Solving the Item-Level Problem on the Web

Published: 2012-01-23 13:57

Digital Collections Services Through Using Web Crawls

Digital libraries have attempted to provide various aggregations of their content. Usually the participants in the aggregation already make that content accessible on the open web. The approaches to aggregating content that have been taken in the past have relied on hosting institutions to provide their metadata in new ways and support additional infrastructure and workflows. An alternative approach to creating aggregations is to perform targeted crawls and reuse the content on the pages. The problem with the crawler approach dentifying items in the collection as opposed to other pages. This document presents a few possibilities for how to identify items.

If you know of prior work with similar critiques and suggested solutions, please let me know. I am eager to improve on the techniques of this approach.

This document was initially written in support of a DPLA Technical Strawman Proposal.

Problems with technical approaches taken so far to achieve aggregations

Leaving aside the usefulness of aggregations, the approaches taken to achieve aggregations of digital collections have had problems for those who want to be a part of such aggregations. These problems fall into the categories of separate standards, separate infrastructure, and metadata dumb-down.

The primary way in which institutions make their resources available is on the Web. Great effort is expended to make web pages that are optimized for search engines and usable and attractive for users. The standards used are common, ubiquitous standards like HTML and HTTP shared by developers throughout the world. The metadata presented provides as rich context as is available for the objects made accessible.

Counter this to the common approach found in aggregations created by libraries. They often rely on an OAI-PMH gateway for harvesting item-level metadata about collections. In order for collections to take part in an OAI-PMH enabled aggregation, institutions which host digital collections must expose their information through special XML rather than the HTML they already have. The different standards and tools are foreign and a barrier to entry for the many developers more familiar with web standards. An OAI-PMH gateway is another separate service which needs to be maintained in addition to the web site. The effort for providing services and harmonizing data can be pushed down from the aggregator to the source collections. It takes extra effort on the part of institutions which are already squeezed. There are maintenance costs to keeping these services up and in sync with the data which is exposed through the website. New aggregators would do well to investigate the problems encountered by previous aggregations using this kind of approach like the DLF Aquifer.

Metadata dumb-down is where metadata goes through a transformation which decreases the level of precision of metadata. It has been a valuable strategy to harmonizing metadata across institutions. The problem is that many institutions have rich, specialized metadata that, when dumbed-down, loses much of its value. This rich content is often exposed on collection websites but cannot make it through the transformation to the shared metadata schema. In order to create the powerful aggregations that we want, we need to look for new ways to leverage more of the rich metadata our institutions have invested in.

Crawling for Data

Instead of relying on separate standards or infrastructure to provide an aggregation, it is possible to crawl websites instead. This significantly lowers the barrier for institutions to participate in an aggregation as it relies on the existing web sites of digital collections. When a robot crawls a web site, it may follow all of the links on a page. Pages like browse, search, about, and contact pages can be crawled along with the pages that describe individual digital objects. For many kinds of search, an index of all of that crawled content could be useful.

Some aggregations, though, prefer to only expose item-level metadata including small surrogates for objects like thumbnails. Crawling digital collections is not incompatible with being able to identify individual items in a digital collection. There are relatively simple solutions for identifying items which could cover many of the digital collections that already exist on the web. Some of these will be set forth below.

Even if an aggregator is able to identify pages that describe individual items, there is still the problem of how to extract useful information to allow for functionality like faceted search interfaces. Until recently search engines have had to be content with using some HTML semantics (<ol> means an ordered list), along with natural language processing, in order to discover the core content and meaning of pages on the web. These approaches can go very far in extracting meaning from the unstructured data on a page, but they have their limits.

Recently there have been renewed efforts to create simple standards for embedding data in HTML. These allow search engines to extract data from the page and make sense of it. While these efforts so far have been targeted toward the use cases of search engines and commercial organizations, there are possibilities for the cultural heritage sector to make use of these same technologies. By digital collections using hidden markup embedded in HTML the crawler approach could also have access to the rich content that is already accessible in a form which would enable more interesting interfaces for aggregations.

