1. II. Middleware for Distributed Systems
Outline
Principles of Object-Oriented Middleware
CORBA, COM and Java/RMI
ORB Runtime
Resolving Heterogeneity
Dynamic Object Requests
Principles of Object-Oriented Middleware

2. Principles of Object-Oriented Middleware
Outline
Computer Networks
Types of Middleware
Object-oriented Middleware
Developing with Object-Oriented Middleware
Principles of Object-Oriented Middleware

3. Principles of Object-Oriented Middleware
Network Operating System
Facilitate physical interchange of electrical or optical signals as packets of information
Detect and correct transmission errors
Implement routing of packets between hosts
Compose packets into messages
Application
Requesting operations from remote objects
Middleware
Shield lower level details from applications
Component-1 …
Component-n
Middleware
Network OS
Hardware
Host-1
Principles of Object-Oriented Middleware

4. Computer Networks ISO/OSI Reference Model (7-layer model) Application
Presentation
Session
Transport
Network
physical layer is concerned with the transmission of bits over a physical circuit
data link layer implements a single connection between two computers
network layer implements the interconnection of multiple nodes in a network.
transport layer adds the ability to route longer messages between two hosts.
session layer establishes and maintains connections between distributed system components.
presentation layer resolves differences in data representation and provides the ability to transport complex data.
application layer provides application functionality (in this book, the application layer will be implemented using distributed objects.)
Data Link
Physical
Principles of Object-Oriented Middleware

5. Computer Networks ISO/OSI Reference Model (7-layer model) Application
Presentation
Middleware
Session
Transport
Network
physical layer is concerned with the transmission of bits over a physical circuit
data link layer implements a single connection between two computers
network layer implements the interconnection of multiple nodes in a network.
transport layer adds the ability to route longer messages between two hosts.
session layer establishes and maintains connections between distributed system components.
presentation layer resolves differences in data representation and provides the ability to transport complex data.
application layer provides application functionality (in this book, the application layer will be implemented using distributed objects.)
Data Link
Physical
Principles of Object-Oriented Middleware

6. Available Middleware Transaction-Oriented Middleware
Support transactions across different distributed db systems
Two-phase commit protocol to implement distributed transaction
Products: IBM’s CICS, etc.
Message-Oriented Middleware
Supports communication between distributed system components by facilitating message exchange
Support asynchronous message delivery naturally
Support multi-casting
Fault tolerance (message queue on temporarily persistent storage)
De-coupling of client and server
Products: IBM’s MQSeries
Object Oriented Middleware
Has transaction-oriented middleware capabilities
Support synchronous communication with at-most-once semantics
Trend: to be integrated with message-oriented middleware
Principles of Object-Oriented Middleware

7. Available Middleware RPC Call across host boundaries
Origin of OO middleware
Interface Definition Language
See notes
Principles of Object-Oriented Middleware

8. Remote Procedure Calls
Presentation Layer
Resolution of data heterogeneity
Common data representation
Transmission of data declaration
Marshalling and Unmarshalling
static
Dynamic
Client and server stubs
Static implementations of marshalling and unmarshalling
Char * marshal() {
char * msg;
msg = new char[4*(sizeof(int) + 1) + strlen(name) + 1];
sprintf(msg, “%d%d%d%d%s”, dob.day, dob.month, dob.year, strlen(name), name;
return(msg); };
Principles of Object-Oriented Middleware

9. Remote Procedure Calls
Session Layer
Enable client to locate an RPC server
Static binding: simple, lost location transparency
Dynamic binding: depending on a deamon
Support location transparency
Print_person (char* host, Player * pers) {
CLIENT * clnt;
clnt = clnt_create(host, , 0, “udp”);
if (clnt == (CLIENT*) NULL) exit (1);
if (print_0(pers, clnt) == NULL)
clnt_perror(clnt, “call failed”);
clnt_destroy (clnt);
Principles of Object-Oriented Middleware

10. General Pattern for Remote Invocation
Client Code
Server Code
Stub
Skeleton
Infrastructure
Call:
marshal arguments
convert to network format
locate server
transmit data
Serve:
receive data
convert & unmarshal
invoke method
marshal return value
transmit data
Principles of Object-Oriented Middleware

