Distributed web based systems

1. Distributed Web-Based Systems Department of Engineering – Information Technology Reza Ghanbari 2010

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 It is 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 document 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 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. Example: http://www.example.sharif.edu/notes/WebBasedDistributedSystem.ppt

5. 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. 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 and XML 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-level Multipurpose Internet Mail Exchange types and some common subtypes.

8. HTTP All communication between the 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 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 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) Using non - persistent connections. ( b ) Using persistent connections.

11. HTTP Operations

12. HTTP MESSAGES (Request)

13. HTTP MESSAGES (Response) Status code (Phrase): 200 (OK), 400 (Bad Request), 403 (Forbidden), and 404 (Not Found).

14. HTTP MESSAGES (Response) There are also various message headers that the client can send to the server explaining what it is able to accept as a response

15. HTTP MESSAGES (Response)

16. HTTP Caching Clients often 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 16

17. Example Cache Check Request 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 Check Response 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 Lowest load : 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 HTTP supports 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 Client does 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 : t ypically the local name server for client 22

23. Web Proxy Caches 23 client Proxy server client HTTP request HTTP request HTTP response HTTP response HTTP request HTTP response origin server 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

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 servers 24 origin server in North America CDN distribution node CDN server In U.S.A CDN server in Europe CDN server in Asia

25. Content Distribution Networks Replicate content on many servers 25 The general organization of a CDN as a feedback-control system

26. Web Service Web Service: “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

28. Web Services Framework 28

29. Why a New Framework? CORBA, DCOM, Java/RMI, ... already exist XML+HTTP: platform/language neutral, widely accepted and utilized  Web service interoperability 29

30. Servlets/CGI vs. Web Services Browser Web Server HTTP GET/POST DB JDBC Web Server DB JDBC Browser Web Server SOAP GUI Client SOAP WSDL WSDL WSDL WSDL 30

31. TRADITIONAL WEB-BASED SYSTEMS Many Web-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

32. TRADITIONAL WEB-BASED SYSTEMS 32

33. MULTITIERED ARCHITECTURES Web documents can be built in two ways: Static locates and returns the object identified in the request. includes predefined HTML pages and JPEG or GIF files. Web servers 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 of the first enhancements is Common Gateway Interface (CGI): user data comes from an HTML form, specifying the program and parameters. 34

35. MULTITIERED ARCHITECTURES Because of the 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 The front 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 A scalable content-aware cluster of Web servers. 38

39. WEB SERVER CLUSTERS Another alternative 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. 39

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 Trojan horses Loss of Information Compromise of Machine MACs and Hashes Confidentiality Eavesdropping Theft of Information Loss of Information Privacy Breach Encryption DoS Stopping Filling up Disks and Resources Stopped Transactions Authentication ImpersonationData Forgery Misrepresentation of User Accept false Data Signatures, MACs

41. Secure the Web There are many strategies to securing the web We may attempt to secure the IP Layer of the TCP/IP Stack: This may be accomplished using IPSec, for example. 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) 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 41

42. Securing the TCP/IP Stack TCP IP/IPSEC HTTP FTP SMTP TCP IP HTTP FTP SMTP SSL/TLS TCP IP S/MIME PGP UDP Kerberos SMTP SET HTTP At the Network Level At the Transport Level At the Application Level 42

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) 43

44. Protocol Structure 44

45. References Distributed Systems Principles and Paradigms, by Maarten van Steen, VU Amsterdam , [email protected] Web Service Composition - Current Solutions and Open Problems, by Biplav Srivastava -IBM India Research Laboratory and Jana Koehler-IBM Zurich Research Laboratory A Reference Architecture for Web Servers, by Ahmed E. Hassan and Richard C. Holt , Software Architecture Group (SWAG), University of Waterloo An Introduction to Web-based Support Systems, by JingTao Yao, University of Regina Semantic Annotation for Web Services and their elevance to Environmental Models, by DumitruRoman University of Innsbruck / STI Innsbruck 45