Cornell CS 502 Identifiers and Types CS 502 – Carl Lagoze – Cornell University

Cornell CS 502 Identity Change Persistence Paradox: reality contains things that persist and change over time –Heraclitus and Plato: can you step into the same river twice? –Ship of Theseus: over the years, the Athenians replaced each plank in the original ship of Theseus as it decayed, thereby keeping it in good repair. Eventually, there was not a single plank left of the original ship. So, did the Athenians still have one and the same ship that used to belong to Theseus

Cornell CS 502 Identity Change Persistence

Cornell CS 502 Identifiers Provide a key or handle linking abstract concepts to physical or perceptible entities Provide us with a necessary figment of persistence They are perhaps the one essential and common form of metadata Why bother? –Finding things –Referring to things –Asserting ownership over things

Cornell CS 502 I have lots of identifiers Carl Jay Lagoze, Dad, Hey you (SSN) (Visa Card) FZBMLH (US Airways locator on Jan 31 flight to San Diego)

Cornell CS 502 Identifier Issues Location independence Global uniqueness Persistent across time Human vs. machine generation Machine resolution Administration (centralized vs. decentralized) Intrinsic semantics Type specific

Cornell CS 502 Two common pre-digital identifiers ISBN (International Standard Book Number) –Uniquely identifies every monograph (book) –One ISBN for each format HP & SS hardback HP & SS softcover X –Number is semantically meaningful (components) –International administration (>150 countries) ISSN (International Standard Serial Number) –Uniquely identifies every serial (not issue or volume) –Semantically meaningless –International administration

Cornell CS 502 URI: Universal Resource Identifier Generic syntax for identifiers of resources Defined by RFC 2396RFC 2396 Syntax: :// ? –Scheme Defines semantics of remainder of URI ftp, gopher, http, mailto, news, telnet –Authority Authority governing namespace for remainder of URI Typically Internet-based server –Path Identification of data within scope of authority –Query String of information to be interpreted by authority

Cornell CS 502 Why is RFC 2396 so big? Character encodings Partial and relative URIs

Cornell CS 502 URL: Universal Resource Locator String representation of the location for a resource that is available via the Internet Use URI syntax Scheme has function of defining the access (protocol) method. Used by client to determine the protocol to “speak”. – - open socket to an.org on port 80 and issue a GET for index.html –ftp://an.org/index.html - open socket to an.org on port 21, open ftp session, issue ftp get for index.html….

Cornell CS 502 URL Issues Persistence Location dependence Valid only at the item level –What about works, expressions, manifestations Multiple resolution –“get the one that is cheapest, most reliable, most recent, most appropriate for my hardware, etc.” Non-digital resources? Disconnection from the entity

Cornell CS 502 URC – Uniform Resource Characteristic (Catalog) Failed but interesting effort –Multiple resolution –Describe resource by its characteristics Provide adequate bundled information about a resource (metadata) to create identification block for any given resource (including locations) –Exactly what are the common set of characteristics for describing different types of resources? –Where are these characteristics stored? Robust URLs – Berkeley –Characteristic of document or metadata is computed automatically via fingerprint of its content.

Cornell CS 502 URN – Universal Resource Name “globally unique, persistent names” Independence from location and location methods ::= "urn:" ":" NID : namespace identifier NSS : namespace-specific string examples: urn:ISSN: urn:isbn: urn:doi: /140

Cornell CS 502 Handles: Names for Internet Resources Naming system for location-independent, persistent names The resource named by a Handle can be: A library item A collection of library items A catalog record A computer An address A public key for encryption etc., etc., etc.....

Cornell CS 502 / or hdl: / Examples / ;9 (date-time stamp) cornell.cs/cstr (mnemonic name) loc/a43v-8940cgr(random string) Syntax of Handles

Cornell CS 502 Example of a Handle and its Data Used to Identify Two Locations URL loc.ndlp.amrlp/ Handle Data typeHandle data RAPloc/repository-1r4589

Cornell CS 502 Use of Handles in a Digital Library Repository Handle System Search System User interface

Cornell CS 502 Scalability and Caching Client Caching Server Handle Servers Hash Cache Hash table

Cornell CS 502 Replication for Performance and Reliability Example: the Global Handle System Washington, DC Los Angeles, CA

Cornell CS 502 Global and Local Handle Servers Global Local Handle Servers

Cornell CS 502 Ways to Resolve Handles I. Resolution by Program Any program can resolve Handles by sending standard format messages to the Handle System. A set of procedures, with Java and C versions, is available to link into applications programs. They are known as the Handle Client Library.

Cornell CS 502 Ways to Resolve Handles II. Web Browsers Browsers modified to recognize Handles. This requires installation of a Handle Extension. 1. Whenever the browser expects a URL, it will recognize "hdl:". 2. The Handle is passed to the Handle System, where it is resolved and a data item of type "URL" is returned. Handle Extensions for Netscape and Internet Explorer are available for most versions of Windows.

Cornell CS 502 Ways to Resolve Handles III. Proxies Any Web browser can resolve Handles, even with no extension, via a proxy. For example, the following URL can be used to resolve the Handle loc.ndlp.amrlp/3a16616:

Cornell CS 502 Proxy Resolution WWW browser HTTP server URL to Proxy URL Resource Handle System hdl.handle.net Proxy server

Cornell CS 502 OCLC's Persistent URL (PURL) A PURL is a URL -> Is fully compatible with today's Internet browsers -> Users need no special software Has some of the desirable features of URNs Lacks some desirable features of URNs -> Resolves only to a URL -> Does not support multiple resolution Developed by OCLC Software openly available

Cornell CS 502 PURL Syntax A PURL is a URL. PURL resolvers use standard http redirects to return the actual URL. protocolresolver addressname

Cornell CS 502 PURL Namespaces A PURL provides a local (not-global namespace) is different from

Cornell CS 502 OCLC PURL Resolution WWW browser PURL server HTTP server PURL database PURL URL Resource

Cornell CS 502 Why haven’t URNs caught on? Complexity of systems One size does not fit all - special purpose URN schemes have been successful, e.g., PubMed ID, Astrophysics BibCode No guarantee of persistence – longevity is an organizational not technical issue Requires well-regulated administrative systems Absence of “killing” applications – although reference linking is emerging

Cornell CS 502 Types: Not all data and content is the same Format or Genre –How you sense it –What you can do with it –E.G. – audio, video, map, book Type –What you need to process it –What is its bit layout Compression or encoding

Cornell CS 502 Multipurpose Internet Mail Extensions RFC 822 – define textual format of messages RFC – Extend textual to allow –Character sets other than US-ASCII –Extensible set of non-ASCII types for message bodies –Definition of multi-part mail (attachments)

Cornell CS 502 MIME Types Two part type hierarchy –Top level type text audio video image application multipart –Examples text/plain image/gif application/postscript Extensions are handled by IANA

Cornell CS 502 MIME in HTTP (Content Negotiation) Accept in request-header –Accept: text/plain; q=0.5, text/html, text/x-dvi; q=0.8, text/xml text/plain and text/xml are preferred, then text/x-dvi, then text/html Content-Type in response-header –Content-Type: text/html