11. Object-Oriented Middleware
IDL
Object types as parameters;
Failure handling;
inheritance
Interface Player: Object {
typedef struct Date {
short day; short onth; short year; };
attribute string name;
readonly attribute Date DoB; }
Interface PlayerStore: Object {
exception IDNotFound{};
short save (in Player p);
Player load(in short id) raises (IDNotfound);
Void print (in Player p); };
Principles of Object-Oriented Middleware

12. Object-Oriented Middleware
Presentation Layer
Similar to RPC
Support client and server stubs
Perform marshalling and unmarshalling
Resolve heterogeneity of data representation
Different from RPC
Define representation of object references
Marshalling/unmarshalling object references
Principles of Object-Oriented Middleware

13. Object-Oriented Middleware
Session Layer
Map object references to hosts
Implements object activation policies in the object adapter
Object adapters need to be able to start up severs, which register in an implementation repository or registry
Implement operation dispatch
Implement synchronization
Object Request Broker
Processes
Object Reference
Hosts
Objects
Principles of Object-Oriented Middleware

14. Developing with Object-Oriented Middleware
Design
Interface Definition
Server Stub Generation
Client Stub Generation
Server Coding
Client Coding
Server Registration
Principles of Object-Oriented Middleware

15. General Architecture for a DOC System
Registration Service
Object Skeleton
Object Storage
Server
Implementation
Object Manager
Object Interface Specification
IDL Compilers
Naming Service
Client Stub
Interface
Client Application
Principles of Object-Oriented Middleware

16. CORBA OMG standard Common architecture framework
Enable interoperability between applications in heterogeneous distributed environment
Common architecture framework
Common framework across heterogeneous hardware platforms and operating systems
Common framework for inter-communication of application objects
Open distributed object computing infrastructure
Automate many common network programming tasks, such as
object registration, location, and activation;
Request demultiplexing
Framing and error-handling
Parameter marshalling and un-marshalling
Operation dispatching
Principles of Object-Oriented Middleware

17. CORBA CORBA ORB Message-passing Object-oriented programming
Automate network functions
Sit on the host between the data and the application layer
Handles request messages from clients to servers in transparent manner
Principles of Object-Oriented Middleware

18. CORBA Objects A CORBA Object has an interface and an implementation
Interface is not bound to a specific implementation PL,
Interface Definition Language
Principles of Object-Oriented Middleware

19. Interface Definition Purpose of Defining Interface Reading Material
Instantiate meta-models
Provide details to class diagrams
Govern interaction between client and server
Provide basis for distributing type information
Provide basis for automatically generate client and server stub
Reading Material
Brose’s book chapter 2
Principles of Object-Oriented Middleware

20. Stub Generation Difference between Method Calls and Object Requests
Caller
Caller
Called
Stub
Stub
Called
Transport Layer (e.g. TCP or UDP)
Principles of Object-Oriented Middleware

21. IDL Compiler For each .idl file, idl compiler generates 4 files
Test.idl
IDL-Compiler
Testcl.hh
Testsv.hh
Testcl.cc
Testsv.cc
Principles of Object-Oriented Middleware

22. Implementation of Client Objects
A static object request is made by a client calling a local method of a client stub
Stubs are typed
Stubs can achieve access transparency
Middleware may shortcut a stub if server and client reside on the same host
Principles of Object-Oriented Middleware

23. Implementation of Server Objects
The generated server stub has to call the server implementation that an application builder designed.
Interfaces and inheritance make server object implementations type safe.
<<uses>>
<<uses>>
Player_Dispatch
Player_Imp
Player_Dispatch
<<Interface>> Player_Imp
<<Implements>>
Player_Server
Player_Server
Inheritance
Interface
Principles of Object-Oriented Middleware

24. Server Registration Server object must be registered
Registry
Implementation repository
System administrator maintain the repository
Middleware provide tools for maintain the repository
Register new server objects
Startup server objects
Stop/delete existing server objects
Principles of Object-Oriented Middleware

25. Summary OO middleware built on top of the transport layer
OO middleware implements the session and presentation layers
Other than OO middleware, there are transactional, message-oriented, remote procedure call middleware
Session layer implements an object adaptor activating/deactivating objects, synchronize client and server objects
Presentation layer resolve data heterogeneity
Development process for distributed objects
Idl, stub generation, coding, registration
Type safety
Implementation repository
Principles of Object-Oriented Middleware