A DataSource is some external entity that puts data into the system. This data generally falls into two categories:

• Bibliographic information about a book, such as its title or cover image. This goes into the bibliographic metadata subsystem.
• Licensing information which can be used to serve actual copies of the book to library patrons. This goes into the licensing subsystem.

Some examples of DataSources:

• Overdrive, Bibliotheca, and Axis 360 license commercially published ebooks to libraries for delivery to patrons. They also provide bibliographic information about the books they license.
• Standard Ebooks provides bibliographic information about books, as well as free copies of the books themselves.
• OCLC and Content Cafe provide bibliographic information about books, but have no way of giving access to the actual books.
• VIAF provides information about the people who write books, but very little about the books themselves.
• The New York Times knows the ISBNs of the books on its best-seller lists, but not much more.

A DataSource may also:

• Provide many Editions
• Provide many Equivalencys
• Provide many Hyperlinks
• Provide many Resources
• Provide many Classifications
• Provide many CustomLists
• Grant access to many LicensePools
• Provide many LicensePoolDeliveryMechanisms
• Generate many CoverageRecords
• Have many associated Credentials
• Have one IntegrationClient

An Identifier provides a way to uniquely refer to a particular book. Common types of Identifier include ISBNs and proprietary IDs such as Overdrive or Bibliotheca IDs.

An Identifier may:

• Have many Classifications representing how the book would be shelved in a bookstore or library. (See the classification subsystem.)
• Have many Measurements of quantities like quality and popularity. (See the measurement subsystem.)
• Have many HyperLinks to associated files such as cover images or descriptions. (See the linked resources subsystem.)
• Participate in many Equivalencys.
• Serve as the primary_identifier for multiple Editions.
• Serve as the identifier for many LicensePools, through Collection.
• Be associated with one Work, through Edition

An Equivalency is an assertion made by a DataSource that two different Identifiers refer to the same book.

• The strength of the Equivalency is a number from -1 to 1 indicating how much we trust the assertion. When Overdrive says that an Overdrive ID is equivalent to an ISBN, we give that Equivalency a strength of 1, because Overdrive got the ISBN from the publisher and assigned the Overdrive ID itself. When OCLC says that two ISBNs represent the same book, we give it a lower strength, because OCLC is frequently wrong about this. A negative strength means that the DataSource is pretty sure two Identifiers represent differentbooks.

An Edition is a collection of information about a book from a particular data source. Like most items in the "bibliographic metadata" section, it represents an opinion. If different data sources give conflicting information about a book, that's fine -- everyone has their opinion. When this happens, we create multiple Editions and we sort it out later, when it's time to make the presentation edition.

An Edition:

• Has one DataSource. This is the data source whose opinions are recorded in the Edition.
• Has one Identifier, the primary_identifier. This identifies the book the data source is talking about.
• Contains basic metadata -- title, series, language, publisher, medium -- for that book.
• May have one or more Contributors, through Contribution.
• May be the presentation edition for a specific Work. The presentation edition is a synthetic Edition created by the system. We look over a bunch of Editions which are all (supposedly) talking about the same book, and consolidate it into a new Edition containing the best or most trusted metadata.