Distributed Programming - CORBA

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Distributed Programming - CORBA

• Marc Conrad
• D104 (Park Square Building)
• Marc.Conrad_at_luton.ac.uk
• See also www.perisic.com for CORBA links.

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The Common Object Request Broker Architecture

CORBA
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PART I CORBA and the OMG

• Explaining the meaning of the acronym CORBA and
  the role of the OMG for CORBA.

4
The Common Object Request Broker Architecture

• CORBA is not a programming language but an
  architecture.
• CORBA is an elegant and sophisticated way to
  describe and link the use of objects in a
  distributed environment.
• The "dirty work", the implementation of the
  behavior of the objects is out of the scope of
  CORBA.

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The Common Object Request Broker Architecture
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The Common Object Request Broker Architecture

• CORBA is a standard which has been devloped by
  the OMG, the Object Management Group.
• The OMG is the world's largest computer industry
  consortium with over 800 members.
• See http//cgi.omg.org/cgi-bin/membersearch.pl

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A Collection Of Rather Bulky Acronyms.

• OMG - Object Management Group
• ORB - Object Request Broker
• IDL - Interface Definition Language
• IOR - Interoperable Object Reference
• POA - Portable Object Adapter
• IIOP - Internet Inter-ORB Protocol

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PART II - An example CORBA implementation

• or
• CORBA says "Hello World"
• (and a first glance at IDL)

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The CORBA Development Process

• Write some IDL that describes the interfaces to
  the object or objects that will be used or
  implemented.
• Compile the IDL file.
• Identify the IDL compiler generated classes and
  interfaces.
• Write additional code.
• Run the application.

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IDL Example

• module Example
• interface Hello
• string sayHello()

This is an example of an IDL which provides one
method, namely sayHello(). This code is saved in
a file hello.idl
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Mapping Hello.idl to Java(Here with idlj)

• Go to a command line prompt and type in the
  following command idlj -fall
  Hello.idl

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Mapping Hello.idl to Java(Here with idlj)

• The command idlj -fall Hello.idl produces 6
  files

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Mapping Hello.idl to Java(Here with idlj)

• The command idlj -fall Hello.idl produces 6
  files
• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

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Writing the Server.

• For the implementation of the Server we have to
  implement two classes
• The Servant, i.e. the object which has been
  specified by the Hello.idl
• The Server itself, i.e. the program running in
  the background which reacts on client requests.

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Writing the Servant

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

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Server

• Initialise the ORB
• Instantiate a HelloServant object.
• Register this object in the ORB.
• Publish the object to the rest of the world
• Wait for client requests.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(helloRef)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new
  PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

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A stringified reference

• Here is an example of an stringified object
  reference, generated by the example code
• IOR000000000000001649444c3a4578616d706c652f48656c
  6c6f3a312e3000000000000001000000000000005800010100
  0000000f3139342e38302e3231352e32323300000682000000
  000018afabcaff0000000225163ffd00000008000000000000
  00000000000100000001000000140000000000010020000000
  000001010000000000

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Client.

• Initialising the ORB.
• Retreiving an object reference.
• Narrowing the object.
• Using the object.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloClient
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• String filename "A//HelloIOR"
• FileInputStream fis new
  FileInputStream(filename)
• java.io.DataInputStream dis new
  java.io.DataInputStream(fis)
• String ior dis.readLine()
• org.omg.CORBA.Object obj
  orb.string_to_object(ior)
• Hello helloRef HelloHelper.narrow(ob
  j)
• String str helloRef.sayHello()
• System.out.println(str)
• catch (Exception e)
• System.out.println("ERROR " e)
• e.printStackTrace(System.out)

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The CORBA services

• Instead of sending a stringified object reference
  it is also possible to use CORBA services for
  obtaining object references
• The CORBA naming service, which finds objects by
  name, similar to a "white page" phone book.
• The CORBA trading service, which finds references
  that match search criteria, similar to a "yellow
  page" service.

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Summary

• The IDL provides class definitions for objects.
• A Compiler compiles the IDL into several files.
• The server and the client are then implemented by
  using these files.
• The ORB plays a crucial role in running the
  application both for the server and for the
  client.

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PART III Learning the IDL

• or
• We learn a language which is not a programming
  language but used to develop programs.

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The OMG IDL Overview.

• IDL supports the following constructs
• Constants
• Data type declarations
• Attributes
• Operations
• Interfaces
• Modules
• Valuetypes

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IDL Types and Constants

• The following basic types are available in IDL
• short, unsigned short, long, unsigned long, long
  long, unsigned long long, float, double, long
  double, fixed, char, wchar, boolean, string,
  wstring, octet, enum, any, native.
• Excercise How are all this types mapped into
  Java? C?

