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Fall 2005 CEG 4183 15-1
Lecture 15:
Frameworks for Application-layer
Communications
Prof. Shervin Shirmohammadi
SITE, University of Ottawa
Fall 2005 CEG 4183 15-2
Background
• We have seen previously that:
– Applications need to exchange messages
– In the labs, the assignments, and the project, we
used application-level framing that uses
application-specific tokens.
• Whiteboard example
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Fall 2005 CEG 4183 15-3
Motivation
• Token-based messaging scheme can become
complicated as the application adds more and
more features.
– Think of the number of commands in a word-processor
applications.
• There should be a way for a more methodical and
structured technique to allow programs to
communicate.
– Ideally, the programmer should not even have to worry
about communication and how it’s done.
• Enter communications frameworks:
– RPC, RMI, CORBA, SOAP
Fall 2005 CEG 4183 15-4
Remote Procedure Call• RPC, used in C and C++ (theoretically language neutral)
• The idea goes back to 1976, with full-scale implementations appearing in the late 1970s.
• We know that in a program, functions can “call”one another.
• Even different programs running on the same computer can call each other’s function.– e.g., OLE on Windows, pipes on UNIX, …
• The idea behind RPC is to allow the same thing for programs running on different computers.– A program on my computer should be able to call a function inside a program on your computer.
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Fall 2005 CEG 4183 15-5
RPCHost 1 Host 2
void main()
{
compute(3, 5);
}
int compute()
{
return x+y/x;
}
int compute()
{
return x+y/x;
}
main.c compute.c
compute.c
• Somehow we have to transmit the return value (which can be anything: integer, array, linked list, …) over the network.
• What else must we take care of?
Fall 2005 CEG 4183 15-6
RPC Details• During compilation, client and server stubs are created.
• Application calls a function from the client stub, which converts arguments to Network Data Representation (NDR) and sends them to the RPC run-time.
• Run-time uses the transport
layer to send data to server.
• Server’s RPC run-time takes
the incoming data and passes
them to server’s stub, which
runs the actual procedure.
• Result is returned similarly.
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Fall 2005 CEG 4183 15-7
Remote Method Invocation
• RMI, used in Java only.
• Can be thought of as “Object-oriented RPC”.
• How to send objects over the network?
Host 1 Host 2
void main()
{
Local.compute(3, 5);
Remote.compute(3,5);
}
int compute()
{
return x+y/x;
}
int compute()
{
return x+y/x;
}
main.java
Local.java Remote.java
Fall 2005 CEG 4183 15-8
RMI Details• Stub (remote object) and skeleton (server object) is created.
• Stubs don’t need to be compiled into the client, they can be downloaded at run-time.
• Remote objects are registered, and clients can download their stub to use their services.
• Application invokes remote object’s stub as if invoking a local object.
• Stub passes the arguments to Remote Reference Layer which then uses the transport layer for network communication.
• RRL at server invokes the skeleton, which invokes the actual object to do the operation. The result is sent back in a similar manner.
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Fall 2005 CEG 4183 15-9
RMI Example• Time server example in RMI
• First, we define the service available by describing its methods.
• No implementation is needed at this stage– users don’t care anyway about the internal implementation, they just want
to be able to call the function.
• Defining the interface:
• TimeInterface.java:
import java.rmi.*;
public interface TimeInterface extends Remote {
public String getTime() throws RemoteException;
}
Fall 2005 CEG 4183 15-10
RMI Example: Remote Object
• Now we need to implement that Interface
• Time.java:
import java.rmi.*;
import java.rmi.server.*;
public class Time extends UnicastRemoteObject implements TimeInterface {
public String getTime() throws RemoteException {
return "It's 12PM";
}
}
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Fall 2005 CEG 4183 15-11
RMI Example: Server• We need to run the service:
• Server.java:
import java.rmi.Naming;
public Server {
public static void main (String[] argv) {
try {
Naming.rebind ("Time", new Time());
System.out.println ("Time Server is running.");
} catch (Exception e) {
System.out.println ("Problem with Time Server: " + e);
}
}
}
Fall 2005 CEG 4183 15-12
RMI Example: Client• Finally, we can write a client program that uses the Time service:
• Client.java:
import java.rmi.Naming;
public Client {
public static void main (String[] argv) {
try {
TimeInterfacce time_object = (TimeInterface) Naming.lookup("137.122.20.16/Time");
System.out.println (time_object.getTime());
}
catch (Exception e) {
System.out.println ("Exception: " + e);
}
}
}
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Fall 2005 CEG 4183 15-13
CORBA• Common Object Request Broker Architecture, an Object
Management Group (OMG) standard
• Cross-platform, language-independent!
