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21 Www Web Services 

21 Www Web Services

 

 
 
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Published:  April 24, 2010
 
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Slide 1: CS 194: Distributed Systems WWW and Web Services Scott Shenker and Ion Stoica Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776
Slide 2: The Web – History (I)  1945: Vannevar Bush, Memex: "a device in which an individual stores all his books, records, and communications, and which is mechanized so that it may be consulted with exceeding speed and flexibility"  Vannevar Bush (1890-1974) (See http://www.iath.virginia.edu/elab/hfl0051.html) Memex 2
Slide 3: The Web – History (II)  1967, Ted Nelson, Xanadu: - A world-wide publishing network that would allow information to be stored not as separate files but as connected literature - Owners of documents would be automatically paid via electronic means for the virtual copying of their documents  Ted Nelson Coined the term “Hypertext” 3
Slide 4: The Web – History (III)  World Wide Web (WWW): a distributed database of “pages” linked through Hypertext Transport Protocol (HTTP) - First HTTP implementation - 1990 • Tim Berners-Lee at CERN - HTTP/0.9 – 1991 • Simple GET command for the Web - HTTP/1.0 –1992 • Client/Server information, simple caching - HTTP/1.1 - 1996 Tim Berners-Lee 4
Slide 5: The Web  Core components: - Servers: store files and execute remote commands - Browsers: retrieve and display “pages” - Uniform Resource Locators (URLs): way to refer to pages  A protocol to transfer information between clients and servers - HTTP 5
Slide 6: Uniform Record Locator (URL) protocol://host-name:port/directory-path/resource  Extend the idea of hierarchical namespaces to include anything in a file system ftp://www.cs.berkeley.edu/~istoica/cs194/05/lecture.ppt  Extend to program executions as well… - http://us.f413.mail.yahoo.com/ym/ShowLetter?box=%40B%40Bulk&MsgId=26 Server side processing can be incorporated in the name 6
Slide 7: Web and DNS  URLs use hostnames Thus, content names are tied to specific hosts This is bad! Uniform Resource Names (URNs) are one proposal to achieve persistence - Not discussed in this lecture    7
Slide 8: Hyper Text Transfer Protocol (HTTP)  Client-server architecture Synchronous request/reply protocol - Runs over TCP, Port 80   Stateless 8
Slide 9: Big Picture Client Establish connection Client request TCP Syn TCP syn + ack Server TCP a ck + H T TP G ET Request response Close connection . . . 9
Slide 10: Hyper Text Transfer Protocol Commands      GET – transfer resource from given URL HEAD – GET resource metadata (headers) only PUT – store/modify resource under given URL DELETE – remove resource POST – provide input for a process identified by the given URL (usually used to post CGI parameters) 10
Slide 11: Response Codes      1x informational 2x success 3x redirection 4x client error in request 5x server error; can’t satisfy the request 11
Slide 12: Client Request  Steps to get the resource: http://www.eecs.berkeley.edu/index.html 1. Use DNS to obtain the IP address of www.eecs.berkeley.edu 1. Send to an HTTP request: GET /index.html HTTP/1.0 12
Slide 13: Server Response HTTP/1.0 200 OK Content-Type: text/html Content-Length: 1234 Last-Modified: Mon, 19 Nov 2001 15:31:20 GMT <HTML> <HEAD> <TITLE>EECS Home Page</TITLE> </HEAD> … </BODY> </HTML> 13
Slide 14: HTTP/1.0 Example Client Request imag e1 ge 1 ransfer ima T Server Request imag e 2 age 2 Transfer im Request text Transfer text Finish display page 14
Slide 15: HHTP/1.0 Performance  Create a new TCP connection for each resource - Large number of embedded objects in a web page - Many short lived connections  TCP transfer - Too slow for small object - May never exit slow-start phase  Connections may be set up in parallel (5 is default in most browsers) 15
