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ArchitecturalOverview
The Library Simplified project consists of a small number of architectural components, joined together by a single protocol -- OPDS. This document lists the main components of the system and shows how they fit together.
The circulation manager mediates between a library's patrons, the library's ILS, and the many external vendors from which libraries license ebooks and audiobooks.
The circulation manager provides patrons with a single consolidated catalog which may contain electronic content from any number of vendors. When the patron expresses interest in a particular title, the circulation manager validates their credentials with the library's ILS. Once the credentials are validated, the circulation manager contacts the external vendor and negotiates the loan and delivery of the title to the patron.
The circulation manager hides the differences between library ILS systems and content vendors. A single circulation manager may manage the circulation for many libraries -- this is most useful for library consortia that have a shared collection.
We expect the Library Simplified ecosystem to contain hundreds or thousands of deployed circulation managers, each managing the collection for a relatively small (1-50) number of libraries.
We also expect the Library Simplified ecosystem to contain many software installations which authenticate patrons, negotiate loans, and deliver books, but which are not based on the circulation manager code base. An example is the Internet Archive's book server. The circulation manager software is less important than the architectural role it fulfills.
The library registry is a discovery mechanism for matching human beings to the libraries that serve them. People are matched to libraries primarily by geographic proximity, but they may also be matched to libraries that serve specific audiences, such as students, or people with print disabilities.
A properly configured circulation manager can automatically register itself with a library registry, through the OPDS Directory Registration Protocol.
We expect the Library Simplified ecosystem to contain only one library registry. We expect every library in the ecosystem to be registered with that registry. If there are multiple library registries, it becomes much more difficult for patrons to find 'their' libraries, since they would first have to find 'their' library registry.
The metadata wrangler exists to find the best available bibliographic metadata for every book in the world, and to share this metadata with circulation managers.
A properly configured circulation manager can be registered with a metadata wrangler. When a title is added to the circulation manager's collection, it will notify the metadata wrangler, and 'subscribe' to metadata updates for that title. Whenever the metadata wrangler finds improved metadata for that title, the circulation manager will hear about it and start using the new metadata.
We expect the Library Simplified ecosystem to contain a relatively small number of metadata wranglers, since bibliographic information about books can be shared across libraries. However, unlike with the library registry, nothing is lost if individual organizations decide to set up their own metadata wranglers -- it's just inefficient.
A patron of a library uses a client component to communicate with their library's circulation manager. Client components are generally designed for the use of library patrons. Patrons use client components to browse a library's catalog, to borrow books and audiobooks, and to read or listen to the books and audiobooks.
A client component may also communicate with the library registry. This would allow patrons to find their library from within the client, rather than having to locate their library first (e.g. by a web search) and finding out if that particular library has a client available.
The most commonly used client is SimplyE, a mobile application that communicateswith the library registry and with circulation managers. There is an iOS client and an Android client, both of which are called "SimplyE".
SimplyE is the preferred client for most purposes, because it's able to negotiate DRM licenses for ebooks and audiobooks. Someone using another client will only be able to consume media that is not encrypted with DRM.
The patron-facing web client is a web application that offers much of the functionality of SimplyE within a web browser. Patrons using the web client can browse a catalog, authenticate with their library's ILS, see their current loans, and borrow books and audiobooks. However, the patron-facing web client cannot negotiate DRM licenses for ebooks or audiobooks, so this client can only retrieve titles that are not encrypted with DRM.
Like the circulation manager, the patron-facing web client must be installed for a specific library. The patron-facing web client does not communicate with the library registry and does not have an interface to locate "your library".
Since circulation managers are OPDS servers (see below), to some extent you can use any OPDS client to access a circulation manager. This includes mobile clients such as Aldiko.
In practical terms, the most useful general OPDS client is the OPDS browser demo. By putting in the URL to any OPDS server, you can browse that server and conduct transactions from your web browser. Here's an example using the Internet Archive's book server.
Here's a simple architectural diagram showing how the user of a client (a library patron) gets a list of libraries from the library registry and a catalog of books from a circulation manager. It shows how bibliographic metadata flows from the metadata wrangler to the circulation manager, and how ebooks might flow from a distributor through the circulation manager to the patron.
+--------+ +------------------+
| Client |<--OPDS---| Library registry |
+--------+ +------------------+
^
|
| +---------------------+ +-------------------+
+--OPDS---| Circulation manager |<--OPDS---| Metadata wrangler |
+---------------------+ +-------------------+
^
| +-------------------+
+---OPDS---------------| Ebook distributor |
+-------------------+
Every communication in this diagram uses the same protocol: OPDS. OPDS is basically a general way of talking about books. OPDS can represent anything to do with books. This includes bibliographic metadata (like that produced by the metadata wrangler), catalogs (like those generated by the circulation manager, or links to the books themselves (like those provided by distributors).
There are two different versions of OPDS: OPDS 1.2, which is an XML-based format and the forthcoming OPDS 2.0, which is a JSON-based format. Both versions of OPDS serve the same purpose -- to convey information about books.
OPDS doesn't cover absolutely everything that Library Simplified needs to do. The OPDS community have collectively invented a lot of extensions to OPDS, like ODL, as well as totally separate protocols like the OPDS Directory Registration Protocol. But the core design principle of Library Simplified is that the components should communicate using OPDS as much as possible.