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Kerko is a web application component that provides a user-friendly search and browsing interface for sharing a bibliography managed with the Zotero reference manager.

The combination of Kerko and Zotero gives you the best of both worlds: a rich but easy to use web interface for end-users of the bibliography, and a well-established and powerful bibliographic reference management tool for individuals or teams working on the bibliography's content.

How it works

A Kerko-powered bibliography is managed using Zotero, and stored in the cloud on, while Kerko itself is incorporated into an application which is installed on a web server. The bibliographic references may reside in a Zotero group library, where multiple users may collaborate to manage the content, or in a Zotero private library. On the web server, Kerko maintains a search index, which is a copy of the Zotero library that is optimized for search. When users interact with the web application, Kerko gets all the required data from that search index, without ever contacting It is through a scheduled task, which runs at regular intervals, that Kerko automatically brings its search index up to date by using the Zotero Web API to retrieve the latest data from

Kerko is implemented in Python using the Flask framework. As an application component, Kerko only works when incorporated into a Flask application. Such an application, KerkoApp, is available for you to use, and can be deployed on a web server. KerkoApp can be seen as just a thin layer over Kerko. It might work for you if you like the default appearance, and if the provided configuration options are sufficient for your needs.

For more advanced needs, you could consider building a custom application, possibly derived from KerkoApp, where additional customizations could be implemented in Python. In this scenario, it is not recommended that you change Kerko itself, but that you use Kerko's Python API to implement your customizations. Kerko is actually a Flask "blueprint" (similar to what some other systems might call a plugin or an extension), which allows a modular architecture where the application integrates and extends Kerko, and possibly other blueprints as well; the Kerko-powered bibliography could be just one component of a larger Flask application.

Demo site

A demo site, which is based on KerkoApp, is available for you to try. You may also view the Zotero library that contains the source data for the demo site.

Sites using Kerko

The following sites are powered by Kerko:

Are you using Kerko?

If you would like to add your Kerko-powered site to this list, please e-mail us or submit a pull request.


The main features provided by Kerko are:

  • Faceted search interface: allows exploration of the bibliography both in search mode and in browsing mode, potentially suiting different user needs, behaviors, and abilities. For example, users with a prior idea of the topic or expected results may enter keywords or a more complex query in a search field, while those who wish to become familiar with the content of the bibliography or discover new topics may choose to navigate along the proposed facets, to narrow or broaden their search. Since both modes are integrated into a single interface, it is possible to combine them.
  • Keyword search features:
    • Boolean operators:
      • AND: matches items that contain all specified terms. This is the default relation between terms when no operator is specified, e.g., a b is the same as a AND b.
      • OR: matches items that contain any of the specified terms, e.g., a OR b.
      • NOT: excludes items that match the term, e.g., NOT a.
      • Boolean operators must be specified in uppercase and may be translated in other languages.
    • Logical grouping (with parentheses), e.g., (a OR b) AND c.
    • Sequence of words (with double quotes), e.g., "a b c". The default difference between word positions is 1, meaning that an item will match if it contains the words next to each other, but a different maximum distance may be selected (with the tilde character), e.g. "web search"~2 allows up to 1 word between web and search, meaning it could match web site search as well as web search.
    • Term boosting (with the caret), e.g., faceted^2 search browsing^0.5 specifies that faceted is twice as important as search when computing the relevance score of results, while browsing is half as important. Boosting may be applied to a logical grouping, e.g., (a b)^3 c.
    • Keyword search is case-insensitive, accents are folded, and punctuation is ignored. To further improve recall (albeit at the cost of precision), stemming is also performed on terms from most text fields, e.g., title, abstract, notes. Stemming relieves the user from having to specify all variants of a word when searching, e.g., terms such as search, searches, and searching all return the same results. The Snowball algorithm is used for that purpose.
    • Full-text search: the text content of PDF attachments can be searched.
    • Scope of search: users may choose to search everywhere, in author/contributor names, in titles, in all fields (i.e., in metadata and notes), or in documents (i.e., in the text content of attachments). Applications may provide additional choices.
  • Faceted browsing: allows filtering by topic (Zotero tag), by resource type (Zotero item type), by publication year. Moreover, you may define additional facets modeled on collections and subcollections; in such case, any collection can be represented as a facet, and each subcollection as a value within that facet. By taking advantage of Zotero's ability to assign any given item to multiple collections, a faceted classification scheme can be designed, including hierarchical subdivisions within facets.
  • Relevance scoring: provided by the Whoosh library and based on the BM25F algorithm, which determines how important a term is to a document in the context of the whole collection of documents, while taking into account its relation to document structure (in this regard most fields are neutral, but the score is boosted when a term appears in specific fields, e.g., DOI, ISBN, ISSN, title, author/contributor). Any keyword search asks the question "how well does this document match this query clause?", which requires calculating a relevance score for each document. Filtering with facets, on the other hand, has no effect on the score because it asks "does this document match this query clause?", which leads to a yes or no answer.
  • Sort options: by relevance score (only applicable to keyword search), by publication date, by author, by title.
  • Citation styles: any from the Zotero Style Repository, or custom stylesheet defined in the Citation Style Language (stylesheet must be accessible by URL).
  • Language support: the default language of the user interface is English, but some translations are provided. Additional translations may be created using gettext-compatible tools (see Translating Kerko). Also to consider: locales supported by the Zotero Data Schema (which provides the names of fields, item types and author types displayed by Kerko); languages supported by Whoosh (which provides the search capabilities), i.e., ar, da, nl, en, fi, fr, de, hu, it, no, pt, ro, ru, es, sv, tr.
  • Semantic markup: pages generated by Kerko embed HTML markup that can be detected by web crawlers (helping the indexing of your records by search engines) or by web browsers (allowing users of reference management tools to easily import metadata in their library). Supported schemes are:
    • OpenURL COinS, in search results pages and individual bibliographic record pages. COinS is recognized by many reference management tools, including the Zotero Connector browser extension.
    • Highwire Press tags, in the individual bibliographic record pages of book, conference paper, journal article, report or thesis items. These tags are recommended for indexing by Google Scholar, and are recognized by many other databases and reference management tools, including the Zotero Connector browser extension.
  • Web feeds: users of news aggregators or feed readers may get updates when new bibliographic records are added. They may subscribe to the main feed, or to one or more custom feeds.
    • The main feed lists the most recently added bibliographic records.
    • Any search page has a related custom feed that lists the most recently added bibliographic records that match the search criteria. Thus, a user can obtain a custom feed for a particular area of interest simply by entering keywords to search and/or selecting filters.
    • Feeds are provided in the Atom syndication format.
    • Basic metadata is provided directly in the feeds, using both Atom and unqualified Dublin Core elements.
    • An age limit may be configured to exclude older items from the feeds. This may be useful to bibliographies that are frequently updated and mostly meant to promote recent literature (all resources still remain visible to the search interface regardless of their age).
  • Sitemap: an XML Sitemap is automatically generated, and you may use it to help search engines discover your bibliographic records.
  • Exporting: users may export individual records as well as complete bibliographies corresponding to search results. By default, download links are provided for the RIS and BibTeX formats, but applications may be configured to export any format supported by the Zotero API.
  • Printing: stylesheets are provided for printing individual bibliographic records as well as lists of search results. When printing search results, all results get printed (not just the current page of results).
  • Notes and attachments: notes, attached files, and attached links to URIs are synchronized from and made available to users of the bibliography. Regular expressions may be used to include or exclude such child items from the bibliography, based on their tags.
  • DOI, ISBN and ISSN resolver: items that have such identifier in your library can be referenced by appending their identifier to your Kerko site's base URL.
  • Relations: bibliographic record pages show links to related items, if any. You may define such relations using Zotero's Related field. Moreover, Kerko adds the Cites and Cited by relation types, which can be managed in Zotero through notes (see guide). Custom applications can add more types of relations if desired.
  • Badges: custom applications can have icons conditionally displayed next to items.
  • Responsive design: the simple default implementation works on large monitors as well as on small screens. It is based on Bootstrap.
  • Google Analytics integration: just provide a Google Analytics stream ID to have Kerko automatically include the tracking code into its pages.
  • Integration: as a Flask blueprint, Kerko can be integrated into any Flask application. For a standalone application, however, you may simply install KerkoApp.
  • Customizable front-end: applications may partly or fully replace the default templates, scripts and stylesheets with their own.
  • Command line interface (CLI): Kerko provides commands for synchronizing or deleting its data.

