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Unit-5_3 PPT on Distributed Web based System.pdf
- 1. PPT On Unit-5 DistributedWeb-Based Systems
- 2. Outline • WWW • URL •Web Documents • HTTP – Connections – Methods – Messages – Caching • Content Distribution Network • Web Service – Terminology • Architecture – Traditional Web Based Systems – Multi-tiered Web Based Systems • Web Server Clusters • Web Security – SSL • References
- 3. World Wide Web Itis a wide distributed system with millions of clients and servers for accessing linked documents. Servers maintain collections of documents while clients provide users an easy- to-use interface for presenting and accessing those documents. A document is fetched from a server, transferred to a client, and presented on the screen. There is conceptually no difference between a documents stored locally or in another part of the world for any user. 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.
- 4. Uniform Resource Locator Areference 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. Example: http://www.example.sharif.edu/notes/WebBasedDistributedSystem.ppt
- 5. WEB DOCUMENTS A Webdocument 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. The main part of Web documents are written in a markup language, such as HyperText Markup Language (HTML) and eXtensible Markup Language (XML)
- 6. WEB DOCUMENTS HTML andXML can include tags that refer to embedded documents, which are references to other files. An embedded document can be a complete program executed on- the-fly as part of displaying information. Multipurpose Internet Mail Exchange (MIME) is used to specify the type of an embedded document. MIME was originally developed to provide information on the content of e-mail messages.
- 7. WEB DOCUMENTS Six top-levelMultipurpose Internet Mail Exchange types and some common subtypes.
- 8. HTTP All communication betweenthe clients and servers is based on the HTTP. Servers listen on port 80. HTTP is a simple protocol; a client sends a request to a server and waits for a response. HTTP is based on TCP; whenever a client issues a request to a server, it first sets up a TCP connection and sends the message on that connection. The same connection is used for receiving the response. One of the problems with the first versions of HTTP was its inefficient use of TCP connections. HTTP 1.0 vs. HTTP 1.1
- 9. HTTP CONNECTIONS A Webdocument 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 non-persistent. 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.
- 10. HTTP CONNECTIONS (a) Usingnon-persistent connections. (b) Using persistent connections.
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- 13. HTTP MESSAGES (Response) Statuscode (Phrase): 200 (OK), 400 (Bad Request), 403 (Forbidden), and 404 (Not Found).
- 14. HTTP MESSAGES (Response) Thereare also various message headers that the client can send to the server explaining what it is able to accept as a response
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- 16. HTTP Caching • Clientsoften cache documents – Challenge: update of documents – If-Modified-Since requests to check • HTTP 0.9/1.0 used just date • HTTP 1.1 has an opaque “entity tag” (could be a file signature, etc.) as well • When/how often should the original be checked for changes? – Check every time? – Check each session? Day? Etc? – Use “Expires” header • If no Expires, often use Last-Modified as estimate
- 17. Example Cache CheckRequest GET / HTTP/1.1 Accept: */* Accept-Language: en-us Accept-Encoding: gzip, deflate If-Modified-Since: Mon, 29 Jan 2001 17:54:18 GMT If-None-Match: "7a11f-10ed-3a75ae4a" User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0) Host: www.intel-iris.net Connection: Keep-Alive 17
- 18. Example Cache CheckResponse HTTP/1.1 304 Not Modified Date: Tue, 27 Mar 2001 03:50:51 GMT Server: Apache/1.3.14 (Unix) (Red-Hat/Linux) mod_ssl/2.7.1 OpenSSL/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24 Connection: Keep-Alive Keep-Alive: timeout=15, max=100 ETag: "7a11f-10ed-3a75ae4a" 18
- 19. Problems • Over 50%of all HTTP objects are un-cacheable . • Not easily solvable – Dynamic data : stock prices, scores, web cams – CGI scripts : results based on passed parameters – SSL : encrypted data is not cacheable • Most web clients don’t handle mixed pages well : many generic objects transferred with SSL – Cookies : results may be based on passed data – Hit metering : owner wants to measure # of hits for revenue, etc. 19
- 20. Server Selection • Lowestload : – to balance load on servers • Best performance : – to improve client performance • Any alive node : – to provide fault tolerance • How to direct clients to a specific server? – Cluster load balancing : TCP hand-off – As part of application : HTTP redirect – As part of naming : DNS 20
- 21. Application-Based Redirection • HTTPsupports simple way to indicate that Web page has moved (30X responses) • Server receives Get request from client – Decides which server is best suited for particular client and object – Returns HTTP redirect to that server • May introduce additional overhead : – multiple connection setup, name lookups, etc. 21
- 22. Naming Based • Clientdoes name lookup for service • Name server chooses appropriate server address – A record returned is “best” one for the client • Name server could base decision on – Server load/location must be collected – Information in the name lookup request • Name service client : – typically the local name server for client 22
- 23. Web Proxy Caches Proxy server client origin server client origin server •User configures browser: Web accesses via cache • Browser sends all HTTP requests to cache – Object in cache: cache returns object – Else cache requests object from origin server, then returns object to client 23
