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63-502: Originally, PURLs were recognizable for being hosted at purl.org or other hostnames containing purl . Early on many of those other hosts used descendants of the original OCLC PURL system software. Eventually, however, the PURL concept came to be generic and was used to designate any redirection service (named PURL resolver ) that: PURLs are used to curate the URL resolution process, thus solving

126-462: A namespace is an abstract domain to which a collection of element and attribute names can be assigned. The namespace name is a character string which must adhere to the generic URI syntax. However, the name is generally not considered to be a URI, because the URI specification bases the decision not only on lexical components, but also on their intended use. A namespace name does not necessarily imply any of

189-456: A "hypertext name" or "document name". Over the next three and a half years, as the World Wide Web's core technologies of HTML , HTTP , and web browsers developed, a need to distinguish a string that provided an address for a resource from a string that merely named a resource emerged. Although not yet formally defined, the term Uniform Resource Locator came to represent the former, and

252-475: A PURL service is independent of the publisher of information. PURL services thus allow the management of hyperlink integrity. Hyperlink integrity is a design trade-off of the World Wide Web, but may be partially restored by allowing resource users or third parties to influence where and how a URL resolves. A simple PURL works by responding to an HTTP GET request by returning a response of type 302 (equivalent to

315-518: A forked pre-1.0 release of Apache HTTP Server . The software was modernized and extended in 2007 by Zepheira under contract to OCLC and the official website moved to http://purlz.org (the 'Z' came from the Zepheira name and was used to differentiate the PURL open-source software site from the PURL resolver operated by OCLC). PURL version numbers may be considered confusing. OCLC released versions 1 and 2 of

378-495: A fragment identifier is already present in a target URL, any fragment in the original URL should be abandoned. Bos' suggestion failed to navigate the IETF standards track and expired without further work. Dubost et al. resurrected Bos' suggestions in a W3C Note (not a standard, but guidance in the absence of a standard). Makers of Web clients such as browsers have "generally" failed to follow Bos' guidance. Starting with PURLz 1.0 series,

441-420: A manner identical to a 301 or 302 redirection, with the difference that a PURL server will handle the redirection internally for greater efficiency. This efficiency is useful when many redirections are possible; since some Web browsers will stop following redirections once a set limit is encountered (in an attempt to avoid loops). A PURL of type "200" is an "Active PURL", in which the PURL actively participates in

504-454: A means of locating and retrieving information resources on a network (either on the Internet or on another private network, such as a computer filesystem or an Intranet ) are Uniform Resource Locators ( URLs ). Therefore, URLs are a subset of URIs, ie. every URL is a URI (and not necessarily the other way around). Other URIs provide only a unique name, without a means of locating or retrieving

567-571: A minor update to RFC 2396, allowing URIs to accommodate IPv6 addresses. A number of shortcomings discovered in the two specifications led to a community effort, coordinated by RFC 2396 co-author Roy Fielding , that culminated in the publication of IETF RFC   3986 in January 2005. While obsoleting the prior standard, it did not render the details of existing URL schemes obsolete; RFC 1738 continues to govern such schemes except where otherwise superseded. IETF RFC   2616 for example, refines

630-410: A network failure prevented it or because it did not exist. PURLs are themselves valid URLs, so their components must map to the URL specification. The scheme part tells a computer program, such as a Web browser, which protocol to use when resolving the address. The scheme used for PURLs is generally HTTP. The host part tells which PURL server to connect to. The next part, the PURL domain, is analogous to

693-462: A pointer to more specific information within a resource and are designated as following a # separator in URIs. Partial redirection in the presence of a fragment identifier is problematic because two conflicting interpretations are possible. If a fragment is attached to a PURL of type "partial", it is unclear whether a PURL service should assume that the fragment has meaning on the target URL, or discard it in

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756-588: A registered PURL. If so, a redirection occurs with the remainder of the requested URL appended to the target URL. For example, consider a PURL with a URL of http//purl.org/some/path/ with a target URL of http://example.com/another/path/. An attempt to perform an HTTP GET operation on the URL http//purl.org/some/path/and/some/more/data would result in a partial redirection to http://example.com/another/path/and/some/more/data. The concept of partial redirection allows hierarchies of Web-based resources to be addressed via PURLs without each resource requiring its own PURL. One PURL

819-408: A resource path in a URL. The domain is a hierarchical information space that separates PURLs and allows for PURLs to have different maintainers. One or more designated maintainers may administer each PURL domain. Finally, the PURL name is the name of the PURL itself. The domain and name together constitute the PURL's "id". Both permalink and PURL are used as permanent/persistent URL and redirect to

