally redirect a user to the resource location of a DOI. For example, the DOI "10.1126/science.169.3946.635" redirects to a landing page describing the article, "The Structure of Ordinary Water". Content negotiated requests to doi.org that ask for a content type which isn't "text/html" will be redirected to a metadata service hosted by the DOI's registration agency. Crossref, DataCite and mEDRA support content negotiated DOIs via their respective resolvers.

       GET "Accept: text/html"
https://doi.org/10.1126/science.169.3946.635

                   |
                   |
                   |
                   V

       Publisher landing page
https://www.sciencemag.org/content/169/3946/635

Normal browser requests or explicit requests for text/html redirect to the content's landing page.

       GET "Accept: application/rdf+xml"
https://doi.org/10.1126/science.169.3946.635

                   |
                   |
                   |
                   V

    Crossref metadata service
https://api.crossref.org/10.1126/science.169.3946.635/transform

Requests for a data type redirect to a registration agency's metadata service.

3 What is Content Negotiation?

Content negotiation allows a user to request a particular representation of a web resource. DOI resolvers use content negotiation to provide different representations of metadata associated with DOIs.

A content negotiated request to a DOI resolver is much like a standard HTTP request, except server-driven negotiation will take place based on the list of acceptable content types a client provides.

3.1 The Accept Header

Making a content negotiated request requires the use of a HTTP header, "Accept". Content types that are acceptable to the client (those that it knows how to parse), each with an optional "quality" value indicating its relative suitability. For example, a client that wishes to receive citeproc JSON if it is available, but which can also handle RDF XML if citeproc JSON is unavailable, would make a request with an Accept header listing both "application/citeproc+json" and "application/rdf+xml":

$ curl -LH "Accept: application/rdf+xml;q=0.5, application/vnd.citationstyles.csl+json;q=1.0" https://doi.org/10.1126/science.169.3946.635

{
  "volume" : "169",
  "issue" : "3946",
  "DOI" : "10.1126/science.169.3946.635",
  "URL" : "https://doi.org/10.1126/science.169.3946.635",
  "title" : "The Structure of Ordinary Water: New data and interpretations are
           yielding new insights into this fascinating substance",
  "container-title" : "Science",
  "publisher" : "American Association for the Advancement of Science AAAS (Science)",
  "issued" : { "date-parts" : [ [ 1970,8,14 ] ] },
  "author" : [ { "family" : "Frank", "given" : "H. S."} ],
  "editor" : [],
  "page" : "635-641",
  "type" : "article-journal"
}

This request favours citeproc JSON but will accept RDF XML if citeproc is unavailable. The q values are optional. The request could have been written without them. The order of content types then becomes important; more suitable content types should be placed at the front of the Accept header.

$ curl -LH "Accept: application/vnd.citationstyles.csl+json, application/rdf+xml" https://doi.org/10.1126/science.169.3946.635

3.2 Response Codes

Code	Meaning
200	The request was OK.
204	The request was OK but there was no metadata available.
404	The DOI requested doesn't exist.
406	Can't serve any requested content type.

Individual RA metadata services may utilise additional response codes but they will always use the response codes above in event of the case described.

If multiple content types specified by the client are supported by a DOI then the content type with the highest "q" value (or, if no "q" values are specified, the one that appears first in the "accept" header) will be returned.

4 Supported Content Types

Currently three DOI registration agencies have implemented content negotation for their DOIs: Crossref, DataCite and mEDRA. They support a number of metadata content types, some of which are common to the three RAs.

Format	Content Type	Crossref	DataCite	mEDRA
RDF XML	application/rdf+xml	Yes	Yes	Yes
RDF Turtle	text/turtle	Yes	Yes	Yes
Citeproc JSON	application/vnd.citationstyles.csl+json	Yes	Yes	Yes
Schema.org in JSON-LD	application/vnd.schemaorg.ld+json	No	Yes	No
Formatted text citation	text/x-bibliography	Yes	Yes	Yes
RIS	application/x-research-info-systems	Yes	Yes	No
BibTeX	application/x-bibtex	Yes	Yes	Yes
Crossref Unixref XML	application/vnd.crossref.unixref+xml	Yes	No	No
Crossref UNIXSD XML	application/vnd.crossref.unixsd+xml	Yes	No	No
DataCite XML	application/vnd.datacite.datacite+xml	No	Yes	No
ONIX for DOI	application/vnd.medra.onixdoi+xml	No	No	Yes

Using content negotiation it is possible to make a request that favours content types specific to a particular registration agency but which will also degrade to respond with a more standard content type for other registration agencies. For example:

$ curl -LH "Accept: application/vnd.crossref.unixref+xml;q=1, application/rdf+xml;q=0.5" https://doi.org/10.1126/science.169.3946.635

This request will return Crossref XML for Crossref DOIs and RDF XML for non-Crossref DOIs, such as DataCite DOIs.

4.1 Formatted Citations

Crossref, DataCite and mEDRA support formatted citations via the text/bibliography content type. These are the output of the Citation Style Language processor, citeproc-js. The content type can take two additional parameters to customise its response format. A "style" can be chosen from the list of style names found in the CSL style repository. Many styles are supported, including common styles such as apa and harvard3:

$ curl -LH "Accept: text/x-bibliography; style=apa" https://doi.org/10.1126/science.169.3946.635

Frank, H. S. (1970). The Structure of Ordinary Water: New data and interpretations are yielding
  new insights into this fascinating substance. Science, 169(3946), 635-641. American Association
  for the Advancement of Science AAAS (Science). doi:10.1126/science.169.3946.635

A locale can also be specified. Use one of the locale names from the CSL locales repository:

$ curl -LH "Accept: text/x-bibliography; style=harvard3; locale=fr-FR" https://doi.org/10.1126/science.169.3946.635

Frank, HS 1970, « The Structure of Ordinary Water: New data and interpretations are yielding new
  insights into this fascinating substance ». Science, vol. 169, no. 3946, p. 635-641. Consulté
  de https://doi.org/10.1126/science.169.3946.635

5 Link-based Content Type Requests

DataCite supports link-based content type requests, for example:

$ curl https://data.datacite.org/vnd.datacite.datacite+xml/10.5524/100005

For documentation see https://data.datacite.org.

Crossref also supports link-based content type requests via their REST API:

$ curl https://api.crossref.org/works/10.5555/12345678/transform/application/x-bibtex

For documentation see https://api.crossref.org.

6 Getting Help

Please contact labs@crossref.org, tech@datacite.org or tech@medra.org for support.