• Provides services to enable an application in a given programming language to talk to a remote application using another programming language, both running on different platforms.
• Includes many services for registry, lookup, naming, and so on.
• Uses Interface Definition Language (IDL) to specify method signatures (return type, name, arguments).– IDL is platform-independent, and defines its own types.
• Stub (client) and skeleton (server) is created during compilation. These have to be implemented in a language-specific manner.
• Binding is done during run-time.
Fall 2005 CEG 4183 15-14
CORBA Scenario
Host 1Win 95
Host 2Solaris
void main()
{
compute(3, 5);
}
int compute()
{
return x+y/x;
}
main.cRemote.java
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Fall 2005 CEG 4183 15-15
SOAP• Simple Object Access Protocol, a W3C standard, is a communications protocol to exchange messages among applications.
• Typically used on top of HTTP.– Doesn’t have to be HTTP.
• An XML-based application-layer communications technique.
• The idea is, similar to RPC RMI and CORBA, to hide the communication part and let the application-layer on one side to talk to the application layer on the other side.
• Part of the Web Services paradigm.
• What is a Web Service? A Web page that provides a a service?
Fall 2005 CEG 4183 15-16
IBM
Evolution of Networked Applications
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Fall 2005 CEG 4183 15-17
Web Evolution
XMLXML
ProgrammabilityProgrammabilityConnectivityConnectivity
HTMLHTML
PresentationPresentation
TCP/IP
TCP/IP
Techno
logy
Techno
logy
Innovation
Innovation
FTP,FTP, EE--mail, Gopher
mail, GopherWeb Pages
Web Pages
Browse Browse the Webthe Web
Program Program the Webthe Web
Web Services
Web Services
Fall 2005 CEG 4183 15-18
Web Services are:• a solution for providing application-to-application
communication over the Internet.
• a URL-addressable software resource that performs functions.
• Web services communicate using standard protocol known as
SOAP (Simple Object Access Protocol)
•A Web services is located by its listing in a Universal
Discovery, Description and Integration (UDDI) directory.
•A Web Service is typically defined using the Web Service
Definition Language (WSDL)
SOAP
Bind
WSDL
UDDI
Publish /
describe
WSDL
UDDI
Discover /
describeService
Requester
Service
Broker
Service
Provider
XML
Requester Broker Provider
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Fall 2005 CEG 4183 15-19
Architecture – Request & Response
Listener
SOAP or XML-RPC
SOAP or XML-RPCWeb
Services
HTTP
HTTP
middleware
Service Tier
Business
Services
API
– SOAP and XML-RPC have become accepted standards for
XML-based messaging.
– HTTP is the protocol for the Internet.
– The Web Service will parse the request and either fulfill the
request directly, or invoke one or more business services via
some middleware API.
CORBA,
RMI,
DCOM,
JMS…
Fall 2005 CEG 4183 15-20
Extensible Markup Language (XML)• XML is a metadata language: data that describes data.
• It is used as a standard way for information exchange.
– W3C standard
– In some ways it can be thought as part of the presentation layer.
• It is a subset of SGML (Standard Generalized Markup
Language)
– SGML is the “parent” of HTML
• The key to success of XML: it lets you define your own
data types in a standard way.
• Processing XML:
– APIs (DOM, SAX)
– addressing XML: XPath, XLink, XPointer
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Fall 2005 CEG 4183 15-21
XML Element and Attributes
<resource>
<tutorial name=“cooking">
<authors>
<author>Jane Smith</author>
</authors>
<title>The Joy of Cooking</title>
</tutorial>
</resource>
Nice, but how do we know what are resource, tutorial, title, …?