882-516: A scheme component followed by a colon ( : ). A path segment that contains a colon character (e.g., foo:bar ) cannot be used as the first path segment of a relative reference if its path component does not begin with a slash ( / ), as it would be mistaken for a scheme component. Such a path segment must be preceded by a dot path segment (e.g., ./foo:bar ). Web document markup languages frequently use URI references to point to other resources, such as external documents or specific portions of

945-487: A specific edition of the William Shakespeare play Romeo and Juliet . The URN for that edition would be urn:isbn:0-486-27557-4 . However, it gives no information as to where to find a copy of that book. A Uniform Resource Locator (URL) is a URI that specifies the means of acting upon or obtaining the representation of a resource, i.e. specifying both its primary access mechanism and network location. For example,

1008-403: A suitable replacement has not been identified. PURLs of type "clone" are used solely during PURL administration as a convenient method of copying an existing PURL record into a new PURL. The PURL service includes a concept known as partial redirection. If a request does not match a PURL exactly, the requested URL is checked to determine if some contiguous front portion of the PURL string matches

1071-415: A web browser and be taken to the identified source. Of course, the issue of persistent identification predates the Internet. Over centuries, writers and scholars developed standards for citation of paper-based documents so that readers could reliably and efficiently find a source that a writer mentioned in a footnote or bibliography. After the Internet started to become an important source of information in

1134-610: A webpage, mail address, phone number, books, real-world objects such as people and places, concepts. URIs are used to identify anything described using the Resource Description Framework (RDF), for example, concepts that are part of an ontology defined using the Web Ontology Language (OWL), and people who are described using the Friend of a Friend vocabulary would each have an individual URI. URIs which provide

1197-463: Is undefined if it has an associated delimiter and the delimiter does not appear in the URI; the scheme and path components are always defined. A component is empty if it has no characters; the scheme component is always non-empty. The authority component consists of subcomponents : This is represented in a syntax diagram as: [REDACTED] The URI comprises: The scheme- or implementation-specific reserved character + may be used in

1260-655: Is a superset of the syntax of all URI schemes. It was first defined in RFC   2396 , published in August 1998, and finalized in RFC   3986 , published in January 2005. A URI is composed from an allowed set of ASCII characters consisting of reserved characters (gen-delims: : , / , ? , # , [ , ] , and @ ; sub-delims: ! , $ , & , ' , ( , ) , * , + , , , ; , and = ), unreserved characters ( uppercase and lowercase letters , decimal digits , - , . , _ , and ~ ), and

1323-535: Is a technique by which a command is appended to a URL, usually at the end, after a "?" token . It is commonly used in WebDAV as a mechanism of adding functionality to HTTP . In a versioning system, for example, to add a "checkout" command to a URL, it is written as http://editing.com/resource/file.php?command=checkout . It has the advantage of both being easy for CGI parsers and also acts as an intermediary between HTTP and underlying resource, in this case. In XML ,

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1386-430: Is a useful but informal concept: a URL is a type of URI that identifies a resource via a representation of its primary access mechanism (e.g., its network "location"), rather than by some other attributes it may have. As such, a URL is simply a URI that happens to point to a resource over a network. However, in non-technical contexts and in software for the World Wide Web, the term "URL" remains widely used. Additionally,

1449-494: Is analogous to a person's name, while a URL is analogous to their street address. In other words, a URN identifies an item and a URL provides a method for finding it. Technical publications, especially standards produced by the IETF and by the W3C, normally reflect a view outlined in a W3C Recommendation of 30 July 2001, which acknowledges the precedence of the term URI rather than endorsing any formal subdivision into URL and URN. URL

1512-459: Is simply an address of a resource on the World Wide Web. A Persistent URL is an address on the World Wide Web that causes a redirection to another Web resource. If a Web resource changes location (and hence URL), a PURL pointing to it can be updated. A user of a PURL always uses the same Web address, even though the resource in question may have moved. PURLs may be used by publishers to manage their own information space or by Web users to manage theirs;

1575-502: Is sufficient to serve as a top-level node for a hierarchy on a single target server. The new PURL service uses the type "partial" to denote a PURL that performs partial redirection. Partial redirections at the level of a URL path do not violate common interpretations of the HTTP 1.1 specification. However, the handling of URL fragments across redirections has not been standardized and a consensus has not yet emerged. Fragment identifiers indicate