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The IDL language mapping

• The OMG so far specifies mappings to the
  following languages
• C, C, Smalltalk, COBOL, Lisp, Python, and Java.

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PART IV - The IDL to Java mapping

• or
• How to translate an IDL to a language where you
  can really write programs.

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Mapping IDL to Java

• IDL supports the following constructs
• Constants
• Data type declarations
• Attributes
• Operations
• Interfaces
• Modules
• Valuetypes

• A constant is mapped to a Java public static
  final variable within a class of its own.

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Mapping IDL to Java

• IDL supports the following constructs
• Constants
• Data type declarations
• Attributes
• Operations
• Interfaces
• Modules
• Valuetypes

• IDL operations are mapped to Java methods as seen
  in the HelloWorld example.

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Mapping IDL to Java

• IDL supports the following constructs
• Constants
• Data type declarations
• Attributes
• Operations
• Interfaces
• Modules
• Valuetypes

• Interfaces are mapped to Java interfaces/classes
  as seen in the example.
• Note One interface generates more then one Java
  class/interface (Helper classes, )

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IDL to Java mapping

• Identifiers are mapped to Java identifiers with
  some extra rules for special cases, e.g. class
  which is not an IDL keyword will be mapped to
  _class in Java.

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IDL Types and Constants

• This is the complete list of the basic types
  available in IDL
• short, unsigned short, long, unsigned long, long
  long, unsigned long long, float, double, long
  double, fixed, char, wchar, boolean, string,
  wstring, octet, enum, any, native.
• The blue ones do also exist in Java. What about
  the other?

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IDL Java mapping- Signed integer types

• short (16 bit)
• Mapped to Java short.
• long (32 bit)
• Mapped to Java int
• long long (64 bit)
• Mapped to Java long

• Caveat!
• The CORBA type long and the Java type long are
  different!

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IDL Java Mapping- Unsigned integer types

• There is an obvious type mismatch here.
• However, the mapping is defined from unsigned
  CORBA types to signed Java types.

• unsigned short (16 bit)
• Mapped to Java short.
• unsigned long (32 bit)
• Mapped to Java int
• unsigned long long (64 bit)
• Mapped to Java long

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IDL Types and Constants- Floating point types

• float
• 16-bit IEEE floating point number
• mapped to the Java type float
• double
• 32-bit IEEE floating point number
• mapped to the Java type double
• long double
• 64-bit IEEE floating point number
• so far not mapped!

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IDL to Java mapping- Fixed point type

• fixed
• fixed-point deximal number of up to 31 digits,
  e.g.
• typedef fixedlt5,2gt priceTag
• The IDL type fixed is mapped to the Java class
  java.math.BigDecimal.
• Range checking is performed at run time.
• Exceptions are raised if values are outside of
  the range.

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IDL to Java mapping- characters

• char, wchar
• are both mapped to the Java type char.
• Note IDL char is 8-bit, IDL wchar is 16-bit, the
  Java char is 16-bit.
• Also The IDL wchar can hold characters which are
  not part of Javas native Unicode set.

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IDL Types and Constants- strings

• string
• Variable-length string of characters whose length
  is available at run time.
• wstring
• Variable-length string of wchar characters.
• Strings may be bounded or unbounded, e.g. typedef
  octstring stringlt8gt for a bounded string of 8
  characters.

• These are mapped to java.lang.String with
  possible exceptions raised for bounded strings.

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IDL Types and Constants- miscancelous

• boolean (TRUE or FALSE)
• mapped to the Java type boolean.
• octet (8-bit uninterpreted type)
• mapped to the Java type byte.
• enum (enumerated type with named integer values.)
• mapped to a final Java class emulating the
  required properties.

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IDL mapping of any

• any (Can represent a value from any possible IDL
  type, basic or constructed, object or nonobject.
  any has an application interface describing how
  values are inserted, extracted, and how the type
  can be discovered.)
• mapped to org.omg.CORBA.any

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PART V the ORB

• or
• The thing which does the job.

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The ORB

• The ORB plays a central role in CORBA.
• We have already met the ORB in different
  contexts, so the following slides are slides
  already seen, but we emphasise now more on the
  role of the ORB.

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The Common Object Request Broker Architecture

• The Object Request Broker (ORB) is itself an
  object which provides the services for accessing
  remote objects and transmitting data.
• Note that the implementation of the ORB depends
  on the specific vendor. The CORBA standard only
  defines the interface.

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The ORB A Pseudo Object

• CORBA is primarily a specification for accessing
  remote objects.
• However, we can as well use the IDL for the
  specification of local objects.
• These objects are called pseudo objects.

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The Common Object Request Broker Architecture