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-# Reference Graph
-
-Since 08/2021 references are available on an "inbound" and "outbound" basis in
-the web interface.
-
-The backend reference graph is available via the [Search Index](./search_api.md)
-under the `fatcat_ref` index.
-
-## Background and Mode of Operation
-
-Release entities in fatcat have a [refs fields](./entity_release.md) which
-contains citations, which in turn may be identified in different ways. Another
-source of reference metadata is provided by structured data extraction from PDF
-with tools such as [GROBID](https://grobid.readthedocs.io). The raw reference data combined
-amounts to over 2B documents which we take as input for a batch process, that
-derives the graph structure.
-
-Two main modes of citation matching are employed: identifier based matching and
-fuzzy matching. Identifier based matching currently works with DOI, Arxiv ids,
-PMID and PMCID and ISBN. Fuzzy matching employs a scalable way to cluster
-documents (with pluggable clustering algorithms). For each cluster of match
-candidates we run a more extensive verification process, which yields a match
-confidence category, ranging from weak over strong to exact. Strong and exact
-matches are included in the graph.
-
-The current reference search index contains both matches and yet unmatched
-references. We expect this dataset to be iterated over regularly as there are
-a few dimensions along which the dataset can be improved and extended.
+# Reference Graph (refcat)
+
+In Summer 2021, the first version of a reference graph dataset, named "refcat",
+was released and integrated into the fatcat.wiki web interface. The dataset
+contains billions of references between papers in the fatcat catalog, as well
+as partial coverage of references from papers to books, to websites, and from
+Wikipedia articles to papers. This is a first step towards identifying links
+and references between scholarly works of all types preserved in archive.org.
+
+The refcat dataset can be downloaded in JSON lines format from the archive.org
+"[Fatcat Database Snapshots and Bulk Metadata Exports](https://archive.org/details/fatcat_snapshots_and_exports)"
+collection, and is released under a CC-0 license for broad reuse.
+Acknowledgement and attribution for both the aggregated dataset and the
+original metadata sources is strongly encouraged (see below for provenance
+notes).
+
+References can be browsed on fatcat.wiki on an "outbound" ("References") and
+"inbound" ("Cited By") basis for individual release entities. There are also
+special pages for Wikipedia articles ("outbound", such as
+[Internet](https://fatcat.wiki/wikipedia/en:Internet/refs-out)) and Open
+Library books ("inbound", such as [The
+Gift](https://fatcat.wiki/openlibrary/OL2670078W/refs-in)). JSON versions of
+these pages are available, but do not yet represent a stable API.  The backend
+reference graph is available via the [Elasticsearch API](./search_api.md) under
+the `fatcat_ref` index, but these schema and semantics of this index are also
+not yet stable.
+
+
+## How It Works
+
+Raw reference data comes from multiple sources (see "provenance" below), but
+has the common structure of a "source" entity (which could be a paper,
+Wikipedia article, etc) and a list of raw references. There might be duplicate
+references for a single "source" work coming from different providers (eg, both
+Pubmed and Crossref reference lists). The goal is to match as many references
+as possible to the "target" work being referenced, creating a link from source
+to target. If a robust match is not found, the "unmatched" reference is
+retained and displayed in a human readable fashion if possible.
+
+Depending on the source, raw references may be a simple "raw" string in an
+arbitrary citation style; may have been parsed or structured in fields like
+"title", "year", "volume", "issue"; might include a URL or identifier like an
+arxiv.org identifier; or may have already been matched to a specific target
+work by another party. It is also possible the reference is vague, malformed,
+mis-parsed, or not even a reference to a specific work (eg, "personal
+communication"). Based on the available structure, we might be able to do a
+simple identifier lookup, or may need to parse a string, or do "fuzzy" matching
+against various catalogs of known works. As a final step we take all original
+and potential matches, verify the matches, and attempt to de-duplicate
+references coming from different providers into a list of matched and unmatched
+references as output. The refcat corpus is the output of this process.
+
+Two dominant modes of reference matching are employed: identifier based
+matching and fuzzy matching. Identifier based matching currently works with
+DOI, Arxiv ids, PMID and PMCID and ISBN. Fuzzy matching employs a scalable way
+to cluster documents (with pluggable clustering algorithms). For each cluster
+of match candidates we run a more extensive verification process, which yields
+a match confidence category, ranging from weak over strong to exact. Strong and
+exact matches are included in the graph.