Slide 16: HTTP/1.0 Caching Support   Exploit locality of reference A modifier to the GET request: - If-modified-since – return a “not modified” response if resource was not modified since specified time  A response header: - Expires – specify to the client for how long it is safe to cache the resource  A request directive: - No-cache – ignore all caches and get resource directly from server  These features can be best taken advantage of with HTTP proxies - Locality of reference increases if many clients share a proxy 16
Slide 17: HTTP/1.1 (1996)  Performance: - Persistent connections - Pipelined requests/responses -…  Efficient caching support - Network Cache assumed more explicitly in the design - Gives more control to the server on how it wants data cached  Support for virtual hosting - Allows to run multiple web servers on the same machine 17
Slide 18: Persistent Connections  Allow multiple transfers over one connection Avoid multiple TCP connection setups Avoid multiple TCP slow starts   18
Slide 19: Pipelined Requests/Responses  Buffer requests and responses to reduce the number of packets Multiple requests can be contained in one TCP segment Note: order of responses has to be maintained Client Reques t1 Reques t2 Reques t3 Server   r1 Transfe r2 Transfe r3 Transfe 19
Slide 20: Caching and Replication  Problem: You are a web content provider - How do you handle millions of web clients? - How do you ensure that all clients experience good performance? - How do you maintain availability in the presence of server and network failures?  Solutions: - Add more servers at different locations  If you are CNN this might work! - Caching - Content Distribution Networks (Replication) 20
Slide 21: “Base-line”  Many clients transfer same information - Generate unnecessary server and network load - Clients experience unnecessary latency Server Backbone ISP ISP-1 ISP-2 Clients 21
Slide 22: Reverse Caches   Cache documents close to server  decrease server load Typically done by content providers Server Reverse caches Backbone ISP ISP-1 ISP-2 Clients 22
Slide 23: Forward Proxies   Cache documents close to clients  reduce network traffic and decrease latency Typically done by ISPs or corporate LANs Server Reverse caches Backbone ISP ISP-1 Forward caches ISP-2 Clients 23
Slide 24: Content Distribution Networks (CDNs)  Integrate forward and reverse caching functionalities into one overlay network (usually) administrated by one entity - Example: Akamai  Documents are cached both - As a result of clients’ requests (pull) - Pushed in the expectation of a high access rate  Beside caching do processing, e.g., - Handle dynamic web pages - Transcoding 24
Slide 25: CDNs (cont’d) Server CDN Backbone ISP ISP-1 Forward caches ISP-2 Clients 25
Slide 26: Example: Akamai  Akamai creates new domain names for each client content provider. - e.g., a128.g.akamai.net   The CDN’s DNS servers are authoritative for the new domains The client content provider modifies its content so that embedded URLs reference the new domains. - “Akamaize” content, e.g.: http://www.cnn.com/image-of-theday.gif becomes http://a128.g.akamai.net/image-of-the-day.gif. 26
Slide 27: Example: Akamai www.nhc.noaa.gov “Akamaizes” its content. akamai.net DNS servers a lookup a128.g.akamai.net DNS server for nhc.noaa.gov Akamai servers store/cache secondary content for “Akamaized” services. b get http://www.nhc.noaa.gov local DNS server c “Akamaized” response object has inline URLs for secondary content at a128.g.akamai.net and other Akamai-managed DNS names. 27
Slide 28: Core Web Technologies    HTML CGI XML 28
Slide 29: What is HTML?  HTML is the lingua franca for web publishing. Hyper Text Markup Language is based on SGML (Standard Generalized Markup Language) - HTML 4.0: http://www.w3.org/TR/html4/intro/intro.html   Initial version invented by Tim Berners-Lee Originally developed for sharing scientific documents on the web 29 
Slide 30: What is HTML?  HTML documents are plain text files Contain text and HTML mark-up tags Markup tags describe elements representing the style and structure of the visual document   30
