Web Technologies

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Web Technologies

• Plattner Melanie
• Leschinger Bernhard

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What is this lecture about?

• Introduction to the basic Web technologies
• that are used to implement the Web portion of
  Web Services
• Some historical details
• Core Web Technologies
• Wide area integration
• Tunneling, Firewalls, EDIFACT

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Some historical details

• Start of standard groups
• TCP
• handles conversion between messages and streams
  packets
• IP
• handles addressing of packets across networks
• TCP/IP
• enables packets to be sent across multiple
  networks using multiple standards
• Telnet
• One of the earliest standards for exchanging
  transmission, directly connect accounts on
  different systems.
• SMTP
• specifies another way of direct connection

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Some historical details

• MIME
• Extension to the SMTP Protocol which supports the
  exchange of richer data files such as audio-,
  video-, and images data.
• FTP
• (1973) supports file transfer between Internet
  sites
• and allows a system to publish a set of files by
  hosting an FTP sever
• innovation ? permits anonymous users to transfer
  files
• Archie
• Late 1980s distributed file system based on FTP
• Gopher
• First simple system, providing GUI

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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

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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

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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

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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

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REST(Representational State Transfer)

• architectural style, defining the principles of
  distributed network systems.
• is the underlying architectural model, guiding
  the design and development of the current and
  next generation web architectures

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REST

• Provides a set of architectural constraints, that
  emphasizes
• Scalability of component interaction
• Generality of interfaces
• Independent deployment of components
• Enforce Security
• Etc.

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REST vs. Web Services

• Rest promotes and recommends generic operations
  on resources
• HTTP methods PUT GET POST DELETE
• SQL select create drop ect.
• Utilizes the caching mechanism
• WS does not promote generic operations
• First generation only utilizes HTTP POST
• Each service defines its own application specific
  operations
• Requires additional means of discrption,discovery
  mechanisms on top of the web
• No caching capabilities

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Rest Principles

• Web consists of addressable resources
• ? a user, utilizing an application selects a
  specific address(URL)
• ?a specific representation of that resource is
  returned over the web
• ? places the client application into a specific
  state.
• ?On accessing another URL, the client application
  gets another representation of the resource and
  in turn, transferring from the current to the new
  state.

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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

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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.

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B2C (Business to consumer)

• Conventional 3-tier architectures are designed to
  operate within a single company ? data exchanges
  occur within the safe boundaries of the company
• in principle there are no reasons why the system
  could not be opened to other users if the need
  arises
• ATM(automatic teller machines are an excellent
  example of the advantages if doing so

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B2C

• ATM(automatic teller machines are an excellent
  example of the advantages if doing so
• client/server system
• a PC with a network connection to the information
  services of the bank
• gives customers easier access to their accounts
  without the bank incurring
• a significant part of manual work disappears
• more efficient interactions with the customer
• ? great service, but there are limitations

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B2C

• Limitations
• Customers must travel to the nearest ATM,
• would not be necessary,
• access to their bank accounts any time,
• helps extend its functionality.
• This architecture is called B2C, indicating that
  the business allows consumers to access their
  information services directly

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Problem

• Users wanting to take advantage of this
  opportunity would need to have specialized
  clients for every company they want to interact
  with
• Complexity would grow enormous -gt administration

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Solution

• One of the biggest contributions of the Web ?
  providing a universal client for such extensions
• Nowadays such architectures are implemented by
  letting the remote computer use a Web Browser as
  a client
• since Web Browsers are standard tools, no
  application specific client has to be installed

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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

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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

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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

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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

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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.

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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

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Application Servers short overview

• Equivalent to middleware platforms
• Main difference ? intercorporation of the Web as
  a key access channel to the services implemented
  using the middleware ? several important
  implications
• The presentation layer acquires a much more
  relevant role
• Direct consequence of how HTTP and the Web work

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Application servers short overview

• Realized by merging the presentation layer
  related to the Web with the application layer of
  the middleware platform
• Reason ? to allow the efficient delivery of
  content trough the Web as well as to simplify the
  management of Web applications

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J2EE

• There are two competing frameworks for Web-based
  middleware
• Suns J2EE
• Microsoft's .NET
• very similar

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J2EE

• A significant aspect of application servers is
  the bundling of more and more functionality
  within the middleware platform
• Providing integrated support for many different
  middleware abstractions
• Therefore blurring the borders between
  application servers and other middleware

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Application server - Application Layer

• At the application layer, application servers
  conceptually resemble conventional middleware,
  provided functionality similar to CORBA, TP
  monitors and message brokers
• Goal of application server vendors
• providing a unique environment for hosting all
  kinds of application logic, whether Web-based or
  otherwise,

