Unit-5_3 PPT on Distributed Web based System.pdf

Distributed Files: Introduction, File System Architecture, Sun
Network File System and HDFS.
Distributed Multimedia Systems: Characteristics of Multimedia Data,
Quality of Service Management, Resource Management.
Distributed Web Based Systems: Architecture of Traditional Web-
Based Systems, Apache Web Server, Web Server Clusters,
Communication by Hypertext Transfer Protocol, Synchronization,
Web Proxy Caching.
Case Study: The Global Name Service, The X.500 Directory Service,
Bit Torrent.

1. PPT On Unit-5










Distributed Web-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




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 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







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

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 :

– 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




• 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
Browser GUI
Client
HTTP GET/POST Web WSDL
Server
SOAP
WSDL


Web
SOAP
WSDL

Server
WSDL

Server Web
JDBC
JDBC
DB
DB


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 ARCH ITECTURES


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.


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/IP Stack






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

Handshake

Cipher
Data


Spec








Record

Layer








TCP