+
+All the code for this process is available open source:
+
+- [cgraph](https://gitlab.com/internetarchive/cgraph): batch processing and matching pipeline, in Python and Go
+- [fuzzycat](https://gitlab.com/internetarchive/fuzzycat): Python verification code and "live" fuzzy matching
+
+
+## Metadata Provenance
+
+The provenance for each reference in the index is tracked and exposed via the
+`match_provenance` field. A `fatcat-` prefix to the field means that the
+reference came through the `refs` metadata field stored in the fatcat catalog,
+but originally came from the indicated source. In the absence of `fatcat-`, the
+reference was found, updated, or extracted at indexing time and is not recorded
+in the `release` entity metadata.
+
+Specific sources:
+
+* `crossref` (and `fatcat-crossref`): citations deposited by publishers as part
+  of DOI registration. Crossref is the largest single source of citation
+  metadata in refcat. These references may be linked to a specific DOI; contain
+  structured metadata fields; or be in the form of a raw citation string.
+  Sometimes they are "complete" for the given work, and sometimes they only
+  include references which could be matched/linked to a target work with a DOI.
+* `fatcat-datacite`: same as `crossref`, but for the Datacite DOI registrar.
+* `fatcat-pubmed`: references, linked or not linked, from Pubmed/MEDLINE
+  metadata
+* `fatcat`: references in fatcat where the original provenance can't be infered
+  (but could be manually found by inspecting the release edit history)
+* `grobid`: references parsed out of full-text PDFs using
+  [GROBID](https://github.com/kermitt2/grobid)
+* `wikipedia`: citations extracted from Wikipedia (see below for details)
+
+Note that sources of reference metadata which have formal licensing
+restrictions, even CC-BY or ODC-BY licenses as used by several similar
+datasets, are not included in refcat.
+
+
+## Current Limitations and Known Issues
+
+The initial Summer 2021 version of the index has a number of limitations.
+Feedback on features and coverage are welcome! We expect this dataset to be
+iterated over regularly as there are a few
+dimensions along which the dataset can be improved and extended.
+
+The reference matching process is designed to eventually operate in both
+"batch" and "live" modes, but currently only "batch" output is in the index.
+This means that references from newly published papers are not added to the
+index in an ongoing fashion.
+
+Fatcat "release" entities (eg, papers) are matched from a Spring 2021 snapshot.
+References to papers published after this time will not be linked.
+
+Wikipedia citations come from the dataset [Wikipedia Citations: A comprehensive
+dataset of citations with identifiers extracted from English
+Wikipedia](https://zenodo.org/record/3940692), by Singh, West, and Colavizza.
+This is a one-time corpus based on a May 2020 snapshot of English Wikipedia
+only, and is missing many current references and citations. Additionally, only
+direct identifier lookups (eg, DOI matches) are used, not fuzzy metadata
+matching.
+
+Open Library "target" matches are based on a snapshot of Open Library works,
+and are matched either ISBN (extracted from citation string) or fuzzy metadata
+matching.
+
+Crossref references are extracted from a January 2021
+[snapshot](https://archive.org/details/crossref_doi_dump_2021-01) of Crossref
+metadata, and do not include many updates to existing works.
+
+Hundreds of millions of raw citation strings ("unstructured") have not been
+parsed into a structured for for fuzzy matching. We plan to use GROBID to parse
+these citation strings, in addition to the current use of GROBID parsing for
+references from fulltext documents.
+
+The current GROBID parsing used version v0.6.0. Newer versions of GROBID have
+improved citation parsing accuracy, and we intend to re-parse all PDFs over
+time. Additional manually-tagged training datasets could improve GROBID
+performance even further.
+
+In a future update, we intend to add Wayback (web archive) capture status and
+access links for references to websites (distinct from references to online
+journal articles or books). For example, references to an online news article
+or blog post would indicate the closest (in time, to the "source" publication
+date) Wayback captures to that web page, if available.
+
+References are only displayed on fatcat.wiki, not yet on scholar.archive.org.
+
+There is no current or planned mechanism for searching, sorting, or filtering
+article search results by (inbound) citation count. This would require
+resource-intensive transformations and continuous re-indexing of search
+indexes.
+
+It is unclear how the batch-generated refcat dataset and API-editable release
+refs metadata will interact in the future. The original refs may eventually be
+dropped from the fatcat API, or at some point the refcat corpus may stabilize
+and be imported in to fatcat refs instead of being maintained as a separate
+dataset and index. It would be good to retain a mechanism for human corrections
+and overrides to the machine-generated reference graph.
+