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J2EE - EJB

• EJB (Enterprise JavaBeans) specification is at
  the heart of J2EE ? there the bulk of the
  application logic resides
• An EJB is a server-side component, that delivers
  application-specific funktionality(responding to
  a request for a quote)

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J2EE - EJB

• The EJB specification defines 3 different types
  of beans, based on how they interact with other
  components and how they manage state and
  persistence
• Session beans
• Entity beans
• Message-driven beans

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J2EE EJB container

• Provides the environment in which the beans run ?
  all interactions go through the container
• Provides a number of services
• Supports transactions ? freeing a developer from
  having to define transaction boundaries and
  implement the related code

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J2EE - JNDI

• Defines an interface for directory services,
  without mandating any implementation
• Clients can bind to servers based on the object
  name (EJB ? binding to a server involves binding
  to an object that provides the interface for
  interacting with a server)

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J2EE - JDBC

• is an API that enables developers to access
  almost any tabular data source by executing SQL
  commands from a java program
• methods can be called from an EJB or directly
  from a servlet

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Application Server

• Offer services that simplify administration and
  management of the application
• Caching frequently needed objects
• Checking that an application is running and
  restarting
• Object administration and security, defining user
  has access to which application and enforcing
  access restrictions

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Application Server

• Cannot match the performance of TP monitors but
  they try to make systems easier to develop and
  easier to evolve.

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Application Server - Presentation Layer

• the support for the presentation layer and for
  the document as the basic unit of transfer is
  what differentiates application servers from
  conventional middleware
• Application servers
• implement mechanisms which make the transaction
  between documents and arguments more efficient,
  flexible and manageable
• provide a variety of presentation features to
  support the delivery of dynamically generated,
  personalized content to different types of clients

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Application Server - Presentation Layer

• A modern application server supports the
  following types of clients
• Web Browsers(most common types of clients)
• Applications
• Such as those encountered in conventional
  middleware
• Devices
• Such as mobile phones and PDAs
• E-mail programs
• Web service clients

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Wide area integration

• A number of strategies
• 3 layers
• Client
• Middleware
• Server(resource manager)

The available strategies are given by all
possible combinations of these three layers
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Wide area integration - strategies

• integrating systems at the client level
• at the middleware level
• by connecting clients directly to the remote
  middleware platform,
• by connecting resource managers to the remote
  middleware platform.
• Which of these strategies is the most
  appropriate, depends on a number of factors

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Middleware Extensions

• The internet requires additional middleware
  layers between clients and servers or between
  servers.
• ? existing platforms were simply extended to
  allow them to interact through the internet, most
  middleware platforms were designed to work on a
  single LAN(Local area network)

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B2B Business to Business

• The number of LANs started to grow
• ?different branches of the same company
  implemented their own middleware-based systems
• ? the need for different middleware platforms to
  communicate with each other, arose.
• such interactions are called B2B
• ? fully automate the interactions

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Firewalls and Tunneling

• Firewalls
• Acts as a barrier against unwanted network
  traffic
• Blocks many communication channels
• Can change the design space in two ways
• No direct communication between the system to be
  integrated
• Parties outside the firewall are not trusted

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Firewalls and Tunneling

• How to get through a firewall and why?
• Tunneling
• Tricking the firewall into believing that
  traffic, which otherwise should be blocked, is
  actually allowed
• Protocols which would be blocked are hidden under
  protocols that are accepted by the firewall
• Why ? not having a direct communication channel
  is compounded by a necessary lack of trust on all
  traffic generated outside the firewall

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EDIFACT

• Another important challenge
• Identifying a common syntax and semantics for the
  data exchanged between applications
• In conventional middleware platforms, this
  problem is hidden behind IDLs ? fulfill two roles
• ? used to define interfaces
• ?uses an intermediate data representation
  that specifies how each data type used in
  IDL is represented in a machine-independent
  manner

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EDIFACT

• In message-based systems, format and semantics of
  the messages or files exchanged are typically
  determined by the EDIFACT standard.
• Provides standard templates for messages and for
  the contents of the message

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EDIFACT

• A EDIFACT message typically contain the following
  fields
• Interchange Header
• Version of EDIFACT,IDs of sender end recipient,
  passwords,date,time
• Message Header
• Type of message
• User Data segments
• payload
• Message Trailer
• Check message completeness
• Interchange Trailer
• Check interchange completeness

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EDIFACT Pros Cons

• standard structure with 3 letter codes
• universal standard defined by EDIFACT
• parsers can be constructed easily
• - very complex standard, often unnecessarily
  complicated
• - often only a fraction of the possible
  information is used
• - hast to be standardized by EDIFACT before use
• -requires adhoc development on the systems