Slide 31: Markup Tags   An HTML element may include a name, some attributes and some text or hypertext, and will appear in an HTML document as <tagName> text </tagName> <tagName attribute=argument> text </tagName>, or just <tagName> Examples: <title> My Document </title> <a href=http://www.cs.berkeley.edu/>Berkeley CS Web page<a> 31
Slide 32: A trivial HTML document Nesting structure <HTML> <HEAD> <TITLE> My web page </TITLE> </HEAD> <BODY> Welcome to my webpage! This is on the same line. </BODY> </HTML> HTML HEAD TITLE My web page BODY Welcome to my webpage! This is on the same line. 32
Slide 33: Common Gateway Interface (CGI)  CGI – general standard specifying how programs can be run on server, from the WWW Any program in any language can be a CGI program it just has to follow the CGI rules These rules define how programs get data (e.g., HTML form data) and how to make sure web server knows it’s a CGI program Call of a CGI program (like any HTML page): <a href=“http://www.mysite/cgi-bin/myprog”> Run my CGI program </a>    33
Slide 34: Client-Server CGI Architecture 34
Slide 35: CGI Examples  Any programming language can be used for CGI (e.g., shell script) Every CGI program must write out data to send back to web browser. The first thing they must write out is MIME type of file (e.g., text/plain, text/html) #!/bin/sh echo “Content-type: text/plain” echo echo “Hello World” 35  
Slide 36: CGI and Forms  CGI programs can process data from forms: <form method="get" action="http://www.foo.org/cgi-bin/cgiwrap/example.cgi"> <p> Name: <input type="text" name="username" /> </p> <p> Age: <input type="text" name="age" /> </p> <p> <input type="submit" value="Do it" /> </p> </form>  If method=“get” then the form data gets put in variable QUERY_STRING available to CGI programs 36
Slide 37: GET vs POST  Using “get” method: - Data added to URL as ..prog?var=val etc. - This data is put in QUERY_STRING variable available to CGI programs - E.g.: http://us.f413.mail.yahoo.com/ym/ShowLetter?box=%40B Nhead=f&YY=31454&order=down&sort=date&pos =0&view=a&head=b  Alternative is to use “post” method: - Data is sent separately to URL. - CGI program reads this data from its standard input. 37
Slide 38: CGI Security  CGI programs let anyone in the world run a program on your system Special wrapper programs may be used to do some security checks  38
Slide 39: XML: eXtensible Markup Language  A simple, very flexible text format derived from SGML  Rapidly emerging as the language of choice for data sharing on the Internet 39
Slide 40: XML Example  (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) An XML definition for referring to a journal article. <!ELEMENT article (title, author+,journal)> <!ELEMENT title (#PCDATA)> <!ELEMENT author (name, affiliation?)> <!ELEMENT name (#PCDATA)> <!ELEMENT affiliation (#PCDATA)> <!ELEMENT journal (jname, volume, number?, month? pages, year)> <!ELEMENT jname (#PCDATA)> <!ELEMENT volume (#PCDATA)> <!ELEMENT number (#PCDATA)> <!ELEMENT month (#PCDATA)> <!ELEMENT pages (#PCDATA)> <!ELEMENT year (#PCDATA)> 40
Slide 41: XML Example (cont’d)  XML document using XML definitions from previous slide (1) <?xml = version "1.0"> (2) <!DOCTYPE article SYSTEM "article.dtd"> (3) <article> (4) <title> Prudent Engineering Practice for Cryptographic Protocols</title> (5) <author><name>M. Abadi</name></author> (6) <author><name>R. Needham</name></author> (7) <journal> (8) <jname>IEEE Transactions on Software Engineering</jname> (9) <volume>22</volume> (10) <number>12</number> (11) <month>January</month> (12) <pages>6 – 15</pages> (13) <year>1996</year> (14) </journal> (15) </article> 41
Slide 42: XML vs HTML?  HTML combines structure and display, while XML separates them - HTML – presentation markup language: it describes the look, feel, and actions of web pages - XML describes document structure: what words in documents are  Flexibility: - HTML – only one standard definition of all of the tags - XML – custom documents defining the meaning of tags  XML may replace HTML in the future 42
