COMMON OBJECT REQUEST BROKER ARCHITECTURE

1. Common Object Request
Broker Architecture
Ali Ghodsi
aligh@imit.kth.se
2004-02-02 A. Ghodsi aligh@imit.kth.se 1
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2. Goal of lecture
Go a bit more into depth on the core
architecture of CORBA
Less breadth
Read van Steen’s book
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3. Reading suggestions
Tanenbaum & van Steen
CORBA
Section 2.3 page page 85-98
Section 3.2.2 page 152-158
Section 9.1
Read chapter 9 and compare other systems with CORBA
Compare RPC and DCE Remote Objects with CORBA
Links
Nice CORBA tutorial:
http://www.omg.org/gettingstarted/
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4. Outlook
General Overview
General Information
Applications
Quick Architectural Overview
OOP plus Distribution Transparency
CORBA main overview
Interface Definition Language (IDL)
Types
Examples
Mappings
ORB
DII (and DSI)
ORB interface
Object Reference
POA
Persistent and Transient Objects
Conclusions
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5. General CORBA Information
Distributed Object Model (more later)
It is a middleware
Difference between Network OS Middleware?
Only a standard (v 2.7, 3.0)
No reference implementation!
Many independent implementations
OMG - Non-profit organization
800 members!
Standardized UML and more…
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6. Real World Applications?
Support ”dinosaurs”
Companies have invested years of development in
projects done in ADA, C, Smalltalk…
CORBA enables interoperability with new languages
Languages with small user-base
Eg Erlang, again interoperability
Big ERM, ERP, IS
Many different architectures, languages, platforms…
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7. Outlook
General Overview
Quick Architectural Overview
OOP with Distribution Transparency
CORBA overview
Interface Definition Language (IDL)
Types
Examples
Mappings
ORB
Conclusions
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8. CORBA builds on the DOM
Provides a nice model
Encapsulation
Inheritance
Polymorphism
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9. Exploiting Encapsulation
Encapsulation enables:
Distribution Transparency
Have stubs and skeletons that together with ORBs
enable distribution*.
Inter-operability**
Define interfaces in a standardised way
Interface Definition Language (IDL)
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10. Goal 1: Distribution Transparency
Encapsulation: black-box principle
Has an interface
Implementation details
public interface MathBox {
hidden
int add(int x, int y);
} Transparently
distribute
public class MathBoxCL
… implements MathBox {
MathBox obj = new MathBoxCL(); MathBoxCL() {}
System.out.println(obj.add(10,20)); int add(int x, int y)
… { return x+y; }
}
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11. Distribution Transparency
Missing parts:
Client • Marshalling Server Implementation
… int add(int x, int y)
• Unmarshalling
MathBox obj = new MathBoxCL(); { return x+y; }
Integer result = obj.add(10,20);
• References
…
MathBoxCL (SKELETON)
• Binding client to server
int invoke(msg msg)
MathBoxCL (PROXY) {
{ int x, y;
x=msg.Unmarshall(INT);
x=msg.Unmarshal(INT);
int add(int x, int y)
y=msg.Unmarshall(INT);
y=msg.Unmarshal(INT);
{ Msg msg=new Msg();
res=serverImpl.add(x,y);
msg.Marshall(x);
msg.Marshal(x);
Msg msg=new Msg();
msg.Marshall(y);
msg.Marshal(y);
msg.marshall(res);
msg.marshal(res);
SendReqMsg(HOST,IP,msg);
SendReqMsg(HOST,IP,msg);
SendRespMsg(HOST,IP,msg);
SendRespMsg(HOST, IP, msg);
}
}
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12. Goal 2: Inter-operability
Use a language with standardized syntax to
define the interface
Generate the stub and the skeleton
Programming Language Independent
C++
JAVA
MathBoxCL (SKELETON)
int invoke(Msg msg)
{ int x, y;
MathBoxCL (STUB) msg=GetMsg();
int add(int x, int y)
{ Msg msg=new Msg(); x=msg.Unmarshal(INT);
msg.Marshal(x); y=msg.Unmarshal(INT);
msg.Marshal(y); res=serverImpl.add(x,y);
SendReqMsg(HOST,IP,msg); Msg msg=new Msg();
} msg.marshal(res);
SendRespMsg(HOST, IP, msg);
}
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13. Overview
operation()
Client args + return Object Implementation
value
Object
SKELETON
STUB Adapter
Network
ORB Core ORB Core
ORB-dependent implementation
Application specific Stub and Skeleton
Same inteface. ORB-independent
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14. Outlook
General Overview
Architecture Overview
Interface Definition Language (IDL)
Types
Example
Language Mappings
ORB
Conclusions
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15. Interface Definition Language
Builds on OOP principle of encapsulation
Clear boundary between implementation and interface
Independent
Programming Language (Only OO?)
