Naming in Distributed Web-based Systems Madhuri Siddula

Outline Introduction Naming: URI Two forms of URI Syntax of URI Examples of URI Other uses of URI References

Introduction The World Wide Web (WWW) is a huge client-server system with millions of servers. Each server hosting thousands of documents. A document is fetched from a server, transferred to a client, and presented on the screen. Now, web has become more than just a simple document based system. With the emergence of web services, it is becoming a system of distributed services rather than just documents offered to any user or machine.

Naming The Web uses a single naming system to refer to documents. The names used are called Uniform Resource Identifiers (URIs). According to Berners-Lee et al., 2005, URI is the project implemented to make this world possible despite many different protocols.

Two forms of URI URL URI URN

Uniform Resource Locator (URL) URL is a URI that identifies a document by including information on how and where to access the document. The DNS name of its associated server along with a file name is specified. Example: https://grid.cs.gsu.edu/~cscyqz/courses/aos/student- lecture/student-class-lecture-fall2017.pdf

Uniform Resource Name (URN) A URN is a URI that identifies a resource by name in a particular namespace. In contrast to URL, URN acts as true identifier. A URN is used as a globally unique, location-independent, and persistent reference to a document.

URL Vs URN A Uniform Resource Name (URN) may be compared to a person's name, while a Uniform Resource Locator (URL) may be compared to their street address. In other words, a URN identifies an item and a URL provides a method for finding it.

Syntax of URI A generic URI is of the form: scheme:[//[user[:password]@]host[:port]][/path][?query][#fragment] Scheme: consisting of a sequence of characters beginning with a letter and followed by any combination of letters. It is also followed by a colon (:). Examples are http, ftp, mailto and so on.

Syntax of URI

Syntax of URI Location information for a document is embedded in a URL by means of the DNS name of the server to which an access request can be sent, although an IP address can also be used. The number of the port on which the server will be listening for such requests is also part of the URL. When port number is left out, a default port is used.

Resolving an URL If the server is referred to by its DNS name, that name will need to be resolved to the server's IP address. Using the port number contained in the URL, the client can then contact the server using the protocol named by the scheme, and pass it the document's name that forms the last part of the URL.

Examples of URI

Other uses of URI URIs are often used as well for purposes other than referring to a document. For example, a telnet URI is used for setting up a telnet session to a server. There are also URIs for telephone-based communication . The tel URI as shown in previous figure essentially embeds only a telephone number and simply lets the client to establish a call across the telephone network. In this case, the client will typically be a telephone. The modem URI can be used to set up a modem-based connection with another computer. In the example, the URI states that the remote modem should adhere to the ITU-T V32 standard.

References Tim Berners-Lee, Robert Cailliau, Ari Luotonen, Henrik Frystyk Nielsen, and Arthur Secret. 1994. The World-Wide Web. Commun. ACM 37, 8 (August 1994), 76-82. Andrew S. Tanenbaum , and Maarten Van Steen. Distributed Systems Principles and paradigms. Second Edition, 2007.

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