Transcript Hypertext Markup Language HTML

Beyond DHTML
So far we have seen and used:
CGI programs (using Perl ) and SSI on server side
Java Script, VB Script, CSS and DOM on client side.
For some applications involving animations and graphics
scripting languages are not powerful enough. Other
technologies are used in such cases.
Java Applets.
COM objects.
Java Applets
Can be considered as a software component that can be
embedded into an HTML page. An applet cannot run
independently. Needs an applet viewer or a web page to call it.
A file compiled to byte code (.class extension) contains the
applet on the server and is downloaded by the browser along
with HTML page to run on the local machine
Security Issues
Java virtual machine
Applet example.
Component Object Model (COM) Objects
Microsoft technology
Provides a specification and infrastructure for use of compiled
ActiveX components. Each component is designed to provide
specific functionality and can be reused many times. An ActiveX
control is an object that supports a customizable,
programmatic interface. Using the methods, events, and
properties exposed by a control, Web authors can automate
their HTML pages. Examples of ActiveX Controls include
text boxes, command buttons, audio players, video players,
stock tickers, and so on.
You can develop ActiveX Controls using Microsoft Visual
Basic, Microsoft Visual C++, and Java.
Security Issues
ActiveX Data objects for Database connectivity
ODBC (Open Database Connectivity) driver is a low level
program that translates messages from an application into the
syntax expected by a specific database. Each different type of
database (access, Oracle, Db2, etc) requires a different ODBC
driver.
Applications that need to access databases have to be able to
communicate with ODBC drivers. To simplify this
communication Microsoft has developed some COM objects:
Data Access Object (DAO) - does not support queries that
change the content in some databases.
Remote Data Object (RDO) -is full-featured but only
available in Microsoft high-end programming environments.
ActiveX Data Object (ADO) - is full featured and available in
most Microsoft environments.
Microsoft Data Access Architecture using ODBC
Application
Data
Objects
DAO
ODBC
Drivers
Access
RDO
Oracle
Database
ADO
DB2
Beyond DHTML
DHTML along with CGI scripting works well for most
applications but more complex applications need
additional technologies.
Consider a scenario:
An HTML page has a form that asks the user to input
mailing address. In order to make the page look dynamic
we want the postal code field be filled automatically when
the user fills in the street and city fields.
Last Name:
First Name:
Street Address:
City:
Postal Code:
Beyond DHTML
How to achieve the functionality described in previous slide?
1. Send form contents to server w/o postal code. Server has
complete list of postal codes in the country. It finds the
correct postal code and creates a new page with a form
containing all data including postal code, sends it to browser
where it is displayed.
Additional effort, time, annoying wait for user.
2. When the first page is sent by the server the entire file
containing complete list of postal codes in the country is also
sent to the browser. Now browser can look into the list
locally and update the relevant field in the form.
Long download time, heavy network traffic.
A more efficient solution is to use Distributed Objects.
Distributed Objects
Remote procedure call (RPC)
A process on client machine can execute a procedure that
exists on the server and get the results back.
Remote Method Invocation (RMI)
Based on Java Remote Method Protocol (JRMP). An
object on a client machine can invoke a method of an
object that exists on the server machine and get results.
Common Object Request Broker Architecture (CORBA)
Is a specification from Object Management Group (OMG)
governing the remote communication between objects.
Implementations from many vendors. Interoperability.
Uses Object Request Broker (ORB) that handles requests
and responses from / to different objects.
Remote Procedure Call (RPC)
A process on client machine can execute a procedure that exists
on the server and get the results back.
RMI (Remote Method Invocation)
Based on Java Remote Method Protocol (JRMP). An object on a client machine can
invoke a method of an object that exists on the server machine and get results.
Client Object
Server Object
Stubs
Skeletons
RMI Infrastructure
JRMP/IIOP transport layer
Applet using RMI
Web Server
with HTML
pages
Client Browser
HTTP
Application Server
HTML
Remote
Object
Applet
Stub
RMI
Infrastructure
Skeleton
CORBA(Common Object Request Broker Architecture)
Is a specification from Object Management Group (OMG) governing the remote
communication between objects. Implementations from many vendors. Interoperability.
Uses Object Request Broker (ORB) that handles requests and responses from / to
different objects.
Client
Object
Implementations
Request
ORB
Use of CORBA in Web Application
Web Server
with HTML
pages
Client Browser
HTTP
Application Server
HTML
Remote
Object
Applet
ORB
Internet
Inter-ORB
Protocol
(IIOP)
ORB
Distributed Component Object Model (DCOM)
Microsoft’s solution to distributed objects is to use COM
objects by suitably deploying and registering them.
COM objects are assigned a special class identifier (CLSID).
Clients wanting instances of a particular COM request them
with CLSID from the operating system. When a client
machine supports DCOM, it is possible to locate these
objects on a remote server.
A module called “Service control manager” SCM has the
responsibility to locate the objects.
DCOM (Distributed Component Object Model)
Server
Client
Client
SCM
Proxy
Stub
DCOM network protocol
Component
SCM