Distributed Web Based Systems - Distributed Operating Systems - Lecture Slides, Slides of Operating Systems

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Distributed Web-Based

Systems

INTRODUCTION

• What is World Wide Web?

INTRODUCTION

 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.  What can we get from WWW?  Read news, listen to music and watch video;  Buy or sell goods such as books, airline tickets;  Make reservations on hotel room, rental car, restaurant, etc.;  Pay bills and transfer money from one bank account to another;  …

TRADITIONAL WEB-BASED SYSTEMS

 Many Web-based systems are still organized as simple client-server architectures.

TRADITIONAL WEB-BASED SYSTEMS

 How to refer to a document?  URL ( Uniform Resource Locator )?

Uniform Resource Locator

 A reference called Uniform Resource Locator (URL) is used to refer a document.  The DNS name of its associated server along with a file name is specified.  The URL also specifies the protocol for transferring the document across the network.  Example: http://www.cse.unl.edu/~ylu/csce855/notes/web- system.ppt

WEB DOCUMENTS

 A Web document does not only contain text, but it can include all kinds of dynamic features such as audio, video, animations, etc.  In many cases special helper applications (interpreters) are needed, and they are integrated into the browser.  E.g., Windows Media Player and QuickTime Player for playing streaming content  The variety of document types forces browser to be extensible. As a result, plug-ins are required to follow a standard interfaces so that they can be easily integrated with the browsers.

MULTITIERED ARCHITECTURES

 Web documents can be built in two ways: Static – locates and returns the object identified in the request. Static objects include predefined HTML pages and JPEG or GIF files. does not require web servers to communication with any server-side application. Dynamic – the request is forwarded to an application system where the reply is generated dynamically, i.e. data is generated through a server-side program execution.  Although Web started as simple two-tiered client-server architecture for static Web documents, this architecture has been extended to support advanced type of documents.

• What is the most popular Web server software?
  • By far the most popular Web server is Apache. As of March 2007, 58% of all websites are using it.

• How to make a web site scalable?

WEB SERVER CLUSTERS

 The front end can be designed in two ways: Transport-layer switch – simply passes data sent along the TCP connection to one of the servers, depending on some measurement of the server’s load. Content-aware request distribution – it first inspects the HTTP request and decides which server it should forward that request to. For example, if the front end always forwards requests for the same document to the same server, the server may cache the document resulting in better response times.  Approach that combines the efficiency of transport-layer switch and the functionality of content-aware distribution has been developed.

WEB SERVER CLUSTERS

 Another alternative to set up a Web server cluster is to use round-robin DNS.  With round-robin DNS a single domain name is associated with multiple IP addresses.  When resolving a host name, a browser would receive a list of multiple addresses, each address corresponding to a server.  Normally, browsers choose the first address on the list, but most DNS servers circulate the entries.  As a result, simple distribution of requests over the servers in the cluster is achieved.

HTTP CONNECTIONS

 A Web document is constructed from a collection of different files from the same server.  In HTTP version 1.0 and older, each request to a server required setting up a separate connection. When server had responded, the connection was broken down. These connections are referred as nonpersistent.  In HTTP version 1.1, several requests and their responses can be issued without the need for a separate connection. These connections are referred as persistent.  Furthermore, a client can issue several requests in a row without waiting for the response to the first request which is referred as pipelining.

HTTP CONNECTIONS

(a) Using non-persistent connections. (b) Using persistent connections.