- 24. Content Distribution Networks (CDNs) •The content providers are the CDN customers. Content replication • CDN company installs hundreds of CDN servers throughout Internet – Close to users • CDN replicates its customers’ content in CDN servers. When provider updates content, CDN updates origin server in North America CDN distribution node CDN server CDN server In U.S.A CDN server in Asia in Europe servers 24
- 25. Content Distribution Networks •Replicate content on many servers The general organization of a CDN as a feedback-control system 25
- 26. Web Service • WebService: – “software that makes services available on a network using technologies such as XML and HTTP” • Service-Oriented Architecture (SOA): – “development of applications from distributed collections of smaller loosely coupled service providers” 26
- 27. Web Services Terminology •SOAP – Simple Object Access Protocol – exchanging XML messages on a network • WSDL – Web Service Description Language – describing interfaces of Web services • UDDI – Universal Description, Discovery and Integration – managing registries of Web services 27
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- 29. Why a NewFramework? • CORBA, DCOM, Java/RMI, ... already exist • XML+HTTP: platform/language neutral, widely accepted and utilized Web service interoperability 29
- 30. Servlets/CGI vs. WebServices Browser Browser GUI Client HTTP GET/POST Web WSDL Server SOAP WSDL Web SOAP WSDL Server W S D L Server Web JDBC JDBC DB DB 30
- 31. TRADITIONAL WEB-BASED SYSTEMS ManyWeb-based systems are still organized as simple client- server architectures. The core of a Web site: a process that has access to a local file system storing documents. A client interacts with Web servers through a special application known as browser. What’s the key function of a browser? Responsible for displaying documents. 31
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- 33. MULTITIERED ARCHITECTURES Web documentscan be built in two ways: Static locates and returns the object identified in the request. includes predefined HTML pages and JPEG or GIF files. Webservers do not require communication with any server-side application. Dynamic The request is forwarded to an application system where the resulting reply is generated dynamically. (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. 33
- 34. MULTITIERED ARCHITECTURES One ofthe first enhancements is Common Gateway Interface (CGI): user data comes from an HTML form, specifying the program and parameters. 34
- 35. MULTITIERED ARCHITECTURES Because ofthe server-side processing many Web sites are now organized as three-tiered architectures consisting of a Web server, an application server, and a database server. Server-side scripting technologies are used to generate dynamic content: Microsoft: Active Server Pages (ASP.NET) Sun: Java Server Pages (JSP) Netscape: JavaScript Free Software Foundation: PHP Most popular Web server software – Apache. As of March 2007, 58% of all websites are using it. 35
- 36. WEB SERVER CLUSTERS •Web servers are replicated and combined with a front end to improve performance. 36
- 37. WEB SERVER CLUSTERS Thefront end can be designed in two ways: Transport-layer switch simply passes data sent along the TCP connection to one of the server’s, 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. 37
- 38. WEB SERVER CLUSTERS Ascalable content-aware cluster of Web servers. 38
- 39. WEB SERVER CLUSTERS Anotheralternative to set up a Web Server Cluster is to use 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 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.
- 40. Web Security Issues •The Web has become the visible interface of the Internet Many corporations now use the Web for advertising, marketing and sales • Web servers might be easy to use but Complicated to configure correctly and difficult to build without security flaws They can serve as a security hole by which an adversary might be able to access other data and computer systems Threats Consequences Countermeasures Integrity Modification of Data Loss of Information MACs and Hashes Trojan horses Compromise of Machine Confidentiality Eavesdropping Loss of Information Encryption Theft of Information Privacy Breach DoS Stopping Stopped Transactions Filling up Disks and Resources Authentication ImpersonationData Misrepresentation of Signatures, MACs Forgery User Accept false Data
- 41. Secure the Web •There are many strategies to securing the web 1. We may attempt to secure the IP Layer of the TCP/IP Stack: This may be accomplished using IPSec, for example. 2. We may leave IP alone and secure on top of TCP: This may be accomplished using the Secure Sockets Layer (SSL) or Transport Layer Security (TLS) 3. We may seek to secure specific applications by using application- specific security solutions: For example, we may use Secure Electronic Transaction (SET) • The first two provide generic solutions, while the third provides for more specialized services
- 42. Securing the TCP/IPStack HTTP FTP SMTP TCP IP/IPSEC At the Network Level HTTP FTP SMTP SSL/TLS TCP IP At the Transport Level S/MIME PGP SET Kerberos SMTP HTTP UDP TCP IP At the Application Level
- 43. Secure Sockets Layer(SSL) • Originally developed (1994) by Netscape in order to secure http communications • Slight variation became Transport Layer Security (TLS) – Backward compatible with SSL • TCP provides a reliable end-to-end service • Consists of two sublayers: – SSL Record Protocol (where all the action takes place) – SSL Management (Handshake/Cipher Change/ Alert Protocols)
- 44. Application SSL TCP IP Protocol Structure Application Change Alert HandshakeCipher Data Spec Record Layer TCP