1638-451: Is to inform the Web client and end user that the PURL should always be used to address the requested resource, not the final URI resolved. This is to allow continued resolution of the resource if the PURL changes. Some operators prefer to use PURLs of type 301 (indicating that the final URI should be addressed in future requests). A PURL of type "chain" allows a PURL to redirect to another PURL in

1701-469: Is used to direct a Web client to a resource that provides additional information regarding the resource they requested, without returning the resource itself. This subtlety is useful when the HTTP URI requested is used as an identifier for a physical or conceptual object that cannot be represented as an information resource. PURLs of type 303 are used most often to redirect to metadata in a serialization format of

1764-557: The http scheme. Simultaneously, the IETF published the content of RFC 3986 as the full standard STD 66, reflecting the establishment of the URI generic syntax as an official Internet protocol. In 2001, the World Wide Web Consortium's (W3C) Technical Architecture Group (TAG) published a guide to best practices and canonical URIs for publishing multiple versions of a given resource. For example, content might differ by language or by size to adjust for capacity or settings of

1827-525: The RFC   1630 attempted to summarize the syntaxes of URL schemes in use at the time. It acknowledged -- but did not standardize —the existence of relative URLs and fragment identifiers. In December 1994, RFC   1738 formally defined relative and absolute URLs, refined the general URL syntax, defined how to resolve relative URLs to absolute form, and better enumerated the URL schemes then in use. The agreed definition and syntax of URNs had to wait until

1890-482: The Resource Description Framework (RDF) and have relevance for Semantic Web and linked data content. This use of the 303 HTTP status code is conformant with the http-range-14 finding of the Technical Architecture Group of the World Wide Web Consortium . A PURL of type "307" informs a user that the resource temporarily resides at a different URL from the norm. PURLs of types 404 and 410 note that

1953-462: The 1990s, the issue of citation standards became important in the online world as well. Studies have shown that within a few years of being cited, a significant percentage of web addresses go "dead", a process often called link rot . Using a persistent identifier can slow or stop this process. An important aspect of persistent identifiers is that "persistence is purely a matter of service". That means that persistent identifiers are only persistent to

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2016-701: The Apache-based source tree, initially in 1999 under the OCLC Research Public License 1.0 License and later under the OCLC Research Public License 2.0 License ( http://opensource.org/licenses/oclc2 ). Zepheira released PURLz 1.0 in 2007 under the Apache License, Version 2.0 . PURLz 2.0 was released in Beta testing in 2010 but the release was never finalized. The Callimachus Project implemented PURLs as of its 1.0 release in 2012. The oldest PURL HTTP resolver

2079-671: The HTTP status code 302, meaning "Found"). The response contains an HTTP "Location" header, the value of which is a URL that the client should subsequently retrieve via a new HTTP GET request. PURLs implement one form of persistent identifier for virtual resources. Other persistent identifier schemes include Digital Object Identifiers (DOIs), Life Sciences Identifiers (LSIDs) and INFO URIs . All persistent identification schemes provide unique identifiers for (possibly changing) virtual resources, but not all schemes provide curation opportunities. Curation of virtual resources has been defined as, "the active involvement of information professionals in

2142-499: The OCLC-hosted service for several months. The service hosted on Internet Archive servers supports access via purl.org , purl.net , purl.info , and purl.com . OCLC now redirects DNS requests for purl.oclc.org to purl.org . The PURL concept allows for generalized URL curation of HTTP URIs on the World Wide Web . PURLs allow third party control over both URL resolution and resource metadata provision. A URL

2205-430: The PURL service implements partial redirections inclusive of fragment identifiers by writing fragments onto target URLs in an attempt to comply with and avoid problematic and inconsistent behavior by browser vendors. URI scheme A Uniform Resource Identifier ( URI ), formerly Universal Resource Identifier , is a unique sequence of characters that identifies an abstract or physical resource, such as resources on

2268-614: The Semantic Web , which explained the use of content negotiation and the HTTP 303 response code for redirections in more detail. A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace. A URN may be used to talk about a resource without implying its location or how to access it. For example, in the International Standard Book Number (ISBN) system, ISBN 0-486-27557-4 identifies

2331-513: The URL http://example.org/wiki/Main_Page refers to a resource identified as /wiki/Main_Page , whose representation is obtainable via the Hypertext Transfer Protocol ( http: ) from a network host whose domain name is example.org . (In this case, HTTP usually implies it to be in the form of HTML and related code. In practice, that is not necessarily the case, as HTTP allows specifying arbitrary formats in its header.) A URN