Following sections will make the case for how this could be accomplished.

Collection Profiles

The first problem encountered in using crawlers to create item-level aggregations is curatorial. It is infeasible for most institutions to crawl the whole web looking for appropriate digital collections content. Instead there needs to be a store of metadata about digital collection web sites to know what is accessible and where to find it. Such a system can store metadata about collections that enable crawling and other services.

Collection Profiles are a compilation of metadata about digital collections on the open web. The other solutions set out below could use such a metadata store, though the approaches themselves do not rely on it. I have done prior work, along with Tito Sierra, in setting out what a system for compiling metadata about digital collections might look like and how it might function. For more information on this work see the documentation for the Collection Achievements and Profiles System.

Possible Solutions for Detecting Item-Level Pages

Identifying Item Pages

The first problem is determining which pages in a crawl of a site are item-level pages rather than search or browse pages. Below are different solutions for being able to discover item-level pages and make the best use of them.

URL Template

The URLs for item-level pages often have a standard pattern on a single site. Knowing the pattern would allow a system to only use those crawled URLs which fit the pattern to be included in an search aggregation. For example, it would be simple to determine that URLs of this pattern are item-level:

While these are not:

We could do something like the following to create templates for those URLs which are about individual items:{identifier}{identifier}

These URL templates could be stored with a centralized Collection Profile or made available through metadata in the head of web pages. This is a simple, low-cost solution that could work for many digital collections sites.

This is a pattern inspired by OpenSearch.

Meta extension

A site which can add content to the head of the HTML on item pages could add a meta element with a particular name and a value specifying that the page is an item page. The HTML5 specification has a way to officially extend the values allowed for the name attribute to meta.

    <meta name="itempage" content="true">

The problem with this approach is that consumers would have to know to look for this particular extension.


The Sitemap Protocol is a simple standard to allow crawlers to see a list of the pages that a site suggests could be crawled. In most cases a site will only want to expose the most important pages. Item-level pages would be included, while search pages or pages with duplicate content could be excluded from a sitemap. These non-item-level pages could still could be crawled by search engines, but by excluding them from the sitemap makes it easier for a crawler to zero in on the item-level pages.

Many sites are already publishing sitemaps, making them discoverable through their robots.txt, and sending them to search engines. There are common tools for creating sitemaps based on existing sites.

An aggregation could specify that the sitemap provided to their crawler should only contain item-level URLs. Since this is something that many sites are already doing, it could be a low-cost way for many digital collections to give an indication as to what pages are item-level.

Extracting Meaning

Once item-level pages are identified, there are various ways to extract meaning from the pages or otherwise communicate the metadata.

One mechanism which has been available for a long time is the ability to add links to alternative representations of resources in the head of the HTML. For instance, a page which describes a book could advertise that an RIS formatted representation is available.

    <link rel="alternate" type="application/x-research-info-systems"
      href="/search?q=cartoons&format=ris" />

This communicates that there is an alternative representation of a particular type at the given URL. The metadata in the alternative representation could be retrieved, parsed, and indexed.

A problem with this approach is that the alternative representation could go out of sync with the public-facing HTML or the API could be neglected. Nevertheless this approach could be a bridge between a specialized API and a crawl-centered approach.

Meta head content

Digital collections may already make some of their structured data about digital objects available by exposing Dublin Core terms through meta elements in the head of the HTML.

  <meta name="DC.title" content="Title of Digital Object" />

The content of the meta elements could be supplemented with parsing and indexing of the visible content of the page.

Semantic HTML Markup

Semantic markup is a promising future direction for digital collections aggregators to be able to extract richer metadata out of the visible content on web pages.

Semantic markup is the use within HTML of additional, hidden markup and vocabularies to more fully express the meaning of the visible content of the page. Cultural heritage organizations often have detailed metadata about the digital and physical objects they curate. Digital collections metadata are often stored in relational databases with rich schema. When they currently expose their metadata through the Web, search engines for the most part get plain text and need to attempt to make sense of the page with natural language processing.