OS
Platform
Network Connection
etc
Can be converted to a binary format and stored in a
database (i.e. well-defined schema, iterators)
Interface Repository (IR)
A Repository ID for each interface
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16. IDL’s Type System
Two levels:
Interfaces for CORBA objects!
One interface per CORBA object
Official types for variables
integers, floats
struct, enum
array
string
binary values
…and more!
Scoped types
modules
exceptions
Interfaces
structs
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17. Examples
DSLABS.IDL:
typedef string GroupMembers[4];
interface DS_project {
long register_project(in long groupId, in string status, inout string date);
long get_status(in long groupId, out string state, out string date, out
GroupMembers gm);
};
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18. IDL language mappings
OMG defines mappings to different languages
C, C++, Java, Smalltalk, COBOL, Ada, Lisp, PL/1,
Python, and IDLscript
Proprietary mappings exist for obscure languages,
though not standardized!
Every ORB has an IDL compiler
Creates
A STUB and
A SKELETON
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19. Outlook
General Overview
Architecture Overview
Interface Definition Language (IDL)
ORB
DII (and DSI)
ORB interface
Object References
POA
Persistent and Transient Objects
Conclusions
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20. Compile time vs Runtime?
What if interfaces change?
Recompile everything? Unfeasable
Dynamic interface definitions required:
IS (Information Systems)
ERM (Enterprise Resource Management systems)
Batch Service
etcetera
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21. Dynamic Invocation Interface (DII)
Generic run-time invocation
No compile-time knowledge of CORBA
object interfaces
No stub and skeleton needed a-priori
Instead, a generic interface is used
Of course defined in IDL
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22. Dynamic Invocation Interface (DII) cont.
In essence:
Search and fetch an IDL from an Interface
Repository. (remember binary presentation
of IDL)
Construct a request
Specify target object, operation, and
parameters
Invoke the request
C++ (not entirely true)
invoke(remoteObj, ”getStatus”, paramters)
Java uses reflection/introspection (transparent):
remoteObj.getStatus(paramters);
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23. Complete picture
operation()
Client args + return Object Implementation
value
Dynamic
Static Object
Dynamic Static Skeleton
Skeleton Adapter
Invocation Stub Interface
Network
ORB Core ORB Core
ORB-dependent implementation
Application specific Stub and Skeleton
Same inteface. ORB-independent
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24. Object References
Remote object references
Enable clients to invoke CORBA objects
Three incarnations
Language specific implementation
E.g. pointer to a stub in C++ implementing the
IDL
Not valid outside local computation space
Language independent ORB representation
IOR, Inter-operable Object Referenece
Supported by all ORBs
Textual representation
Send by e-mail, store in DB, textfiles and so on.
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25. Inter-operable Object References (IOR)
Type Name Protocol
”Reference toObject ReferenceObject Key Name)
Remote an object on a server
(Repository ID) Hostname & Port (Adapter & Object
*GIOP, address, port ex: ”IIOP v1.0”,”ripper.it.kth.se”, 8765
*Which object adapter, which object?
ex: ”OA5”, ”_DSD”
Repository ID ex: ”IDL:KTH/imit/DSD:1.0”
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26. ORB Interface
operation()
Client args + return Object Implementation
value
Dynamic
ORB Static Object
Dynamic Static Skeleton
Interface Skeleton Adapter
Invocation Stub Interface
Network
ORB Core ORB Core
ORB-dependent implementation
Application specific Stub and Skeleton
Same inteface. ORB-independent
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27. ORB Interface
Standard interface (defined in IDL)
All ORBs implement this interface
Important services provided:
Bootstrapping, getting initial references
Converting Object References to Strings and
vice versa
Object Reference Counting
Distributed garbage collection
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28. How do I get an IOR?
All ORBs implement:
string_to_object()
file, e-mail, phone :)
resolve_initial_references()
Returns an IOR for naming service, interface
repository
Continue to search for IOR’s in a naming
service
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29. Portable Object Adapter (POA)
operation()
Client args + return Object Implementation
value
Dynamic
Dynamic
ORB Skeleton
Static Object
Static Skeleton
Interface Interface Adapter
Invocation Stub
Network
ORB Core ORB Core
ORB-dependent implementation
Application specific Stub and Skeleton
Same inteface. ORB-independent
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30. Why Object Adapters?
Client 1 Server
dsObject.calculate(); DsObject::calculate()
{
Client 2 ...
dsObject.calculate(); }
Several clients call the same object, what
to do?
Demultiplex requests
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31. Why Object Adapters? (2)
Client 1 Server
dsObject.calculate(); DsObject::calculate()
{
Client 2 ...
dsObject.calculate(); }
Queue requests or run in separate threads?
Serialize all requests
One thread per object
One thread per invocation
Pool of threads
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32. Why Object Adapters? (2)
Client 1 Server
dsObject.calculate(); DsObject::calculate()
{
Client 2 ...
dsObject.calculate(); }
Security between the objects?