2394-430: The allowed set or is being used as a delimiter of, or within, the component. A percent-encoding of an identifying data octet is a sequence of three characters, consisting of the character % followed by the two hexadecimal digits representing that octet's numeric value. The URI generic syntax consists of five components organized hierarchically in order of decreasing significance from left to right: A component

2457-431: The basis for namespace names in addition to URI references. Persistent identifier A persistent identifier ( PI or PID ) is a long-lasting reference to a document, file, web page, or other object. The term "persistent identifier" is usually used in the context of digital objects that are accessible over the Internet. Typically, such an identifier is not only persistent but actionable: you can plug it into

2520-407: The character % . Syntax components and subcomponents are separated by delimiters from the reserved characters (only from generic reserved characters for components) and define identifying data represented as unreserved characters, reserved characters that do not act as delimiters in the component and subcomponent respectively, and percent-encodings when the corresponding character is outside

2583-402: The context of an HTTP conversation but do not apply to the process of HTTP redirection. Three additional types of PURLs ("chain", "partial' and "clone") are given mnemonic names related to their functions. Most PURLs are so-called "simple PURLs", which provide a redirection to the desired resource. The HTTP status code, and hence of the PURL type, of a simple PURL is 302. The intent of a 302 PURL

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2646-509: The creation or aggregation of the metadata returned. An Active PURL includes some arbitrary computation to produce its output. Active PURLs have been implemented in PURLz 2.0 and The Callimachus Project . They may be used to gather runtime status reports, perform distributed queries or any other type of data collection where a persistent identifier is desired. Active PURLs act similar to a stored procedure in relational databases. A PURL of type "303"

2709-500: The debate over defining URLs and URNs, it became evident that the concepts embodied by the two terms were merely aspects of the fundamental, overarching, notion of resource identification . In June 1994, the IETF published Berners-Lee's first Request for Comments that acknowledged the existence of URLs and URNs. Most importantly, it defined a formal syntax for Universal Resource Identifiers (i.e. URL-like strings whose precise syntaxes and semantics depended on their schemes). In addition,

2772-527: The degree that someone commits to resolving them for users. No identifier can be inherently persistent, however many persistent identifiers are created within institutionally administered systems with the aim to maximise longevity. However, some regular URLs (i.e. web addresses), maintained by the website owner, are intended to be long-lasting; these are often called permalinks . People and organisations: Publications: Uniform Resource Identifiers : Combined persistent identifier and archiving functionality

2835-448: The device used to access that content. In August 2002, IETF RFC   3305 pointed out that the term "URL" had, despite widespread public use, faded into near obsolescence, and serves only as a reminder that some URIs act as addresses by having schemes implying network accessibility, regardless of any such actual use. As URI-based standards such as Resource Description Framework make evident, resource identification need not suggest

2898-422: The location of the requested web resource . Roughly speaking, they are the same. Their differences are about domain name and time scale : The most common types of PURLs are named to coincide with the HTTP response code that they return. Not all HTTP response codes have equivalent PURL types and not all PURL servers implement all PURL types. Some HTTP response codes (e.g. 401, Unauthorized) have clear meanings in

2961-472: The management, including the preservation, of digital data for future use." PURLs have been criticized for their need to resolve a URL, thus tying a PURL to a network location. Network locations have several vulnerabilities, such as Domain Name System registrations and host dependencies. A failure to resolve a PURL could lead to an ambiguous state: It would not be clear whether the PURL failed to resolve because

3024-493: The more contentious Uniform Resource Name came to represent the latter. In July 1992 Berners-Lee's report on the Internet Engineering Task Force (IETF) "UDI (Universal Document Identifiers) BOF " mentions URLs (as Uniform Resource Locators), URNs (originally, as Unique Resource Numbers), and the need to charter a new working group. In November 1992 the IETF "URI Working Group" met for the first time. During

3087-409: The old PURL-services and PURL-technologies. On 27 September 2016 OCLC announced a cooperation with Internet Archive resulting in the transfer of the resolver service and its administration interface to Internet Archive. The service is supported on newly created software, separate from all previous implementations. The transfer re-enabled the ability to manage PURL definitions that had been disabled in

3150-468: The path, query and fragment, and the generic reserved character ? may be used in the query and fragment. The following figure displays example URIs and their component parts. DOIs ( digital object identifiers ) fit within the Handle System and fit within the URI system, as facilitated by appropriate syntax . A URI reference is either a URI or a relative reference when it does not begin with

