OpenCitations Structured open citation data as a part of the Commons

OpenCitations ( employs Semantic Web technologies to publish scholarly citation data in an open repository. The main service developed by OpenCitations is the OpenCitations Corpus (OCC), in which RDF-based scholarly citation data (i.e. bibliographic references) are stored and made freely available under a Creative Commons public domain dedication so that others may use and build upon them. The other resources of OpenCitations – namely the ontologies describing the data and the software developed to build the OCC – are also available with open licenses. OpenCitations follows the FAIR principles for data-intensive science, namely that the data should be findable, accessible, interoperable and re-usable, and employs the three basic criteria promoted by the Initiative for Open Citations, namely that the citation data must be structured, separable and open.

In this presentation, we will explain how OpenCitations harvests citations and associated metadata from several external APIs (those of Europe PubMed Central, Crossref and ORCID), describes them using the SPAR Ontologies, the Provenance Ontology and FOAF according to the OCC metadata model, and stores all the data in JSON-LD. The stored citations are made accessible via the OCC SPARQL interface, by means of the HTTP requests (using content negotiation, e.g., and as monthly data dumps. As of 5th August 2017, the OCC had ingested the references from 211,196 citing bibliographic resources and contained information about 8,986,030 citation links to 5,106,542 cited resources.

We will exemplify how OpenCitations can benefit the scholarly community at large, being used by international initiatives and projects including Wikidata/WikiCite, OpenAIRE, LOC-DB and eLife, and will describe future plans, made possible by funding from the Sloan Foundation, to increase the rate of citation ingest thirty-fold to 15 million citations per month, and to develop novel user interfaces to access and visualize the citation data.

What is a citation

A citation is a conceptual directional link from a citing entity to a cited entity, instantiated by a bibliographic reference + an in-text reference pointer denoting such reference

Citation indexes

A citation index is a kind of bibliographic index, an index of citations between publications, allowing the user to easily establish which later documents cite which earlier documents – from Wikipedia

  • Some are freely accessible but not downloadable

  • Others can be accessed only by paying significant access fees

Human readability first

Towards machine readability?

Semantic Publishing concerns the use of Web and Semantic Web technologies and standards for enhancing a scholarly work semantically (by means of plain RDF statements, nano-publications, etc.) so as to improve its discoverability, interactivity, openness and (re-)usability...

... for both humans and machines

David Shotton (2009). Semantic publishing: the coming revolution in scientific journal publishing. Learned Publishing, 22 (2): 85–94.

SPAR Ontologies

Back to 2010: Semantic Publishing and Referencing (SPAR) Ontologies form a suite of orthogonal and complementary OWL 2 DL ontology modules for the creation of comprehensive machine-readable RDF metadata for every aspect of semantic publishing and referencing

FAIR citation data


The OpenCitations Corpus (OCC, is a Linked Open Data (LOD) repository of CC0 citation data described with SPAR Ontologies (BiRO, C4O, CiTO, DataCite Ontology, FaBiO, PRO), FOAF, PROV-O, DCAT, VOID

It provides >11M citation links from ~260,000 citing articles to ~6M cited resources + provenance information

OCC data are FAIR:

Ingestion workflow

We developed several scripts for implementing the ingestion workflow that populates the OpenCitations Corpus

All the software is available on the OpenCitations GitHub repository and released as open source code with the ISC License

These scripts implement a live and iterative process

  • Live: it's working while I'm speaking. It doesn't sleep, never. It's like a sentient, relentless, fast zombie – watch out!

  • Iterative: the ingestion workflow continuously calls several external APIs to obtain new reference lists and clean metadata of the citing and cited papers

External APIs

At present, all the reference lists are taken by processing the XML sources of the papers in the PubMed Central Open Access subset

Europe PubMed Central API for retrieving the XML sources

  • We ask for the most recent papers first

  • Citing papers includes articles published in 2016 and 2017

  • There are 1.75M OA articles available in PubMed, according to their API. We have harvested 15% so far...

Crossref APIs to obtain additional information (title, authors, venues, etc.) about citing/cited papers, and then call the ORCID APIs to obtain ORCIDs of the authors

Data model

Available at and implemented in the OpenCitations Ontology

Resources in the OCC

Entity typeWhat it describesCount in the OCC
Bibliographic resource (br)Conference papers, book chapters, journal articles, academic proceedings, books, journals, etc. ~7.3M
Resource embodiment (re)Digital vs. print, first and ending pages, etc. ~4.3M
Bibliographic entry (be)Textual content of a reference in a reference list ~10.5M
Resposible agent (ra)Given name, family name and ORCID of the agent involved ~22.5M
Agent role (ar)Author, publisher, etc. ~28.6M
Identifier (id)DOI, PubMed ID, PubMed Central ID, ORCID, ISSN, etc.~15.1M

Data statementsProvenance statements

Entities in the OCC

Entity typeKinds of entityEntities with identifiers
Bibliographic resource

Academic Proceedings: 13,804

Book: 47,824

Book Chapter: 68,798

Book Series: 15

Data File: 4,155

Expression Collection: 10,321

Journal: 27,373

Journal Article: 4,481,224

Journal Issue: 1,029,369

Journal Volume: 365,286

Part: 6

Proceedings Paper: 28,125

Reference Book: 66

Reference Entry: 7,917

Report Document: 1,946

Series: 155

Specification Document: 600

Thesis: 768

Generic kind: 1,281,627

DOI: 4,697,326

ISBN: 82,329

ISSN: 38,359

PMCID: 1,280,229

PMID: 3,954,683

URL: 4,835,585

Responsible agent -

ORCID: 240,989

URL: 5,642


DOI: 4,697,326

ISBN: 82,329

ISSN: 38,359

ORCID: 240,989

PMCID: 1,280,229

PMID: 3,954,683

URL: 4,841,227


Accessing OCC data

Dump views and downloads

Usage of the OCC

Some known adopters: Wikidata, OpenAIRE, LOC-DB, eLife, Ontotext, independent researchers (Anna Kamińska, Daniel Himmelstein, Thiago Nunes and Daniel Schwabe)

Website accesses

Use by country: United States (33.7%), France (31.3%), Bulgaria (14.3%)


OpenCitations Blog on Wordpress at


OpenCitations Twitter account at


Current infrastructure

It's a virtual machine - it doesn't exist!

International advisory panel

  • Geoffrey Bilder, Crossref

  • Jonathan Dugan, Berkeley Institute for Data Science

  • Catriona MacCallum, Hindawi

  • Johanna McEntyre, Europe PubMed Central

  • Cassidy Sugimoto, Indiana University

  • Dario Taraborelli, Wikimedia Foundation

Future infrastructure

We have recently received a small grant from the Alfred P. Sloan Foundation for

  • one year's salary for a postdoc (Ivan Heibi, who will start in November) to develop new user interfaces

  • new hardware to enhance the OCC performance
    ⇒ from 8M citations per year to 240M

DELL PowerEdge R730xd V4, 512GB RAM, 22TB HD + 30 Raspberry Pi 3

Citation data are coming

END OpenCitations: structured open citation data as a part of the Commons