Element
Attribute
Fall 2005 CEG 4183 15-22
Document Type Definitions (DTD)
• recursive types<!ELEMENT A (B|C)> "an A can contain a B..."
<!ELEMENT B (A|C)> "... which contains an A!"
<!ELEMENT C (#PCDATA)>
• loose typing – <!ELEMENT A ANY>
• no context-sensitive types:DTDs cannot distinguish between the publisher in
– <journal> <publisher>... </publisher> </journal>
– <website> <publisher> ... </publisher> </website>
• What do to in terms of conflicts such as this?
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Fall 2005 CEG 4183 15-23
XML Namespaces
• My element may not be your element:
– geometry context: <element>line</element>
– chemistry context: <element>oxygen</element>
– use XML namespaces to identify the vocabulary
<?xml version="1.0" encoding="utf-8" ?>
<lists xmlns=“http://deltabis.com/products” xmlns:it="http://deltabis.com/itinerary">
<product sku="8822N" size="small" type="trouser">
<it:itinerary>
<it:sold>120</it:sold>
. . .
Fall 2005 CEG 4183 15-24
Example
<?xml version="1.0" encoding="utf-8" ?>
<lists xmlns=“http://deltabis.com/products” xmlns:it="http://deltabis.com/itinerary">
<product sku="8822N" size="small" type="trouser">
<it:itinerary>
<it:sold>120</it:sold>
<it:onhold>45</it:onhold>
<it:returned>10</it:returned>
</it:itinerary>
</product>
<product sku="9820Y" size="small" type="tshirt">
<it:itinerary>
<it:sold>283</it:sold>
<it:onhold>232</it:onhold>
<it:returned>23</it:returned>
</it:itinerary>
</product>
</lists>
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Fall 2005 CEG 4183 15-25
WSDL
– Web Services Definition Language
– Standard for defining a Web Service
• Defines an abstract interface and bindings to particular message
formats
• Defines how to locate the service (URLs for HTTP)
• Defines what protocol the service uses (HTTP, SMTP, FTP)
• Written in XML
– Used to publish services in UDDI
Fall 2005 CEG 4183 15-26
UDDI• Universal Discovery, Description and Integration
• Standard for publishing Web Services– Directory service that enables clients to locate services
– Services can be found by searching or unique ID
– Servers provide a SOAP based interface (API) for finding and publishing
• Three roles for UDDI data– White pages – technical contact information and addresses
– Yellow pages – various services available from business
– Green pages – technical information about services
including WSDL
Clientapplication
Webserver
UDDI registry nodeUDDI registry node
SOAPprocessor
UDDI registryservice
UDDIdatabase
SOAP request
SOAP response
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Fall 2005 CEG 4183 15-27
Back to Web Services: SOAP
–SOAP provides transfer of structured databetween services.
–SOAP is an XML based transport protocol.
–Protocol binding: HTTP, SMTP…
–One way message: Sender or Receiver
– It is stateless, and therefore the sender and receiver need not maintain a session to communicate.• The application however has to take care of the communications semantics.
Fall 2005 CEG 4183 15-28
SOAP Details
• With SOAP, we’re actually back to a
concept which is similar to the familiar
token-based message passing, but SOAP is
standardized.
Host 1 Host 2void main()
{
getLowestPrice();
}
main.cWeb service provider
Give me lowest price for book
lowest price is $67.99
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Fall 2005 CEG 4183 15-29
SOAP Request
getLowestPrice<?xml version="1.0"?>
<soap:Envelope
xmlns:soap="http://www.w3.org/2001/12/soap-envelope" soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">
<soap:Body>
<m:GetPrice mlns:m="http://www.w3schools.com/prices"> <m:Item>ISBN78932167</m:Item>
</m:GetPrice>
</soap:Body>
</soap:Envelope>
This message is sent to the server using HTTP.
Fall 2005 CEG 4183 15-30
SOAP ResponseResponse:
<?xml version="1.0"?>
<soap:Envelope
xmlns:soap="http://www.w3.org/2001/12/soap-envelope" soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">
<soap:Body>
<m:GetPrice mlns:m="http://www.w3schools.com/prices"> <m:Price>67.99</m:Price>
</m:GetPrice>
</soap:Body>
</soap:Envelope>
This message is returned to the requester.
