1. Web Technologies

2. Typical Web Usage 1. User interacts with graphical browser 2. Browser submits HTTP requests to server 2.1. Request relayed by proxy 3. Server returns HTTP reply 3.1. Proxy caches response 4. Browser displays HTML results

3. Components of Web Technology We are primarily interested in the parts that have implications for (reliable) distributed computing  (HTML)  XML  (URLs)  HTTP  Proxy Servers

4. Core Web Technologies HTML(HyperText Markup Language) Defines a standard set of special textual indicators(markups) specifying how a Web pages words and images should be displayed by the web browser

5. Technologies for Supporting Remote Clients Original intent of core Web Technologies  enable linking and sharing documents It was quickly realized, that by wrapping local information systems to expose their presentation layer by using HTML documents, one could leverage the core Web technologies to have clients that are distributed across the internet.

6. HTML HyperText Markup Language  Text format for publishing hypertexts on the World Wide Web Based on Standard Generalized Markup Language (SGML; ISO 8879) (as is XML) Created in 1991, HTML 2.0 in 1994 (60 pages), HTML 4.01 (> 350 pages) in 1997, now work on XHTML  Representation rather than presentation – sort of... HTML is not XML  E.g., : start tag required, end tag forbidden  XHTML: HTML in XML

7. XML Extensible Markup Language  Extensible XML is a framework for defining languages tailored to application domains  Markup XML documents are made up of entities Entity data contains intermingled character data or markup No fixed set of markup tags An example... Reference  http://www.w3.org/TR/2004/REC-xml- 20040204/ Klaus Marius Hansen Admitted 100 Aspirin 50 Ibuprofen element (end markup) tag character data attribute element name XML declaration

8. XML Well-Formedness and Validity Which patient documents are regarded as describing patients?  The valid ones  Have a reference to a document describing legal documents E.g., using XML Schemas  Fulfil the requirements in these  Are well-formed Well-formed patients...  Matches the ”document” production of the XML spec Including that start and end tags match and that element tags are properly nested  + other well-formedness constraints in the spec <p:patient id="301174-..." xmlns:p="http://ehr.org" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://ehr.org patient.xsd"> Klaus Marius Hansen 100 Aspirin 50 Ibuprofen Admitted Namespace Location of schema

9. (Altova XMLSpy syntax) Patient XML Schema Example

10. XML Schema Constructs Constructs  A complex type definition attribute declarations describe which attributes that may or must appear element references: describe which sub-elements that may or must appear, how many, and in which order  A simple type definition defines a set of strings to be used as attribute values or character data  A global element declaration associates element names with types (in the patient example, the complex type definition was inlined in the patient element declaration) Validity  An element is valid according to a given schema if associated element type rules are satisfied  A document is valid if all its elements are valid

11. Complex Types Attribute declarations  E.g., Content of one of the following content model kinds  Empty content  Simple content... Only character data  Regexp content... e.g., with

12. Namespaces XML languages are typically assigned to namespaces  XML Schema  Uses namespaces itself to distinguish XML Schema constructs from the language being defined  Allows namespace assignments of the language being defined

13. Other XML Technologies Namespaces Linking  XLink Addressing parts of documents  XPath Transformation  XSL Querying  XQuery RPC  WSDL and SOAP

14. XML is Bloated An XML encoding is large  What to do (in particular for RPC using XML)?  Undecided what actually to do in W3C... Compress/decompress XML?  Compression is expensive (often more than decompression) Assign gateway between XML and other formats?  E.g., using XML only for interoperable messaging  May introduce single point-of-failure Another approach  Sacrifice self-description  Create mapping to a more efficient format More efficient to serialize and deserialize and on the wire  Allow applications to choose between formats  Such a format could be described by ASN.1 Formal language for describing messages exchanged in a distributed system Used heavily in telecommunications  More next time...

15. HTTP HyperText Transfer Protocol  RPC-style interface to web servers  Messages represented as user-readable ASCII strings May contain encoded information (e.g., Quoted-Printable or Base64)  MIME types in Content-Type: and Accept: headers  Typically runs on TCP/IP, port 80 as default But may use other reliable transports Behavior  HTTP/1.0 (RFC 1945) behavior Open socket, request, response, close socket  HTTP/1.1 (RFC 2616) behavior Persistent connections Good for user Good for network

16. Core Web Technologies HTTP(HyperText Transfer Protocol) generic, stateless protocol governs the transfer of files across a network developed at CERN (Central European Research Network), they also came up with the name WWW, later W3C supports access to SMTP,FTP and other protocols was designed to support hypertext

17. Core Web Technologies Exchanged information, can be static or dynamic Every resource, accessible over the Web has a URL(Uniform resource locator) HTTP mechanism is based on client/server model typically using TCP/IP sockets

18. Core Web Technologies since Version 1.1 HTTP requires servers to support persistent connections, to minimize overhead associated with opening and closing connections. Typical methods on the server side are: OPTIONS  send information about the communication options GET  retrieve document or document produced by a program POST  Append or attach information PUT  Store information DELETE  Delete the resource indicated in the request

