computer science

Persistent uniform resource locator

Persistent uniform resource locator is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Persistent uniform resource locator rather than just read about it. In short: A persistent uniform resource locator (PURL) is a uniform resource locator (URL, i.e. a uniform resource identifier, or URI, that is location-based) that is used to redirect to the location of the requested web resource. PURLs redirect HTTP clients using HTTP status codes.

Key takeaways

Persistent uniform resource locator belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Persistent uniform resource locator to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Persistent uniform resource locator from memory before moving on to harder problems.

Reference excerpt

A persistent uniform resource locator (PURL) is a uniform resource locator (URL, i.e. a uniform resource identifier, or URI, that is location-based) that is used to redirect to the location of the requested web resource. PURLs redirect HTTP clients using HTTP status codes. 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:

has a "root URL" as the resolver reference (e.g. http://myPurlResolver.example); provides means, to its user-community, to include new names in the root URL (e.g. http://myPurlResolver.example/name22); provides means to associate each name with its URL (to be redirected), and to update this redirection-URL; ensure the persistence (e.g. by contract) of the root URL and the PURL resolver services. PURLs are used to curate the URL resolution process, thus solving 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).

History The PURL concept was developed by Stuart Weibel and Erik Jul at OCLC in 1995. The PURL system was implemented using 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 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 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. 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 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 (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 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 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.

Principles of operation 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 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; 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 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 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 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 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.

Comparing with permalink Both permalink and PURL are used as permanent/persistent URL and redirect to the location of the requested web resource. Roughly speaking, they are the same. Their differences are about domain name and time scale:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Persistent uniform resource locator

Start with the simplest possible case. Write down what Persistent uniform resource locator claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Persistent uniform resource locator before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Persistent uniform resource locator ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Persistent uniform resource locator

In research: Persistent uniform resource locator appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Persistent uniform resource locator in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Persistent uniform resource locator is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1995 software, Cross-platform free software, Free software programmed in Java, so understanding it makes those chapters shorter.
In everyday life: Look for Persistent uniform resource locator outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Persistent uniform resource locator in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Persistent uniform resource locator means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Persistent uniform resource locator out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Persistent uniform resource locator in simple terms?

Why does Persistent uniform resource locator matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Persistent uniform resource locator?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Persistent uniform resource locator.