Semantic markup would allow cultural heritage organizations to expose their metadata in a way that preserves more of the intention and meaning. Rather than dumb-down the metadata to Dublin Core before being used by an aggregator, consumers of structured data could utilize something much closer to the intention of the producer. It also allows organizations to expose this structured data without having the burden of maintaining a separate, little used service. Since effort is already being put into the website, it allows organizations to leverage that representation of the data which they already make available.

Producers of digital collections only need to expose to the public one version of the metadata they have. Everyone wants to have web pages for their digital collections. The semantic HTML enables both human users and machines to access the same data and understand it. Relying on collections to have metadata gateways or APIs to expose alternative representations of their data can be burdensome. It adds greater maintenance costs for making the data available. Having more than one public representation of the data leads to the less public one becoming unmaintained and getting stale or out of sync.

Semantic markup is implemented through various syntaxes. Microformats and RDFa syntaxes have allowed for making embedded, structured data available on the Web through HTML for a while and have existing communities of practice. The syntaxes are sometimes difficult to implement. The newer standard HTML5 Microdata has learned from previous efforts and attempts to be simpler for page authors. RDFa Lite is a profile of RDFa which tries to simplify the RDFa syntax similar to Microdata.

These syntaxes are used in combination with a vocabulary to make meaning. In 2011 the big search engines announced the vocabulary and support for Microdata. When the search engines make an announcement it gets the attention of webmasters. This means that many webmasters will be applying Microdata and The tools and ecosystem will be growing. Already many content management systems are providing a way to implement this kind of markup.

There are simple to complex ways in which semantic markup could be implemented to better support aggregations through crawling. For instance, it is simple in Microdata using to specify that a particular page is an item-level page. Even this amount of information would allow an aggregator to sort item-level pages from others. Layered on top of that, digital collections sites could embed a whole host of other types of data. This structured data could be extracted to create new tools and more powerful search interfaces.

Utilizing semantic markup would allow aggregators, search engines, and others to do more interesting things with the pages of resources that collections already make accessible. This broad benefit for collection producers could encourage more involvement from within and outside the cultural heritage community. So while benefiting digital collections through the aggregation this would also benefit them on the open Web.

Semantic HTML markup and relevant vocabularies are also areas where cultural heritage organizations could still have input on the web. While the predominant vocabularies at use now meet the needs of e-commerce, they could be improved and expanded to better fit cultural heritage materials. Using a common, standard, widely-accepted, and widely-consumed web vocabulary for description of cultural heritage materials would have multiplier effects for these collections. Aggregators could be a strong force for spurring creation of web vocabularies which better fit our data models and the use cases we want to enable. Through using common web standards, aggregators could also encourage consumption of this data at scale in ways that have until now been impossible.


Creating aggregations through crawling can lessen the burden on institutions that want to make their collections more discoverable. With just a little more effort, the crawler approach to aggregations can still be narrowed to item-level resources. Collections can implement semantic markup to make structured data available on their pages, then rich semantic data can be extracted from pages for more powerful aggregations. All of these web-centered approaches to the item-level problem benefit digital collections broadly on the web as well as aggregators, search engines, and developers.

At this point an aggregator may need to use some of the existing infrastructure distributed digital collections have in place for exposing item-level resources, but the nudge of standards ought to be in the direction of doing this in the future in this more Web-centric way. There are possibilities for merging the HTML and alternative (e.g. XML) versions for indexing. While a current implementation of an aggregation is likely to use niche protocols like OAI-PMH to include some collections, these approaches should be deprecated.

Please let me know if you know of other and better ways aggregators could identify item-level pages and extract meaning from those pages. I would also be interested in known aggregators that use these approaches.

This text is copyright Jason Ronallo and is licensed under a Creative Commons Attribution 3.0 Unported License.