3213-414: The presumption that a resource with a changed location may have also changed content, thus invalidating fragments defined earlier. Bos suggested that fragments should be retained and passed through to target URLs during HTTP redirections resulting in 300 (Multiple Choice), 301 (Moved Permanently), 302 (Found) or 303 (See Other) responses unless a designated target URL already includes a fragment identifier. If

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3276-525: The problem of transitory URIs in location-based URI schemes like HTTP. Technically the string resolution on PURL is like SEF URL resolution . The remainder of this article is about the OCLC's PURL system, proposed and implemented by OCLC (the Online Computer Library Center). The PURL concept was developed by Stuart Weibel and Erik Jul at OCLC in 1995. The PURL system was implemented using

3339-514: The publication of IETF RFC   2141 in May 1997. The publication of IETF RFC   2396 in August 1998 saw the URI syntax become a separate specification and most of the parts of RFCs 1630 and 1738 relating to URIs and URLs in general were revised and expanded by the IETF. The new RFC changed the meaning of U in URI from "Universal" to "Uniform." In December 1999, RFC   2732 provided

3402-455: The requested resource could not be found and suggests some information for why that was so. Support for the HTTP 307 (Temporary Redirect), 404 (Not Found) and 410 (Gone) response codes are provided for completeness. PURLs of types "404" and "410" are provided to assist administrators in marking PURLs that require repair. PURLs of these types allow for more efficient indications of resource identification failure when target resources have moved and

3465-402: The resource or information about it; these are Uniform Resource Names (URNs). The web technologies that use URIs are not limited to web browsers . URIs and URLs have a shared history. In 1990, Tim Berners-Lee's proposals for hypertext implicitly introduced the idea of a URL as a short string representing a resource that is the target of a hyperlink . At the time, people referred to it as

3528-493: The retrieval of resource representations over the Internet, nor need they imply network-based resources at all. The Semantic Web uses the HTTP URI scheme to identify both documents and concepts for practical uses, a distinction which has caused confusion as to how to distinguish the two. The TAG published an e-mail in 2005 with a solution of the problem, which became known as the httpRange-14 resolution . The W3C subsequently published an Interest Group Note titled Cool URIs for

3591-406: The same logical document: Resolving a URI reference against a base URI results in a target URI . This implies that the base URI exists and is an absolute URI (a URI with no fragment component). The base URI can be obtained, in order of precedence, from: Within a representation with a well defined base URI of a relative reference is resolved to its target URI as follows: URL munging

3654-430: The scheme http is generally used for interacting with web resources using HTTP, but the scheme file has no protocol. A URI has a scheme that refers to a specification for assigning identifiers within that scheme. As such, the URI syntax is a federated and extensible naming system wherein each scheme's specification may further restrict the syntax and semantics of identifiers using that scheme. The URI generic syntax

3717-425: The scheme, userinfo, host, path, query, and fragment, and the scheme- or implementation-specific reserved characters ! , $ , & , ' , ( , ) , * , , , ; , and = may be used in the userinfo, host, path, query, and fragment. Additionally, the generic reserved character : may be used in the userinfo, path, query and fragment, the generic reserved characters @ and / may be used in

3780-465: The semantics of URI schemes; for example, a namespace name beginning with http: may have no connotation to the use of the HTTP . Originally, the namespace name could match the syntax of any non-empty URI reference, but the use of relative URI references was deprecated by the W3C. A separate W3C specification for namespaces in XML 1.1 permits Internationalized Resource Identifier (IRI) references to serve as

3843-480: The term "web address" (which has no formal definition) often occurs in non-technical publications as a synonym for a URI that uses the http or https schemes. Such assumptions can lead to confusion, for example, in the case of XML namespaces that have a visual similarity to resolvable URIs . Specifications produced by the WHATWG prefer URL over URI , and so newer HTML5 APIs use URL over URI . Standardize on

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3906-422: The term URL. URI and IRI [Internationalized Resource Identifier] are just confusing. In practice a single algorithm is used for both so keeping them distinct is not helping anyone. URL also easily wins the search result popularity contest. While most URI schemes were originally designed to be used with a particular protocol , and often have the same name, they are semantically different from protocols. For example,

3969-563: Was operated by OCLC from 1995 to September 2016 and was reached as purl.oclc.org as well as purl.org , purl.net , and purl.com . Other notable PURL resolvers include the US Government Printing Office ( http://purl.fdlp.gov ), which is operated for the Federal Depository Library Program and has been in operation since 1997. The PURL concept is used in the w3id.org , that may replace

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