Sandboxing?
Share methods, separate data?
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33. Why Object Adapters? (2)
Client 1 Server
dsObject.calculate(); DsObject::calculate()
{
Client 2 ...
dsObject.calculate(); }
Lifespan policy:
Transient objects
Persistent Objects
Continues to exist even if activated/deactivated?
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34. Portable Object Adapter – PL meets
ORB!
POA is generic and CORBA object independent
and implements different activation policies
POA keeps pointers to skeletons*
An Object Identifier is associated with object.
A table called Active Object Map maps between
Object Identifers => Skeletons
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35. Portable Object Adapter
OBJ 1 OBJ 2 OBJ 3
skel1 skel2 skel3
POA1 (policy1) POA2(policy2)
Active Object Map Active Object Map
Invoke right OBJ2 -> skel2 Invoke the right OBJ3 -> skel3
OBJ1 -> skel1
skeleton skeleton
Server Demultiplexer
Dispatch requests POA1
to the right POA POA2
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36. Transient Object Illustration
Client
_dsd->student_operation()
Object Implementation
STUB: Object Reference Reply message
IDL:Institution/IT/DSD:1.0 ”IIOP v1.0”,”ripper”, 8765 ”OA5”, ”_DSD” Unique ID : ”13FABCDA”
return variables, out parameters
Active Object Maps
Stub OA4:
_InfoSec Object
Request message Skeleton
OA5: Adapter
Unique ID : ”13FABCDA” ”OA5”, ”_DSD”
_DSC
student_operation() + *par _DSD
ORB Core ORB Core
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37. Persistent Objects
A IOR to a Persistent Object always points
to the same object
Migration Transparency
Location Transparency
Ceases to exist only if the CORBA object is
logically destroyed
Might move
Might change IP, Port, Machine
Might change POA
etc
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38. Persistent Objects continued
Its life cycle is independent of the objects
I.e. its existence is independent of whether the
object is in the local address-space.
ORBs can automatically startup objects
implementing persistent CORBA objects
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39. How is this possible?
Implementation repository (IMR) is used
Keeps information about
Object Adapter
Startup Command
Current Server
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40. Persistent Objects Illustrated
Client
Implem. Repository Object Implementation
_dsd->student_operation()
IMR Table
Adapter Startup Address
ORB Core xOA_1 rsh x ”/bin/st” bored:131
yOA_2 /startup ir:1444
Reply message
OA5 rsh ripper /run ripper:313
Location Forward Unique ID : ”13FABCDA”
return variables, out parameters
ORB Core
Active Object Maps
Stub OA4:
_InfoSec
Request message
OA5:
Unique ID : ”13FABCDA” ”OA5”, ”_DSD”
_DSC Skeleton
student_operation() + par _DSD
STUB: Object Reference
IDL:KTH/imit/DSD:1.0 ”IIOP v1.0”,”IMR”, 8765
”IIOP v1.0”,”ripper”, 313 ”OA5”, ”_DSD”
ORB Core
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41. Failure and replication (IOR cont)
Multiple locations in one reference. If an invocation fails, go to next location
Type Name Protocol1 Object1 Key
(Repository ID) Hostname1 & Port1 (Adapter1 & Object1 Name)
Protocol2 Object2 Key
Remote Object Port2 on a server
to an object (Adapter2
”Reference Hostname2 & Reference & Object2 Name)
*GIOP, address, port ex: ”IIOP v1.0”,”ripper.it.kth.se”, 8765
Repository ID ex: ”IDL:KTH/imit/DSD:1.0”
HOST1/PORT2/ADAPTER2/OBJECT2 ex: ripper1/1234/oa1/obj1
HOST2/PORT2/ADAPTER2/OBJECT2 ex: ripper2/3233/oa3/obj6
…
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42. Summary-1
CORBA is a standardization effort
Based on the the Distributed Object Model
Provides inter-operability
Uses proprietary interface language: IDL
All CORBA objects have an interface in IDL
Most of the services offered by CORBA have an
interface in IDL
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43. Summary-2
Provides both Dynamic and Static invocations
DII/DSI and STUBS/SKELETONS
Stubs/Skeletons talk to an ORB
The ORB uses a standardized protocol to exchange
Messages(Req/Resp), IORs (persistent/transient)
ORB uses a POA to implement different activation policies
Threading policy
Lifespan policy (Persistent vs. Transient Objects)
Security (sandboxing of object implementations)
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44. What did I miss?
A whole lot! ☺
CORBA facilities/services
Synchronization
Caching
Replication
Fault-tolerance
Security
Comparison of CORBA against
.NET
DCOM
Java RMI
etcetera
Please read chapter 9!
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45. The End
THANK YOU VERY MUCH!
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