Searching
Most digital libraries provide a search interface which allows resources to be found. These resources are typically deep web (or invisible web) resources since they frequently cannot be located by search engine crawlers. Some digital libraries create special pages or sitemaps to allow search engines to find all their resources. Digital libraries frequently use the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) to expose their metadata to other digital libraries, and search engines like Google Scholar, Yahoo! and Scirus can also use OAI-PMH to find these deep web resources.
There are two general strategies for searching a federation of digital libraries:
- distributed searching, and
- searching previously harvested metadata.
Distributed searching typically involves a client sending multiple search requests in parallel to a number of servers in the federation. The results are gathered, duplicates are eliminated or clustered, and the remaining items are sorted and presented back to the client. Protocols like Z39.50 are frequently used in distributed searching. A benefit to this approach is that the resource-intensive tasks of indexing and storage are left to the respective servers in the federation. A drawback to this approach is that the search mechanism is limited by the different indexing and ranking capabilities of each database, making it difficult to assemble a combined result consisting of the most relevant found items.
Searching over previously harvested metadata involves searching a locally stored index of information that has previously been collected from the libraries in the federation. When a search is performed, the search mechanism does not need to make connections with the digital libraries it is searching - it already has a local representation of the information. This approach requires the creation of an indexing and harvesting mechanism which operates regularly, connecting to all the digital libraries and querying the whole collection in order to discover new and updated resources. OAI-PMH is frequently used by digital libraries for allowing metadata to be harvested. A benefit to this approach is that the search mechanism has full control over indexing and ranking algorithms, possibly allowing more consistent results. A drawback is that harvesting and indexing systems are more resource-intensive and therefore expensive.
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Famous quotes containing the word searching:
“Through searching out origins, one becomes a crab. The historian looks backwards, and finally he also believes backwards.”
—Friedrich Nietzsche (1844–1900)
“Our graves that hide us from the searching sun
Are like drawn curtains when the play is done.
Thus march we, playing, to our latest rest,
Only, we die in earnest—that’s no jest.”
—Sir Walter Raleigh (1552?–1618)
“I have been searching history to see if really a woman has any precedent to claim the right to have her rights, and I am compelled to say that we men are not so much ahead of women after all, and the only way we have kept our reputation up is by keeping her down—and don’t you forget it!”
—George E. Foster, U.S. women’s magazine contributor. The Woman’s Magazine, pp. 38-41 (October 1886)