KerkoApp is a standalone application built around Kerko. It inherits all of Kerko's features and it provides a few additions of its own:

  • Configuration files: allow separation of configuration from code and enable the Twelve-factor App methodology. Environment variables and TOML configuration files are supported. Secrets, server-specific parameters, and general parameters can be configured in separate files.
  • Page templates for common HTTP errors.
  • Syslog logging handler (for Unix environments).


Kerko requires Python 3.7 or later.

Required Python packages will be automatically installed when installing Kerko. The main ones are:

  • Babel: utilities for internationalization and localization.
  • Bootstrap-Flask: helper for integrating Bootstrap.
  • Click: command line interface creation kit.
  • Flask: web application framework.
  • Flask-Babel: integration of Flask and Babel.
  • Flask-WTF: integration of Flask and WTForms.
  • Jinja2: template engine.
  • Pydantic: configuration parsing and validation.
  • Pyzotero: Python client for the Zotero API.
  • w3lib: URL and HTML manipulation utilities.
  • Werkzeug: WSGI web application library (also required by Flask).
  • Whoosh: pure Python full-text indexing and searching library.
  • WTForms: web forms validation and rendering library.

The following front-end resources are loaded from CDNs by Kerko's default templates (but could be completely removed or replaced in custom templates):

  • Bootstrap: front-end component library for web applications.
  • FontAwesome: beautiful open source icons.
  • jQuery: JavaScript library (required by Bootstrap).
  • Popper.js: JavaScript library for handling tooltips, popovers, etc. (used by Bootstrap).

Known limitations

Before choosing Kerko for your project, you might want to review the following known limitations of Kerko:

  • The system can probably handle relatively large bibliographies (it has been tested so far with about 15,000 entries), but the number of distinct facet values has more impact on response times than the number of references. For the best response times, it is recommended to limit the number of distinct facet values under a few hundreds.
  • Kerko can only manage a single bibliography per application.
  • Although Kerko can be integrated in a multilingual web application were the visitor may select a language, Zotero does not provide a way to manage tags or collections in multiple languages. Thus, there is no easy way for Kerko to provide those names in the user's selected language.
  • Whoosh does not provide much out-of-the-box support for non-Western languages. Therefore, search might not work very well with such languages.
  • Zotero is the sole reference management tool supported as a back-end to Kerko.