Slide 43: Web Services • WS are applications that communicate using internet-based middleware WS are network-based software applications developed to interact with other applications using Internet standard technologies and connections to seamlessly perform business process • 43
Slide 44: Web Services Architecture Stacks www.w3c.org 44
Slide 45: WS Components 1. A standard way for communication (SOAP) A uniform data representation and exchange mechanism (XML) A standard meta language to describe the services offered (WSDL) A mechanism to register and locate WS based applications (UDDI) 1. 1. 1. 45
Slide 46: What is SOAP?  Lightweight protocol used for exchange of messages in a decentralized, distributed environment Platform-independent Used for Remote Procedure Calls W3C note defines the use of SOAP with XML as payload and HTTP as transport    46
Slide 47: SOAP Elements  Envelope (mandatory) - Top element of the XML document representing the message  Header (optional) - Determines how a recipient of a SOAP message should process the message - Adds features to the SOAP message such as authentication, transaction management, payment, message routes, etc…  Body (mandatory) - Exchanges information intended for the recipient of the message - Typical use is for RPC calls and error reporting 47
Slide 48: SOAP Elements    SOAP Encoding Envelope package Header/Body pattern - Similar to how HTTP works Header Body 48
Slide 49: Simple Example <Envelope> <Header> <transId>345</transId> </Header> <Body> <Add> <n1>3</n1> <n2>4</n2> </Add> </Body> </Envelope> 49 c = Add(n1, n2)
Slide 50: SOAP Request <SOAP-ENV:Envelope xmlns:SOAP-ENV=“http://schemas.xmlsoap.org/soap/envelope/” SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/”> <SOAP-ENV:Header> <t:transId xmlns:t=“http://a.com/trans”>345</t:transId> </SOAP-ENV:Header> <SOAP-ENV:Body> <m:Add xmlns:m=“http://a.com/Calculator”> <n1>3</n1> <n2>4</n2> </m:Add> </SOAP-ENV:Body> </SOAP-ENV:Envelope> 50
Slide 51: SOAP Request <SOAP-ENV:Envelope xmlns:SOAP-ENV=“http://schemas.xmlsoap.org/soap/envelope/” SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/”> <SOAP-ENV:Header> <t:transId xmlns:t=“http://a.com/trans”>345</t:transId> </SOAP-ENV:Header> <SOAP-ENV:Body> Scopes the message to the SOAP namespace describing the SOAP envelope <m:Add xmlns:m=“http://a.com/Calculator”> Establishes the type <n1>3</n1> <n2>4</n2> </m:Add> </SOAP-ENV:Body> </SOAP-ENV:Envelope> of encoding that is used within the message (different data types supported) 51
Slide 52: SOAP Request <SOAP-ENV:Envelope xmlns:SOAP-ENV=“http://schemas.xmlsoap.org/soap/envelope/” SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/”> <SOAP-ENV:Header> Qualifies transaction Id <t:transId xmlns:t=“http://a.com/trans”>345</t:transId> </SOAP-ENV:Header> <SOAP-ENV:Body> Defines the method <m:Add xmlns:m=“http://a.com/Calculator”> <n1>3</n1> <n2>4</n2> </m:Add> </SOAP-ENV:Body> </SOAP-ENV:Envelope> 52
Slide 53: SOAP Response <SOAP-ENV:Envelope xmlns:SOAP-ENV=“http://schemas.xmlsoap.org/soap/envelope/” <SOAP-ENV:Header> <t:transId xmlns:t=“http://a.com/trans”>345</t:transId> </SOAP-ENV:Header> <SOAP-ENV:Body> <m:AddResponse xmlns:m=“http://a.com/Calculator”> <result>7</result> </m:AddResponse> </SOAP-ENV:Body> </SOAP-ENV:Envelope> SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding Response typically uses method name with “Response” appended 53
Slide 54: XML-RPC vs SOAP  XML-RPC: lower common denominator form of communication - Simple, easy to understand (only 7 pages specification)  SOAP: can transfer more sophisticated information (could define virtually any data structure) - Flexible, but complex - Supported by industry 54
Slide 55: WSDL  Web Services Description Language is an XML document Describes WS functionality How WS communicate & where it is accessible (What, Where & How)   55
Slide 56: UDDI  Universal Description Definition Interface A standard discovery mechanism for WS Users can query a UDDI registry (company name, service type, Industry category or other criteria) Provides pointers to WSDL document UDDI is also based on XML 56    

   
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