19. Core Web Technologies Another limitation  HTTP is stateless Does not provide storing of information between requests No indication of any relationship between two different requests  cookies, small data structures that a web server requests the HTTP client to store on the local machine, are used to maintain state information e.g. cookies store recently view items on a web shop

20. HTTP Messages (1) HTTP-message = Request | Response ; HTTP/1.1 messages generic-message = start-line *(message-header CRLF) CRLF [ message-body ] start-line = Request-Line | Status-Line message-header = field-name ":" [ field-value ] Request-Line = Method SP Request-URI SP HTTP-Version CRLF Method = "OPTIONS" ; Section 9.2 | "GET" ; Section 9.3 | "HEAD" ; Section 9.4 | "POST" ; Section 9.5 | "PUT" ; Section 9.6 | "DELETE" ; Section 9.7 | "TRACE" ; Section 9.8 | "CONNECT" ; Section 9.9 | extension-method ’host:’ mandatory

21. An Example GET Interaction

22. HTTP Messages (2) HTTP-message = Request | Response ; HTTP/1.1 messages Response = Status-Line ; Section 6.1 *(( general-header ; Section 4.5 | response-header ; Section 6.2 | entity-header ) CRLF) ; Section 7.1 CRLF [ message-body ] ; Section 7.2 Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF Status-Code = "100" ; Section 10.1.1: Continue | "101" ; Section 10.1.2: Switching Protocols | "200" ; Section 10.2.1: OK | "201" ; Section 10.2.2: Created | "202" ; Section 10.2.3: Accepted... | "300" ; Section 10.3.1: Multiple Choices | "301" ; Section 10.3.2: Moved Permanently... | "400" ; Section 10.4.1: Bad Request | "401" ; Section 10.4.2: Unauthorized | "402" ; Section 10.4.3: Payment Required | "403" ; Section 10.4.4: Forbidden | "404" ; Section 10.4.5: Not Found | "405" ; Section 10.4.6: Method Not Allowed... | "500" ; Section 10.5.1: Internal Server Error | "501" ; Section 10.5.2: Not Implemented... | extension-code Informational Success Redirection Client Error Server Error

23. HTTP Methods GET  Retrieves the resource identified by the request URI. May encode request parameters in URI HEAD  Identical to GET except that a message-body must not be returned. E.g., for testing validity, recent modification, accessibility of links POST  Request that server accepts entity enclosed in request as new subordinate of URI, e.g., to annotate of resources, append to a database,... PUT  Requests for a resource to be stored under the URI DELETE  Removes the resource identified by the request URI OPTIONS  Returns the HTTP methods the server supports CONNECT  Reserved for proxies that can dynamically switch to a tunnel TRACE  Returns the header fields sent with the TRACE request, e.g., for testing

24. Proxies An intermediary program which acts as both a client and a server Caching  E.g., GET cacheable  E.g., HEAD not cacheable (well, sort of) The Web is stateless so stale caches a problem  Age: sum of time resident at caches + time on network  Used for reliable cache expiration But proxy MAY still return stale resource with warning

25. More on Reliability and Web Technology Mostly focussed on security  Authentication and confidentiality Secure Socket Layer (SSL)  Privacy Will get back to this next time on web services

26. Web Browsers One of the first problems  web Browsers were originally intended only to display static documents, returned by HTTP calls Difficult to build sophisticated application specific clients for web browsers

27. Applets One answer to this problem  Applets  Java programs, can be embedded in an HTML document When the document is downloaded, the program is executed by the JVM, presented in the browser, turning the browser into a client by sending the client code as an applet Limitations  download the code Advantage  complexity

28. CGI(Common Gateway Interface) Web servers must be able to server up content from dynamic sources  How can a Web server respond to a request by invoking an application that will automatically generate a document to be returned One of the first approaches to solve this problem, was CGI, a standard mechanism that enables HTTP servers, to interface with external applications, which can serve as „gateways“ to the local information system

29. CGI How does CGI work  it assigns programs to URLs, so that when the URL is invoked, the program is executed  CGI programs often serve as an interface between a database and a Web server, allowing users to submit complex queries over the DB through predefined URLs  When the Web server receives request for the URL, it will run a program, that will act as a client of the database and submit the query  executing and packs the result into a HTML document  returned to remote browser

30. Servlets Performance  CGI programs involve a certain overhead Separate process for each instance  takes time, requires a context switch in the operating system Multiple request results – multiple process To avoid this overhead, Jave servlets can be used instead The idea is exactly the same as in CGI programs, but the implementation differs.

31. Servlets How do they work?  Execution and result is the same, but servlets are invoked directly by embedding servlet- specific information within an HTTP request  run as threads of the Java server process, moreover they run as a part of the Web server  eliminates overhead

32. Summary Web technology underlies web services  HTTP is the basic transport  XML a cornerstone in web services definition